On 5 June 2017 at 00:59, Attila Kinali <attila@kinali.ch> wrote: > Moin, > > This discussion is kind of getting heated. > Let's put some facts in, to steer it away from > opinion based discussion. > I can't find it now, but I know someone said thermocouples are obsolete. I spoke to a friend tonight who services industrial boilders. He said thermocouples are far from obsolesce, at temperatures of a few hundred deg C, as nothing else works. Dave

 On 06 June 2017 at 01:45 Bob kb8tq <kb8tq@n1k.org> wrote:

 Hi

 Well, as part of the process of designing them into OCXO’s you do indeed check their long term stability.
 The test is done in an indirect fashion so you only come up with a “it’s below the limit” sort of number. The
 typical process involves running a group of OCXO’s on turn to check the frequency and then shifting them
 off turn to make a sort of thermometer. After a few months of frequency readings you take them back to turn
 for a while. Relative frequency shift math gives you a stability number for the thermistor and the rest of the
 circuitry. You may repeat the run for months / shift process a couple of times. If the answer isn’t “I can’t see
 a difference” you look for a new thermistor. Since it’s a long drawn out test, the tendency is to stick with a
 vendor’s part for quite a while. The parts also tend to be design specific so what works in my (say SMT)
 design may not work well in your (say chip and wire) design.

 Bob

     On Jun 5, 2017, at 9:20 AM, romeo987 <romeo987@westnet.com.au> wrote:

     Hi, guys
     I have been following time nuts and volt nuts for some time out of interest and fascination. Although my personal backyard hobby is more along a volt nuts line, the two worlds often collide - like in this discussion of temperature sensors, and in particular their long term stability. NTC thermistors appear to be very commonly used in ovens used to stabilize voltage references (solid state as well as chemical) . I have long wondered about their stability. If, as Bruce asserts, "high quality thermistors can achieve drifts of around 1mK/month" then it appears that this level of drift is a significant factor in the "apparent" aging of, say, a bank of Weston cells (which is still my best backyard shot at a voltage reference).

     I have had no luck with Google; Bruce's statement is the first quantified allusion that I have seen to this subject. Is there any actual data available on the long term performance of NTC sensors?

     Roman

         On 5 Jun 2017, at 9:53 AM, Bruce Griffiths <bruce.griffiths@xtra.co.nz> wrote:

         The other issue that needs to be considered is the drift in temperature sensor characteristics when operated at a constant temperature (as is typical in a continuously operated crystal oven). High quality thermistors can achieve drifts of around 1mK/month. Its unlikely that something as complex as an AD590 will achieve a similar drift (1nA/month in a operating current of 300uA or so at 25C). High quality PRT sensors drift even less than thermistors when operating at constant temperature.

         Bruce

             .On 05 June 2017 at 11:59 Attila Kinali <attila@kinali.ch> wrote:

             Moin,

             This discussion is kind of getting heated.
             Let's put some facts in, to steer it away from
             opinion based discussion.

             On Sun, 4 Jun 2017 08:44:33 -0700
             "Donald E. Pauly" <trojancowboy@gmail.com> wrote:

                 I stand by my remark that thermistors have been obsolete for over 40
                 years. The only exception that I know of is cesium beam tubes that
                 must withstand a 350° C bakeout. Thermistors are unstable and
                 manufactured with a witches brew straight out of MacBeth. Their
                 output voltages are tiny and are they inconvenient to use at different
                 temperatures.

                 If you really mean thermistors, and not, as Bob suggested thermocouples,
                 then I have to disagree. The most stable temperature sensors are
                 platinum wire sensors. The standards class PRT's are the gold standard
                 when it comes to temperature measurement, for a quite wide range
                 (-260°C to +960°C) and are considered very stable. They offer (absolute)
                 accuracies in the order of 10mK in the temperature range below 400°C.
                 Even industrial grade PRT sensors give you an absolute accuracy better
                 than 0.1K up to 200-300°C. The "cheap" PT100 are more of the order of 1-10°C
                 accuracy... all numbers just using a two-point calibration.

             For more information on this see [1] chapter 6 and [2] for industrial sensors.

             NTC sensors have a higher variablity of their parameters in production
             and are usually specified in % of temperature relative to their reference
             point, which is usually 25°C. Typical values are 0.1% to 5%. Additionally
             there is a deviation from the reference point, specified in °C, which
             is usually in the order of 0.1°C to 1°C.

             The NTC sensors are less accurate than PT sensors, but offer the advantage
             of higher resistance (thus lower self-heating), higher slope (thus better
             precision). Biggest disadvantage is their non-linear curve. Their price
             is also a fraction of PT sensors and due to that you can have them in
             many different forms, from the 0201 SMD resistor, to a large stainless
             steal pipe that goes into a chemical tank. NTCs are the workhorse in
             todays temperature measurement and control designs.

             The next category are band-gap sensors like the AD590. Their biggest
             advantage is that their 0 point is fix at 0K (and very accurately so).
             Ie they can be used with single point calibration and achieve 1°C accuracy
             this way. Their biggest drawback their large thermal mass and large
             insulating case, because they are basically an standard, analog IC.
             Ie their main use is in devices where there is a lot of convection and
             slow temperature change. Due to their simple and and quite linear
             characteristics, they are often used in purely analog temperature
             control circuits, or where a linearization is not feasible.
             But only if price isn't an issue (they cost 10-1000 times as
             much as an PTC). Their biggest disadvantage, beside their slow
             thermal raction time, is their large noise uncorrelated to the
             supply voltage, and thus cannot be compensated by ratiometric measurement.
             They are also more suceptible to mechanical stress than NTC's and PT's,
             due to their construction. Similar to voltage references (which they
             actually are), their aging is quite substantial and cannot be neglected
             in precision application.
             With a 3 point calibration, better than 0.5°C accuracy can be achieved
             (modulo aging) within their operating temperature range, which is
             rather limited, compared to the other sensor types.

             I don't know enough about thermocouples to say much about them, beside
             that they are cumbersome to work with (e.g. the cold contact) and
             produce a low voltage (several µV) output with quite high impedance,
             which makes the analog electronics difficult to design as well.

             With todays electronics, the easiest sensors to work with are NTC and
             PT100/PT1000 as most high resolution delta-sigma ADCs have direct support
             for 3 and/or 4 wire measurement of those, including compensation for
             reference voltage/current variation. Using a uC as control element
             also opens up the possibility to linearize the curve of NTCs without
             loss of accuracy. Usually measurement precision, with a state-of-the-art
             circuit, is limited by noise coupling into the leads of the sensor
             and noise in and around the ADC. (see [3-5])

                 Where did you get the idea to use a 1 k load for an AD590?

                 Jim was refering to a circuit _he_ used in a satellite. Not to your circuit.

                 The room temperature coefficient of an AT crystal is -cd 100 ppb per
                 reference cut angle in minutes. (-600 ppb/C° for standard crystal)
                 The practical limit in a crystal designed for room temperature is
                 about 0.1' cut accuracy or ±10 ppb/C°. If you have access to an
                 atomic standard, you can use feed forward to get ±1 ppb/C°. If the
                 temperature can be held to ±0.001° C, this is ±1 part per trillion.
                 This kind of accuracy has never been heard of.

                 It has been heard of. The 8607 was spec'ed to <2e-10 p-p deviation over temperature range (-30°C to 60°C). Also, to hold the temperature stable to 0.001K in a room temperature environment (let's say 10K variation), you need a thermal gain of >10k. That's quite a bit and needs considerable
                 design effort. Most OCXO design's I am aware of are in the order of 100
                 (the DIL14 designs) to a few 1000 for single ovens, to a few 10k for
                 double ovens. The only exception is the E1938 which achieves >1M.
                 But that design is not for the faint hearted. I don't remember seeing
                 any number, but i would guess the 8607 has a thermal gain in the
                 order of 100k to 1M as well, considering it being a double oven in
                 a dewar flask.

             Also, what do you mean by atomic standard and feed forward?
             If you have an atomic standard you don't need to temperature
             stabilize your quartz. You can just simply use a PLL to lock
             it to your reference and achieve higher stability than any oven
             design.

                 Feed forward also
                 allows you to incorporate the components of the oscillator into the
                 thermal behavior. It does no good to have a perfect crystal if the
                 oscillator components drift.

                 Beyond tau=100s, the temperature and moisture sensitivity of the
                 electronics, combined with the aging of the electronics and the
                 crystal will be the limit of stability. Of course, this is under
                 the assumption that you achieved a thermal noise limited design
                 and thus the 1/f^a noise of the oscillator is negligible in the
                 time range considered.

             Attila Kinali

             [1] "Traceable Temperatures - An Introduction to Temperature Measurement
             and Calibration", 2nd edition, by Nicholas and White, 2001

             [2] "Thin-film platinum resistance thermometer for use at low temperatures
             and in high magnetic fields", Haruyama, Yoshizaki, 1986

             [3] "Completely Integrated 4-Wire RTD Measurement System Using a Low Power,
             Precision, 24-Bit, Sigma-Delta ADC", Analog Circuit Note CN-0381
             http://www.analog.com/CN0381

             [4] "Completely Integrated 3-Wire RTD Measurement System Using a Low Power,
             Precision, 24-Bit, Sigma-Delta ADC", Analog Circuit Note CN-0383
             http://www.analog.com/CN0383

             [5] "2- 3- 4- Wire RDT (Pt100 to PT1000)Temperature Measurement"
             Ti Presentation
             http://www.ti.com/europe/downloads/2-%203-%204-Wire%20RTD%20Measurement.pdf

             --
             You know, the very powerful and the very stupid have one thing in common.
             They don't alters their views to fit the facts, they alter the facts to
             fit the views, which can be uncomfortable if you happen to be one of the
             facts that needs altering. -- The Doctor

             _______________________________________________
             time-nuts mailing list -- time-nuts@febo.com
             To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
             and follow the instructions there.

             _______________________________________________
             time-nuts mailing list -- time-nuts@febo.com
             To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
             and follow the instructions there.

     _______________________________________________
     time-nuts mailing list -- time-nuts@febo.com
     To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
     and follow the instructions there.

Here's a NIST paper on Thermistor stability: http://nvlpubs.nist.gov/nistpubs/jres/83/jresv83n3p247_A1b.pdf Bruce > > On 06 June 2017 at 01:45 Bob kb8tq <kb8tq@n1k.org> wrote: > > Hi > > Well, as part of the process of designing them into OCXO’s you do indeed check their long term stability. > The test is done in an indirect fashion so you only come up with a “it’s below the limit” sort of number. The > typical process involves running a group of OCXO’s on turn to check the frequency and then shifting them > off turn to make a sort of thermometer. After a few months of frequency readings you take them back to turn > for a while. Relative frequency shift math gives you a stability number for the thermistor and the rest of the > circuitry. You may repeat the run for months / shift process a couple of times. If the answer isn’t “I can’t see > a difference” you look for a new thermistor. Since it’s a long drawn out test, the tendency is to stick with a > vendor’s part for quite a while. The parts also tend to be design specific so what works in my (say SMT) > design may not work well in your (say chip and wire) design. > > Bob > > > > > > On Jun 5, 2017, at 9:20 AM, romeo987 <romeo987@westnet.com.au> wrote: > > > > Hi, guys > > I have been following time nuts and volt nuts for some time out of interest and fascination. Although my personal backyard hobby is more along a volt nuts line, the two worlds often collide - like in this discussion of temperature sensors, and in particular their long term stability. NTC thermistors appear to be very commonly used in ovens used to stabilize voltage references (solid state as well as chemical) . I have long wondered about their stability. If, as Bruce asserts, "high quality thermistors can achieve drifts of around 1mK/month" then it appears that this level of drift is a significant factor in the "apparent" aging of, say, a bank of Weston cells (which is still my best backyard shot at a voltage reference). > > > > I have had no luck with Google; Bruce's statement is the first quantified allusion that I have seen to this subject. Is there any actual data available on the long term performance of NTC sensors? > > > > Roman > > > > > > > > > > On 5 Jun 2017, at 9:53 AM, Bruce Griffiths <bruce.griffiths@xtra.co.nz> wrote: > > > > > > The other issue that needs to be considered is the drift in temperature sensor characteristics when operated at a constant temperature (as is typical in a continuously operated crystal oven). High quality thermistors can achieve drifts of around 1mK/month. Its unlikely that something as complex as an AD590 will achieve a similar drift (1nA/month in a operating current of 300uA or so at 25C). High quality PRT sensors drift even less than thermistors when operating at constant temperature. > > > > > > Bruce > > > > > > > > > > > > > > .On 05 June 2017 at 11:59 Attila Kinali <attila@kinali.ch> wrote: > > > > > > > > Moin, > > > > > > > > This discussion is kind of getting heated. > > > > Let's put some facts in, to steer it away from > > > > opinion based discussion. > > > > > > > > On Sun, 4 Jun 2017 08:44:33 -0700 > > > > "Donald E. Pauly" <trojancowboy@gmail.com> wrote: > > > > > > > > > > > > > > > > > > I stand by my remark that thermistors have been obsolete for over 40 > > > > > years. The only exception that I know of is cesium beam tubes that > > > > > must withstand a 350° C bakeout. Thermistors are unstable and > > > > > manufactured with a witches brew straight out of MacBeth. Their > > > > > output voltages are tiny and are they inconvenient to use at different > > > > > temperatures. > > > > > > > > > > If you really mean thermistors, and not, as Bob suggested thermocouples, > > > > > then I have to disagree. The most stable temperature sensors are > > > > > platinum wire sensors. The standards class PRT's are the gold standard > > > > > when it comes to temperature measurement, for a quite wide range > > > > > (-260°C to +960°C) and are considered very stable. They offer (absolute) > > > > > accuracies in the order of 10mK in the temperature range below 400°C. > > > > > Even industrial grade PRT sensors give you an absolute accuracy better > > > > > than 0.1K up to 200-300°C. The "cheap" PT100 are more of the order of 1-10°C > > > > > accuracy... all numbers just using a two-point calibration. > > > > > > > > > > > > > > > > > > For more information on this see [1] chapter 6 and [2] for industrial sensors. > > > > > > > > NTC sensors have a higher variablity of their parameters in production > > > > and are usually specified in % of temperature relative to their reference > > > > point, which is usually 25°C. Typical values are 0.1% to 5%. Additionally > > > > there is a deviation from the reference point, specified in °C, which > > > > is usually in the order of 0.1°C to 1°C. > > > > > > > > The NTC sensors are less accurate than PT sensors, but offer the advantage > > > > of higher resistance (thus lower self-heating), higher slope (thus better > > > > precision). Biggest disadvantage is their non-linear curve. Their price > > > > is also a fraction of PT sensors and due to that you can have them in > > > > many different forms, from the 0201 SMD resistor, to a large stainless > > > > steal pipe that goes into a chemical tank. NTCs are the workhorse in > > > > todays temperature measurement and control designs. > > > > > > > > The next category are band-gap sensors like the AD590. Their biggest > > > > advantage is that their 0 point is fix at 0K (and very accurately so). > > > > Ie they can be used with single point calibration and achieve 1°C accuracy > > > > this way. Their biggest drawback their large thermal mass and large > > > > insulating case, because they are basically an standard, analog IC. > > > > Ie their main use is in devices where there is a lot of convection and > > > > slow temperature change. Due to their simple and and quite linear > > > > characteristics, they are often used in purely analog temperature > > > > control circuits, or where a linearization is not feasible. > > > > But only if price isn't an issue (they cost 10-1000 times as > > > > much as an PTC). Their biggest disadvantage, beside their slow > > > > thermal raction time, is their large noise uncorrelated to the > > > > supply voltage, and thus cannot be compensated by ratiometric measurement. > > > > They are also more suceptible to mechanical stress than NTC's and PT's, > > > > due to their construction. Similar to voltage references (which they > > > > actually are), their aging is quite substantial and cannot be neglected > > > > in precision application. > > > > With a 3 point calibration, better than 0.5°C accuracy can be achieved > > > > (modulo aging) within their operating temperature range, which is > > > > rather limited, compared to the other sensor types. > > > > > > > > I don't know enough about thermocouples to say much about them, beside > > > > that they are cumbersome to work with (e.g. the cold contact) and > > > > produce a low voltage (several µV) output with quite high impedance, > > > > which makes the analog electronics difficult to design as well. > > > > > > > > With todays electronics, the easiest sensors to work with are NTC and > > > > PT100/PT1000 as most high resolution delta-sigma ADCs have direct support > > > > for 3 and/or 4 wire measurement of those, including compensation for > > > > reference voltage/current variation. Using a uC as control element > > > > also opens up the possibility to linearize the curve of NTCs without > > > > loss of accuracy. Usually measurement precision, with a state-of-the-art > > > > circuit, is limited by noise coupling into the leads of the sensor > > > > and noise in and around the ADC. (see [3-5]) > > > > > > > > > > > > > > > > > > Where did you get the idea to use a 1 k load for an AD590? > > > > > > > > > > Jim was refering to a circuit _he_ used in a satellite. Not to your circuit. > > > > > > > > > > The room temperature coefficient of an AT crystal is -cd 100 ppb per > > > > > reference cut angle in minutes. (-600 ppb/C° for standard crystal) > > > > > The practical limit in a crystal designed for room temperature is > > > > > about 0.1' cut accuracy or ±10 ppb/C°. If you have access to an > > > > > atomic standard, you can use feed forward to get ±1 ppb/C°. If the > > > > > temperature can be held to ±0.001° C, this is ±1 part per trillion. > > > > > This kind of accuracy has never been heard of. > > > > > > > > > > It has been heard of. The 8607 was spec'ed to <2e-10 p-p deviation over temperature range (-30°C to 60°C). Also, to hold the temperature stable to 0.001K in a room temperature environment (let's say 10K variation), you need a thermal gain of >10k. That's quite a bit and needs considerable > > > > > design effort. Most OCXO design's I am aware of are in the order of 100 > > > > > (the DIL14 designs) to a few 1000 for single ovens, to a few 10k for > > > > > double ovens. The only exception is the E1938 which achieves >1M. > > > > > But that design is not for the faint hearted. I don't remember seeing > > > > > any number, but i would guess the 8607 has a thermal gain in the > > > > > order of 100k to 1M as well, considering it being a double oven in > > > > > a dewar flask. > > > > > > > > > > > > > > > > > > Also, what do you mean by atomic standard and feed forward? > > > > If you have an atomic standard you don't need to temperature > > > > stabilize your quartz. You can just simply use a PLL to lock > > > > it to your reference and achieve higher stability than any oven > > > > design. > > > > > > > > > > > > > > > > > > Feed forward also > > > > > allows you to incorporate the components of the oscillator into the > > > > > thermal behavior. It does no good to have a perfect crystal if the > > > > > oscillator components drift. > > > > > > > > > > Beyond tau=100s, the temperature and moisture sensitivity of the > > > > > electronics, combined with the aging of the electronics and the > > > > > crystal will be the limit of stability. Of course, this is under > > > > > the assumption that you achieved a thermal noise limited design > > > > > and thus the 1/f^a noise of the oscillator is negligible in the > > > > > time range considered. > > > > > > > > > > > > > > > > > > Attila Kinali > > > > > > > > [1] "Traceable Temperatures - An Introduction to Temperature Measurement > > > > and Calibration", 2nd edition, by Nicholas and White, 2001 > > > > > > > > [2] "Thin-film platinum resistance thermometer for use at low temperatures > > > > and in high magnetic fields", Haruyama, Yoshizaki, 1986 > > > > > > > > [3] "Completely Integrated 4-Wire RTD Measurement System Using a Low Power, > > > > Precision, 24-Bit, Sigma-Delta ADC", Analog Circuit Note CN-0381 > > > > http://www.analog.com/CN0381 > > > > > > > > [4] "Completely Integrated 3-Wire RTD Measurement System Using a Low Power, > > > > Precision, 24-Bit, Sigma-Delta ADC", Analog Circuit Note CN-0383 > > > > http://www.analog.com/CN0383 > > > > > > > > [5] "2- 3- 4- Wire RDT (Pt100 to PT1000)Temperature Measurement" > > > > Ti Presentation > > > > http://www.ti.com/europe/downloads/2-%203-%204-Wire%20RTD%20Measurement.pdf > > > > > > > > -- > > > > You know, the very powerful and the very stupid have one thing in common. > > > > They don't alters their views to fit the facts, they alter the facts to > > > > fit the views, which can be uncomfortable if you happen to be one of the > > > > facts that needs altering. -- The Doctor > > > > > > > > _______________________________________________ > > > > time-nuts mailing list -- time-nuts@febo.com > > > > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > > > and follow the instructions there. > > > > > > > > _______________________________________________ > > > > time-nuts mailing list -- time-nuts@febo.com > > > > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > > > and follow the instructions there. > > > > > > > > > > > > > > > > > > _______________________________________________ > > time-nuts mailing list -- time-nuts@febo.com > > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > and follow the instructions there. > > > > >

 On 06 June 2017 at 09:49 Bruce Griffiths <bruce.griffiths@xtra.co.nz> wrote:

 Here's a NIST paper on Thermistor stability:

 http://nvlpubs.nist.gov/nistpubs/jres/83/jresv83n3p247_A1b.pdf

 Bruce

     On 06 June 2017 at 01:45 Bob kb8tq <kb8tq@n1k.org> wrote:

     Hi

     Well, as part of the process of designing them into OCXO’s you do indeed check their long term stability.
     The test is done in an indirect fashion so you only come up with a “it’s below the limit” sort of number. The
     typical process involves running a group of OCXO’s on turn to check the frequency and then shifting them
     off turn to make a sort of thermometer. After a few months of frequency readings you take them back to turn
     for a while. Relative frequency shift math gives you a stability number for the thermistor and the rest of the
     circuitry. You may repeat the run for months / shift process a couple of times. If the answer isn’t “I can’t see
     a difference” you look for a new thermistor. Since it’s a long drawn out test, the tendency is to stick with a
     vendor’s part for quite a while. The parts also tend to be design specific so what works in my (say SMT)
     design may not work well in your (say chip and wire) design.

     Bob

             On Jun 5, 2017, at 9:20 AM, romeo987 <romeo987@westnet.com.au> wrote:

         Hi, guys
         I have been following time nuts and volt nuts for some time out of interest and fascination. Although my personal backyard hobby is more along a volt nuts line, the two worlds often collide - like in this discussion of temperature sensors, and in particular their long term stability. NTC thermistors appear to be very commonly used in ovens used to stabilize voltage references (solid state as well as chemical) . I have long wondered about their stability. If, as Bruce asserts, "high quality thermistors can achieve drifts of around 1mK/month" then it appears that this level of drift is a significant factor in the "apparent" aging of, say, a bank of Weston cells (which is still my best backyard shot at a voltage reference).

         I have had no luck with Google; Bruce's statement is the first quantified allusion that I have seen to this subject. Is there any actual data available on the long term performance of NTC sensors?

         Roman

                     On 5 Jun 2017, at 9:53 AM, Bruce Griffiths <bruce.griffiths@xtra.co.nz> wrote:

             The other issue that needs to be considered is the drift in temperature sensor characteristics when operated at a constant temperature (as is typical in a continuously operated crystal oven). High quality thermistors can achieve drifts of around 1mK/month. Its unlikely that something as complex as an AD590 will achieve a similar drift (1nA/month in a operating current of 300uA or so at 25C). High quality PRT sensors drift even less than thermistors when operating at constant temperature.

             Bruce

                             .On 05 June 2017 at 11:59 Attila Kinali <attila@kinali.ch> wrote:

                 Moin,

                 This discussion is kind of getting heated.
                 Let's put some facts in, to steer it away from
                 opinion based discussion.

                 On Sun, 4 Jun 2017 08:44:33 -0700
                 "Donald E. Pauly" <trojancowboy@gmail.com> wrote:

                                     I stand by my remark that thermistors have been obsolete for over 40
                                     years. The only exception that I know of is cesium beam tubes that
                                     must withstand a 350° C bakeout. Thermistors are unstable and
                                     manufactured with a witches brew straight out of MacBeth. Their
                                     output voltages are tiny and are they inconvenient to use at different
                                     temperatures.

                     If you really mean thermistors, and not, as Bob suggested thermocouples,
                     then I have to disagree. The most stable temperature sensors are
                     platinum wire sensors. The standards class PRT's are the gold standard
                     when it comes to temperature measurement, for a quite wide range
                     (-260°C to +960°C) and are considered very stable. They offer (absolute)
                     accuracies in the order of 10mK in the temperature range below 400°C.
                     Even industrial grade PRT sensors give you an absolute accuracy better
                     than 0.1K up to 200-300°C. The "cheap" PT100 are more of the order of 1-10°C
                     accuracy... all numbers just using a two-point calibration.

                                     For more information on this see [1] chapter 6 and [2] for industrial sensors.

                 NTC sensors have a higher variablity of their parameters in production
                 and are usually specified in % of temperature relative to their reference
                 point, which is usually 25°C. Typical values are 0.1% to 5%. Additionally
                 there is a deviation from the reference point, specified in °C, which
                 is usually in the order of 0.1°C to 1°C.

                 The NTC sensors are less accurate than PT sensors, but offer the advantage
                 of higher resistance (thus lower self-heating), higher slope (thus better
                 precision). Biggest disadvantage is their non-linear curve. Their price
                 is also a fraction of PT sensors and due to that you can have them in
                 many different forms, from the 0201 SMD resistor, to a large stainless
                 steal pipe that goes into a chemical tank. NTCs are the workhorse in
                 todays temperature measurement and control designs.

                 The next category are band-gap sensors like the AD590. Their biggest
                 advantage is that their 0 point is fix at 0K (and very accurately so).
                 Ie they can be used with single point calibration and achieve 1°C accuracy
                 this way. Their biggest drawback their large thermal mass and large
                 insulating case, because they are basically an standard, analog IC.
                 Ie their main use is in devices where there is a lot of convection and
                 slow temperature change. Due to their simple and and quite linear
                 characteristics, they are often used in purely analog temperature
                 control circuits, or where a linearization is not feasible.
                 But only if price isn't an issue (they cost 10-1000 times as
                 much as an PTC). Their biggest disadvantage, beside their slow
                 thermal raction time, is their large noise uncorrelated to the
                 supply voltage, and thus cannot be compensated by ratiometric measurement.
                 They are also more suceptible to mechanical stress than NTC's and PT's,
                 due to their construction. Similar to voltage references (which they
                 actually are), their aging is quite substantial and cannot be neglected
                 in precision application.
                 With a 3 point calibration, better than 0.5°C accuracy can be achieved
                 (modulo aging) within their operating temperature range, which is
                 rather limited, compared to the other sensor types.

                 I don't know enough about thermocouples to say much about them, beside
                 that they are cumbersome to work with (e.g. the cold contact) and
                 produce a low voltage (several µV) output with quite high impedance,
                 which makes the analog electronics difficult to design as well.

                 With todays electronics, the easiest sensors to work with are NTC and
                 PT100/PT1000 as most high resolution delta-sigma ADCs have direct support
                 for 3 and/or 4 wire measurement of those, including compensation for
                 reference voltage/current variation. Using a uC as control element
                 also opens up the possibility to linearize the curve of NTCs without
                 loss of accuracy. Usually measurement precision, with a state-of-the-art
                 circuit, is limited by noise coupling into the leads of the sensor
                 and noise in and around the ADC. (see [3-5])

                                     Where did you get the idea to use a 1 k load for an AD590?

                     Jim was refering to a circuit _he_ used in a satellite. Not to your circuit.

                     The room temperature coefficient of an AT crystal is -cd 100 ppb per
                     reference cut angle in minutes. (-600 ppb/C° for standard crystal)
                     The practical limit in a crystal designed for room temperature is
                     about 0.1' cut accuracy or ±10 ppb/C°. If you have access to an
                     atomic standard, you can use feed forward to get ±1 ppb/C°. If the
                     temperature can be held to ±0.001° C, this is ±1 part per trillion.
                     This kind of accuracy has never been heard of.

                     It has been heard of. The 8607 was spec'ed to <2e-10 p-p deviation over temperature range (-30°C to 60°C). Also, to hold the temperature stable to 0.001K in a room temperature environment (let's say 10K variation), you need a thermal gain of >10k. That's quite a bit and needs considerable
                     design effort. Most OCXO design's I am aware of are in the order of 100
                     (the DIL14 designs) to a few 1000 for single ovens, to a few 10k for
                     double ovens. The only exception is the E1938 which achieves >1M.
                     But that design is not for the faint hearted. I don't remember seeing
                     any number, but i would guess the 8607 has a thermal gain in the
                     order of 100k to 1M as well, considering it being a double oven in
                     a dewar flask.

                                     Also, what do you mean by atomic standard and feed forward?
                                     If you have an atomic standard you don't need to temperature
                                     stabilize your quartz. You can just simply use a PLL to lock
                                     it to your reference and achieve higher stability than any oven
                                     design.

                                     Feed forward also
                                     allows you to incorporate the components of the oscillator into the
                                     thermal behavior. It does no good to have a perfect crystal if the
                                     oscillator components drift.

                     Beyond tau=100s, the temperature and moisture sensitivity of the
                     electronics, combined with the aging of the electronics and the
                     crystal will be the limit of stability. Of course, this is under
                     the assumption that you achieved a thermal noise limited design
                     and thus the 1/f^a noise of the oscillator is negligible in the
                     time range considered.

                                     Attila Kinali

                 [1] "Traceable Temperatures - An Introduction to Temperature Measurement
                 and Calibration", 2nd edition, by Nicholas and White, 2001

                 [2] "Thin-film platinum resistance thermometer for use at low temperatures
                 and in high magnetic fields", Haruyama, Yoshizaki, 1986

                 [3] "Completely Integrated 4-Wire RTD Measurement System Using a Low Power,
                 Precision, 24-Bit, Sigma-Delta ADC", Analog Circuit Note CN-0381
                 http://www.analog.com/CN0381

                 [4] "Completely Integrated 3-Wire RTD Measurement System Using a Low Power,
                 Precision, 24-Bit, Sigma-Delta ADC", Analog Circuit Note CN-0383
                 http://www.analog.com/CN0383

                 [5] "2- 3- 4- Wire RDT (Pt100 to PT1000)Temperature Measurement"
                 Ti Presentation
                 http://www.ti.com/europe/downloads/2-%203-%204-Wire%20RTD%20Measurement.pdf

                 --
                 You know, the very powerful and the very stupid have one thing in common.
                 They don't alters their views to fit the facts, they alter the facts to
                 fit the views, which can be uncomfortable if you happen to be one of the
                 facts that needs altering. -- The Doctor

                 _______________________________________________
                 time-nuts mailing list -- time-nuts@febo.com
                 To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
                 and follow the instructions there.

                 _______________________________________________
                 time-nuts mailing list -- time-nuts@febo.com
                 To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
                 and follow the instructions there.

                     _______________________________________________
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                     and follow the instructions there.

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Additional info/papers on Thermistor stability: http://www.digikey.com/en/pdf/u/us-sensor/us-sensor-stability-long-term-aging https://www.thermistor.com/sites/default/files/specsheets/T150-Series-Stability.pdf https://www.vishay.com/docs/49498/ntcs-e3-smt_vmn-pt0283.pdf >From LIGO: http://www.aspe.net/publications/Annual_2008/POSTERS/08UNCER/2643.PDF Bruce > > On 06 June 2017 at 09:49 Bruce Griffiths <bruce.griffiths@xtra.co.nz> wrote: > > Here's a NIST paper on Thermistor stability: > > http://nvlpubs.nist.gov/nistpubs/jres/83/jresv83n3p247_A1b.pdf > > Bruce > > > > > > On 06 June 2017 at 01:45 Bob kb8tq <kb8tq@n1k.org> wrote: > > > > Hi > > > > Well, as part of the process of designing them into OCXO’s you do indeed check their long term stability. > > The test is done in an indirect fashion so you only come up with a “it’s below the limit” sort of number. The > > typical process involves running a group of OCXO’s on turn to check the frequency and then shifting them > > off turn to make a sort of thermometer. After a few months of frequency readings you take them back to turn > > for a while. Relative frequency shift math gives you a stability number for the thermistor and the rest of the > > circuitry. You may repeat the run for months / shift process a couple of times. If the answer isn’t “I can’t see > > a difference” you look for a new thermistor. Since it’s a long drawn out test, the tendency is to stick with a > > vendor’s part for quite a while. The parts also tend to be design specific so what works in my (say SMT) > > design may not work well in your (say chip and wire) design. > > > > Bob > > > > > > > > > > > > > > > > > > On Jun 5, 2017, at 9:20 AM, romeo987 <romeo987@westnet.com.au> wrote: > > > > > > > > > > > > > > Hi, guys > > > I have been following time nuts and volt nuts for some time out of interest and fascination. Although my personal backyard hobby is more along a volt nuts line, the two worlds often collide - like in this discussion of temperature sensors, and in particular their long term stability. NTC thermistors appear to be very commonly used in ovens used to stabilize voltage references (solid state as well as chemical) . I have long wondered about their stability. If, as Bruce asserts, "high quality thermistors can achieve drifts of around 1mK/month" then it appears that this level of drift is a significant factor in the "apparent" aging of, say, a bank of Weston cells (which is still my best backyard shot at a voltage reference). > > > > > > I have had no luck with Google; Bruce's statement is the first quantified allusion that I have seen to this subject. Is there any actual data available on the long term performance of NTC sensors? > > > > > > Roman > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > On 5 Jun 2017, at 9:53 AM, Bruce Griffiths <bruce.griffiths@xtra.co.nz> wrote: > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > The other issue that needs to be considered is the drift in temperature sensor characteristics when operated at a constant temperature (as is typical in a continuously operated crystal oven). High quality thermistors can achieve drifts of around 1mK/month. Its unlikely that something as complex as an AD590 will achieve a similar drift (1nA/month in a operating current of 300uA or so at 25C). High quality PRT sensors drift even less than thermistors when operating at constant temperature. > > > > > > > > Bruce > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > .On 05 June 2017 at 11:59 Attila Kinali <attila@kinali.ch> wrote: > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Moin, > > > > > > > > > > This discussion is kind of getting heated. > > > > > Let's put some facts in, to steer it away from > > > > > opinion based discussion. > > > > > > > > > > On Sun, 4 Jun 2017 08:44:33 -0700 > > > > > "Donald E. Pauly" <trojancowboy@gmail.com> wrote: > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > I stand by my remark that thermistors have been obsolete for over 40 > > > > > > > > > > years. The only exception that I know of is cesium beam tubes that > > > > > > > > > > must withstand a 350° C bakeout. Thermistors are unstable and > > > > > > > > > > manufactured with a witches brew straight out of MacBeth. Their > > > > > > > > > > output voltages are tiny and are they inconvenient to use at different > > > > > > > > > > temperatures. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > If you really mean thermistors, and not, as Bob suggested thermocouples, > > > > > > then I have to disagree. The most stable temperature sensors are > > > > > > platinum wire sensors. The standards class PRT's are the gold standard > > > > > > when it comes to temperature measurement, for a quite wide range > > > > > > (-260°C to +960°C) and are considered very stable. They offer (absolute) > > > > > > accuracies in the order of 10mK in the temperature range below 400°C. > > > > > > Even industrial grade PRT sensors give you an absolute accuracy better > > > > > > than 0.1K up to 200-300°C. The "cheap" PT100 are more of the order of 1-10°C > > > > > > accuracy... all numbers just using a two-point calibration. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > For more information on this see [1] chapter 6 and [2] for industrial sensors. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > NTC sensors have a higher variablity of their parameters in production > > > > > and are usually specified in % of temperature relative to their reference > > > > > point, which is usually 25°C. Typical values are 0.1% to 5%. Additionally > > > > > there is a deviation from the reference point, specified in °C, which > > > > > is usually in the order of 0.1°C to 1°C. > > > > > > > > > > The NTC sensors are less accurate than PT sensors, but offer the advantage > > > > > of higher resistance (thus lower self-heating), higher slope (thus better > > > > > precision). Biggest disadvantage is their non-linear curve. Their price > > > > > is also a fraction of PT sensors and due to that you can have them in > > > > > many different forms, from the 0201 SMD resistor, to a large stainless > > > > > steal pipe that goes into a chemical tank. NTCs are the workhorse in > > > > > todays temperature measurement and control designs. > > > > > > > > > > The next category are band-gap sensors like the AD590. Their biggest > > > > > advantage is that their 0 point is fix at 0K (and very accurately so). > > > > > Ie they can be used with single point calibration and achieve 1°C accuracy > > > > > this way. Their biggest drawback their large thermal mass and large > > > > > insulating case, because they are basically an standard, analog IC. > > > > > Ie their main use is in devices where there is a lot of convection and > > > > > slow temperature change. Due to their simple and and quite linear > > > > > characteristics, they are often used in purely analog temperature > > > > > control circuits, or where a linearization is not feasible. > > > > > But only if price isn't an issue (they cost 10-1000 times as > > > > > much as an PTC). Their biggest disadvantage, beside their slow > > > > > thermal raction time, is their large noise uncorrelated to the > > > > > supply voltage, and thus cannot be compensated by ratiometric measurement. > > > > > They are also more suceptible to mechanical stress than NTC's and PT's, > > > > > due to their construction. Similar to voltage references (which they > > > > > actually are), their aging is quite substantial and cannot be neglected > > > > > in precision application. > > > > > With a 3 point calibration, better than 0.5°C accuracy can be achieved > > > > > (modulo aging) within their operating temperature range, which is > > > > > rather limited, compared to the other sensor types. > > > > > > > > > > I don't know enough about thermocouples to say much about them, beside > > > > > that they are cumbersome to work with (e.g. the cold contact) and > > > > > produce a low voltage (several µV) output with quite high impedance, > > > > > which makes the analog electronics difficult to design as well. > > > > > > > > > > With todays electronics, the easiest sensors to work with are NTC and > > > > > PT100/PT1000 as most high resolution delta-sigma ADCs have direct support > > > > > for 3 and/or 4 wire measurement of those, including compensation for > > > > > reference voltage/current variation. Using a uC as control element > > > > > also opens up the possibility to linearize the curve of NTCs without > > > > > loss of accuracy. Usually measurement precision, with a state-of-the-art > > > > > circuit, is limited by noise coupling into the leads of the sensor > > > > > and noise in and around the ADC. (see [3-5]) > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Where did you get the idea to use a 1 k load for an AD590? > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Jim was refering to a circuit _he_ used in a satellite. Not to your circuit. > > > > > > > > > > > > The room temperature coefficient of an AT crystal is -cd 100 ppb per > > > > > > reference cut angle in minutes. (-600 ppb/C° for standard crystal) > > > > > > The practical limit in a crystal designed for room temperature is > > > > > > about 0.1' cut accuracy or ±10 ppb/C°. If you have access to an > > > > > > atomic standard, you can use feed forward to get ±1 ppb/C°. If the > > > > > > temperature can be held to ±0.001° C, this is ±1 part per trillion. > > > > > > This kind of accuracy has never been heard of. > > > > > > > > > > > > It has been heard of. The 8607 was spec'ed to <2e-10 p-p deviation over temperature range (-30°C to 60°C). Also, to hold the temperature stable to 0.001K in a room temperature environment (let's say 10K variation), you need a thermal gain of >10k. That's quite a bit and needs considerable > > > > > > design effort. Most OCXO design's I am aware of are in the order of 100 > > > > > > (the DIL14 designs) to a few 1000 for single ovens, to a few 10k for > > > > > > double ovens. The only exception is the E1938 which achieves >1M. > > > > > > But that design is not for the faint hearted. I don't remember seeing > > > > > > any number, but i would guess the 8607 has a thermal gain in the > > > > > > order of 100k to 1M as well, considering it being a double oven in > > > > > > a dewar flask. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Also, what do you mean by atomic standard and feed forward? > > > > > > > > > > If you have an atomic standard you don't need to temperature > > > > > > > > > > stabilize your quartz. You can just simply use a PLL to lock > > > > > > > > > > it to your reference and achieve higher stability than any oven > > > > > > > > > > design. > > > > > > > > > > > > > > > > > > > > Feed forward also > > > > > > > > > > allows you to incorporate the components of the oscillator into the > > > > > > > > > > thermal behavior. It does no good to have a perfect crystal if the > > > > > > > > > > oscillator components drift. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Beyond tau=100s, the temperature and moisture sensitivity of the > > > > > > electronics, combined with the aging of the electronics and the > > > > > > crystal will be the limit of stability. Of course, this is under > > > > > > the assumption that you achieved a thermal noise limited design > > > > > > and thus the 1/f^a noise of the oscillator is negligible in the > > > > > > time range considered. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Attila Kinali > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > [1] "Traceable Temperatures - An Introduction to Temperature Measurement > > > > > and Calibration", 2nd edition, by Nicholas and White, 2001 > > > > > > > > > > [2] "Thin-film platinum resistance thermometer for use at low temperatures > > > > > and in high magnetic fields", Haruyama, Yoshizaki, 1986 > > > > > > > > > > [3] "Completely Integrated 4-Wire RTD Measurement System Using a Low Power, > > > > > Precision, 24-Bit, Sigma-Delta ADC", Analog Circuit Note CN-0381 > > > > > http://www.analog.com/CN0381 > > > > > > > > > > [4] "Completely Integrated 3-Wire RTD Measurement System Using a Low Power, > > > > > Precision, 24-Bit, Sigma-Delta ADC", Analog Circuit Note CN-0383 > > > > > http://www.analog.com/CN0383 > > > > > > > > > > [5] "2- 3- 4- Wire RDT (Pt100 to PT1000)Temperature Measurement" > > > > > Ti Presentation > > > > > http://www.ti.com/europe/downloads/2-%203-%204-Wire%20RTD%20Measurement.pdf > > > > > > > > > > -- > > > > > You know, the very powerful and the very stupid have one thing in common. > > > > > They don't alters their views to fit the facts, they alter the facts to > > > > > fit the views, which can be uncomfortable if you happen to be one of the > > > > > facts that needs altering. -- The Doctor > > > > > > > > > > _______________________________________________ > > > > > time-nuts mailing list -- time-nuts@febo.com > > > > > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > > > > and follow the instructions there. > > > > > > > > > > _______________________________________________ > > > > > time-nuts mailing list -- time-nuts@febo.com > > > > > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > > > > and follow the instructions there. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > _______________________________________________ > > > > > > time-nuts mailing list -- time-nuts@febo.com > > > > > > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > > > > > and follow the instructions there. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > _______________________________________________ > > > > time-nuts mailing list -- time-nuts@febo.com > > > > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > > > and follow the instructions there. > > > > > > > > > > > > > > > > > > >

-------- In message <CANX10hA776Qb1Y_j+t7MCAwSN6Vrp0veLv5K13HhS6kdda9Spg@mail.gmail.com> , "Dr. David Kirkby (Kirkby Microwave Ltd)" writes: >I can't find it now, but I know someone said thermocouples are obsolete. I >spoke to a friend tonight who services industrial boilders. He said >thermocouples are far from obsolesce, at temperatures of a few hundred deg >C, as nothing else works. Thermocouples are not obsolete. If nothing else because they are cheap and can be made on the spot and in all sorts of weird shapes. The only thing which competes with thermocouples in high temperature is platinum, which is horribly expensive by comparison and more prone to noise than thermocouples. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 phk@FreeBSD.ORG | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence.

                    Attila Kinali

https://www.febo.com/pipermail/time-nuts/2017-May/105566.html I said that thermistors have been obsolete for 40 years not themocouples. (With a FEW rare exceptions) I do not consider platinum wire to be a thermistor. I own a 100 Ω platinum wire thermometer for the DVM in my 2236 Tekronix. It is not worth much without a Kelvin connection. From 0° C to to 100° C it changes 40 Ω and uses banana plugs. Those are unstable by ~ 0.2Ω. This is 0.5° C of error and intermittent. It is worthless for designing ovens. I use thermocouples in my Fluke 52 stereo thermometers all the time. They will work at nearly red heat and are stable. They are hard to use because they only produce 40 μV/C° and require a cold junction comparison. The cold junction is easily calibrated by an ice bath however. Thermistors depend on the cauldron in which they were stirred by the witches at manufacture. In the range of -55° C to 150° C, I don't think anything can match the AD590 or equivalent for repeatability, accuracy, stability, linearity or convenience. They are not affected by lead resistance and can use tiny wires. It will tolerate 3,000 Ω of lead resistance and can be multiplexed. The chip itself is 52 mils by 42 mils or comparable to a thermocouple bead. I figured out that two of them can be driven back to back by a square wave and two temperatures monitored at once with the same pair of wires. An Analog Devices product engineer split a $100 prize with me for my invention. πθ°μΩω±√·Γλ WB0KV ---------- Forwarded message ---------- From: Attila Kinali <attila@kinali.ch> Date: Sun, Jun 4, 2017 at 4:59 PM Subject: [time-nuts] Temperature sensors and quartz crystals (was: HP5061B Versus HP5071 Cesium Line Frequencies) To: Discussion of precise time and frequency measurement <time-nuts@febo.com> Moin, This discussion is kind of getting heated. Let's put some facts in, to steer it away from opinion based discussion. On Sun, 4 Jun 2017 08:44:33 -0700 "Donald E. Pauly" <trojancowboy@gmail.com> wrote: > I stand by my remark that thermistors have been obsolete for over 40 > years. The only exception that I know of is cesium beam tubes that > must withstand a 350° C bakeout. Thermistors are unstable and > manufactured with a witches brew straight out of MacBeth. Their > output voltages are tiny and are they inconvenient to use at different > temperatures. If you really mean thermistors, and not, as Bob suggested thermocouples, then I have to disagree. The most stable temperature sensors are platinum wire sensors. The standards class PRT's are the gold standard when it comes to temperature measurement, for a quite wide range (-260°C to +960°C) and are considered very stable. They offer (absolute) accuracies in the order of 10mK in the temperature range below 400°C. Even industrial grade PRT sensors give you an absolute accuracy better than 0.1K up to 200-300°C. The "cheap" PT100 are more of the order of 1-10°C accuracy... all numbers just using a two-point calibration. For more information on this see [1] chapter 6 and [2] for industrial sensors. NTC sensors have a higher variablity of their parameters in production and are usually specified in % of temperature relative to their reference point, which is usually 25°C. Typical values are 0.1% to 5%. Additionally there is a deviation from the reference point, specified in °C, which is usually in the order of 0.1°C to 1°C. The NTC sensors are less accurate than PT sensors, but offer the advantage of higher resistance (thus lower self-heating), higher slope (thus better precision). Biggest disadvantage is their non-linear curve. Their price is also a fraction of PT sensors and due to that you can have them in many different forms, from the 0201 SMD resistor, to a large stainless steal pipe that goes into a chemical tank. NTCs are the workhorse in todays temperature measurement and control designs. The next category are band-gap sensors like the AD590. Their biggest advantage is that their 0 point is fix at 0K (and very accurately so). Ie they can be used with single point calibration and achieve 1°C accuracy this way. Their biggest drawback their large thermal mass and large insulating case, because they are basically an standard, analog IC. Ie their main use is in devices where there is a lot of convection and slow temperature change. Due to their simple and and quite linear characteristics, they are often used in purely analog temperature control circuits, or where a linearization is not feasible. But only if price isn't an issue (they cost 10-1000 times as much as an PTC). Their biggest disadvantage, beside their slow thermal raction time, is their large noise uncorrelated to the supply voltage, and thus cannot be compensated by ratiometric measurement. They are also more suceptible to mechanical stress than NTC's and PT's, due to their construction. Similar to voltage references (which they actually are), their aging is quite substantial and cannot be neglected in precision application. With a 3 point calibration, better than 0.5°C accuracy can be achieved (modulo aging) within their operating temperature range, which is rather limited, compared to the other sensor types. I don't know enough about thermocouples to say much about them, beside that they are cumbersome to work with (e.g. the cold contact) and produce a low voltage (several µV) output with quite high impedance, which makes the analog electronics difficult to design as well. With todays electronics, the easiest sensors to work with are NTC and PT100/PT1000 as most high resolution delta-sigma ADCs have direct support for 3 and/or 4 wire measurement of those, including compensation for reference voltage/current variation. Using a uC as control element also opens up the possibility to linearize the curve of NTCs without loss of accuracy. Usually measurement precision, with a state-of-the-art circuit, is limited by noise coupling into the leads of the sensor and noise in and around the ADC. (see [3-5]) > Where did you get the idea to use a 1 k load for an AD590? Jim was refering to a circuit _he_ used in a satellite. Not to your circuit. > The room temperature coefficient of an AT crystal is -cd 100 ppb per > reference cut angle in minutes. (-600 ppb/C° for standard crystal) > The practical limit in a crystal designed for room temperature is > about 0.1' cut accuracy or ±10 ppb/C°. If you have access to an > atomic standard, you can use feed forward to get ±1 ppb/C°. If the > temperature can be held to ±0.001° C, this is ±1 part per trillion. > This kind of accuracy has never been heard of. It has been heard of. The 8607 was spec'ed to <2e-10 p-p deviation over temperature range (-30°C to 60°C). Also, to hold the temperature stable to 0.001K in a room temperature environment (let's say 10K variation), you need a thermal gain of >10k. That's quite a bit and needs considerable design effort. Most OCXO design's I am aware of are in the order of 100 (the DIL14 designs) to a few 1000 for single ovens, to a few 10k for double ovens. The only exception is the E1938 which achieves >1M. But that design is not for the faint hearted. I don't remember seeing any number, but i would guess the 8607 has a thermal gain in the order of 100k to 1M as well, considering it being a double oven in a dewar flask. Also, what do you mean by atomic standard and feed forward? If you have an atomic standard you don't need to temperature stabilize your quartz. You can just simply use a PLL to lock it to your reference and achieve higher stability than any oven design. > Feed forward also > allows you to incorporate the components of the oscillator into the > thermal behavior. It does no good to have a perfect crystal if the > oscillator components drift. Beyond tau=100s, the temperature and moisture sensitivity of the electronics, combined with the aging of the electronics and the crystal will be the limit of stability. Of course, this is under the assumption that you achieved a thermal noise limited design and thus the 1/f^a noise of the oscillator is negligible in the time range considered. Attila Kinali [1] "Traceable Temperatures - An Introduction to Temperature Measurement and Calibration", 2nd edition, by Nicholas and White, 2001 [2] "Thin-film platinum resistance thermometer for use at low temperatures and in high magnetic fields", Haruyama, Yoshizaki, 1986 [3] "Completely Integrated 4-Wire RTD Measurement System Using a Low Power, Precision, 24-Bit, Sigma-Delta ADC", Analog Circuit Note CN-0381 http://www.analog.com/CN0381 [4] "Completely Integrated 3-Wire RTD Measurement System Using a Low Power, Precision, 24-Bit, Sigma-Delta ADC", Analog Circuit Note CN-0383 http://www.analog.com/CN0383 [5] "2- 3- 4- Wire RDT (Pt100 to PT1000)Temperature Measurement" Ti Presentation http://www.ti.com/europe/downloads/2-%203-%204-Wire%20RTD%20Measurement.pdf