uC ADC resolution (was: Poor man's oven)
On Wed, Jun 7, 2017 at 3:18 PM, Bob kb8tq kb8tq@n1k.org wrote:
Hi
Consider that a lot of the heat flow is through the glass wall of the
vacuum gizmo. You want to tie your heater(s) to it in order to create
an iso-thermal “wall”. Often this is done by gluing the whole assembly
together.
Not glass walls. Stainless steel. The metal wall make it uniform.
I occurred to be that you could fill the container with a liquid.
Something that has good thermal conductivity and it would be very uniform
inside. I don't know what, transformer oil perhaps?
--
Chris Albertson
Redondo Beach, California
Hi
Metal actually makes things a bit worse since it has a lower thermal resistance than glass.
This is also why a high performance dewar is made from glass rather than metal. Yes, you
can go to weird stuff like titanium (it has been done). You can’t afford that ….
If you fill the entire dewar with a heat conductor you make things worse still. The idea is to
block heat flow out of the heated area. Even without fill, the wall of the dewar goes from the outside
world to the heart of the heated area. It is the perfect “sneak" path into the oven. Actually
it’s not that much of a sneak path since it’s a well known effect :)
Again, none of this is particularly original. Take a hammer to any dewar based OCXO and
the details are going to be similar.
Bob
On Jun 7, 2017, at 9:51 PM, Chris Albertson albertson.chris@gmail.com wrote:
On Wed, Jun 7, 2017 at 3:18 PM, Bob kb8tq kb8tq@n1k.org wrote:
Hi
Consider that a lot of the heat flow is through the glass wall of the
vacuum gizmo. You want to tie your heater(s) to it in order to create
an iso-thermal “wall”. Often this is done by gluing the whole assembly
together.
Not glass walls. Stainless steel. The metal wall make it uniform.
I occurred to be that you could fill the container with a liquid.
Something that has good thermal conductivity and it would be very uniform
inside. I don't know what, transformer oil perhaps?
--
Chris Albertson
Redondo Beach, California
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 6/7/17 7:35 PM, Bob kb8tq wrote:
Hi
Metal actually makes things a bit worse since it has a lower thermal resistance than glass.
This is also why a high performance dewar is made from glass rather than metal. Yes, you
can go to weird stuff like titanium (it has been done). You can’t afford that ….
If you fill the entire dewar with a heat conductor you make things worse still. The idea is to
block heat flow out of the heated area. Even without fill, the wall of the dewar goes from the outside
world to the heart of the heated area. It is the perfect “sneak" path into the oven. Actually
it’s not that much of a sneak path since it’s a well known effect :)
Again, none of this is particularly original. Take a hammer to any dewar based OCXO and
the details are going to be similar.
And the wires provide a thermal path - you've got to get power to it and
the oscillator signal out.
In dewar OCXOs (like the USOs made by APL), the leads are essentially
the only thermal path in/out.
BTW, you don't want to do oil filled. All oil filled electronics (unless
welded closed) inevitably leaks, oozes, or otherwise puts oil on the
outside. If you must have oil, then use solid, uninsulated wires to
penetrate the surface of the oil. Otherwise you'll gain new appreciation
for what capillary action is.
Hi Folks,
This discussion of temperature measurement with ADCs has crossed into my
professional life and without being too much of a commercial plug just
wanted to mention that I support Linear Technology's temp-to-bits family of
ICs: LTC2983, LTC2984, LTC2986.
Overview: each part has 3 24-bit delta-sigma ADCs along with low leakage
input buffers, excitation sources, an internal mux, and a small
linearization engine. They can measure just about any type of temperature
sensor and digitize it to deg C or F with really good accuracy. Ultimately
accuracy will depend on the sensor, your implementation, environmental
conditions, etc. but the parts have a lot of features that help you get the
most out of a given sensor.
They aren't super cheap (Digi-Key pricing is particularly bad, if you
can order from Linear direct it's much better) but they are to my knowledge
by far the simplest way to interface to a thermistor, RTD, thermocouple,
etc. and get <1degC accuracy.
Ping me off-list if you want to know more or have an application question
in mind.
-Logan
http://www.linear.com/product/LTC2983
On Wed, Jun 7, 2017 at 8:04 PM, jimlux jimlux@earthlink.net wrote:
On 6/7/17 7:35 PM, Bob kb8tq wrote:
Hi
Metal actually makes things a bit worse since it has a lower thermal
resistance than glass.
This is also why a high performance dewar is made from glass rather than
metal. Yes, you
can go to weird stuff like titanium (it has been done). You can’t afford
that ….
If you fill the entire dewar with a heat conductor you make things worse
still. The idea is to
block heat flow out of the heated area. Even without fill, the wall of
the dewar goes from the outside
world to the heart of the heated area. It is the perfect “sneak" path
into the oven. Actually
it’s not that much of a sneak path since it’s a well known effect :)
Again, none of this is particularly original. Take a hammer to any dewar
based OCXO and
the details are going to be similar.
And the wires provide a thermal path - you've got to get power to it and
the oscillator signal out.
In dewar OCXOs (like the USOs made by APL), the leads are essentially the
only thermal path in/out.
BTW, you don't want to do oil filled. All oil filled electronics (unless
welded closed) inevitably leaks, oozes, or otherwise puts oil on the
outside. If you must have oil, then use solid, uninsulated wires to
penetrate the surface of the oil. Otherwise you'll gain new appreciation
for what capillary action is.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/m
ailman/listinfo/time-nuts
and follow the instructions there.
Hi,
I was about to make this very point myself. The resolution of the ADC
needs to be higher than the limit you try to achieve. There is several
ways to reason about it, but one is that the system is a bit slugish you
want to have higher resolution in order to react of changes before they
overshot the limits you want to keep. Another benefit is that you get
away from the bang-bang behavior you get when having too few bits.
For an oven you can however cheat some by not requiring linearity in the
"too cold" region of temperature. You do want some linearity as you
start to come into the right range in order to slow down the heating in
order not to do a big overshot.
I have seen a little too much cases where there been too few bits both
on ADC and DAC sides. Some of it you can overcome, but it runs into
trouble. Get good dynamics, it makes the rest of the design easier.
Cheers,
Magnus
On 06/07/2017 08:32 PM, Bob kb8tq wrote:
Hi
There is a gotcha with the initial assumption: You want the loop to be
quiet at a level well below 0.1C. If it is bouncing around that much,
the second order (rate defendant) tempco of a normal crystal will
become a pretty major issue.
Simple rule of thumb - add at least two bits past whatever the target is.
More or less, if you are after 0.1C and that comes out to 6 bits, you need
eight solid bits to get things to work properly.
Bob
On Jun 7, 2017, at 2:10 PM, Chris Albertson albertson.chris@gmail.com wrote:
One question for the control theory experts.
Assume me goal is to regulate temperer of an aluminum block to within 0.1C,
how good must my ADC be? Is an effective 6-bits good enough?
It seems to me the problem with fewer bits is only quantization noise.
Lets assume 6-bits. This is 1 part in 64. If I scale the input to the
ADC such that it os 1.0C from 0 to 63 counts then each cunt is 1/64 C
which is about 6 times better then my allowed error of 0.6 C.
My gut-feel is that this is marginal but could work ("work" is defined as
holds temperature within the range) but I'd be happier using 8 bits. Im
pretty sure I can get 8-bits by over sampling and filtering.
I don't know how to analyze this but I'm guessing with n-bits each each
sample has a 1/2 bit error so my I and D terms in the PID controller will
accumulate lots of 1/2 bit errors. I thing I want them "a couple orders
of magnitude" smaller then the allied temperature range.
Of cose one could buy the best ADC on the market. But this is POOR MAN's
project. So he asks, "What is the lowers performance/cost part that will
allow the system to meet its specification?
BTW, a related story. I'm on another couple lists that deal with vacuum
tube audio. We see the same things there people correctlypointing out how
to make something better but the question is always how much better and at
what cost an does it matter. So a fun project was proposed. Set a budget
of $200 to build a tube based stereo Hi Fi amplifier. Who can do the
best. Youhade to publish the BOM with prices and suppliers. Extra points
if you came in under budget. This eliminated all the suggestions to buy
high end hand made transformers from Sweden.
IT turrets out that you see MUCH more interesting designs when you lower
the budget. Anyone can make a high performance system even enough money.
They waste half the cost on useless stuff and the product costs double what
it should and is over complex but is works real, really well. That's
easy. Harder and more interesting is "Can you make something just as good
at 1/2 the price?" Answer is usually Yes. Then you say "what much do you
loose if I set the price to 1/4? The answer is surprisingly little if you
get smart about sourcing parts. Turns out about $180 is the minimum
for pretty decent quality HiFi vacuum tube.
An interesting graph would be Oven Specification vs. Price. What is the
minimum cost for keeping temperature to within 1.0 C, for 0.1C, 0.01 C?
Can you do 1.0C for under $5? or 0.1C for under $10. I bet yes.
I did an exercise a while back to see what is the minimum price and
complexity to build a GPSDO that was good enough only to drive the lab
bench instruments I have. I implements only 1/2 od Lars W's design and
cut his lines of code by about 90%. Turns outhe cost is the XO and about
$10. Compared to my Thunderbolt, performance was not nearly as good but
the ratio of performance over parts cost might be better.
On Tue, Jun 6, 2017 at 2:39 PM, Mark Sims holrum@hotmail.com wrote:
Another thing to watch out for on processor ADCs is their performance near
the supply rails... the AVR ADCs are particularly entertaining below
around 300 mV (with a 5V Vref).
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.
--
Chris Albertson
Redondo Beach, California
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.
Hi
The “limited range” part of it is why the op-amp makes so much sense. If the ADC can “see” +/-10C that’s
way more than will ever be useful. You could make a pretty good case for -5/ +1 C as being way more than
you will ever use. The only real issue with the +1 is ok argument is when you offset the oven off turn to balance
out the temperature coefficient of the oscillator circuit. Yes, one could use a bridge circuit to reduce the
need for that offset. I believe it has been suggested and tried :)
Bob
On Jun 10, 2017, at 2:33 PM, Magnus Danielson magnus@rubidium.dyndns.org wrote:
Hi,
I was about to make this very point myself. The resolution of the ADC needs to be higher than the limit you try to achieve. There is several ways to reason about it, but one is that the system is a bit slugish you want to have higher resolution in order to react of changes before they overshot the limits you want to keep. Another benefit is that you get away from the bang-bang behavior you get when having too few bits.
For an oven you can however cheat some by not requiring linearity in the "too cold" region of temperature. You do want some linearity as you start to come into the right range in order to slow down the heating in order not to do a big overshot.
I have seen a little too much cases where there been too few bits both on ADC and DAC sides. Some of it you can overcome, but it runs into trouble. Get good dynamics, it makes the rest of the design easier.
Cheers,
Magnus
On 06/07/2017 08:32 PM, Bob kb8tq wrote:
Hi
There is a gotcha with the initial assumption: You want the loop to be
quiet at a level well below 0.1C. If it is bouncing around that much,
the second order (rate defendant) tempco of a normal crystal will
become a pretty major issue.
Simple rule of thumb - add at least two bits past whatever the target is.
More or less, if you are after 0.1C and that comes out to 6 bits, you need
eight solid bits to get things to work properly.
Bob
On Jun 7, 2017, at 2:10 PM, Chris Albertson albertson.chris@gmail.com wrote:
One question for the control theory experts.
Assume me goal is to regulate temperer of an aluminum block to within 0.1C,
how good must my ADC be? Is an effective 6-bits good enough?
It seems to me the problem with fewer bits is only quantization noise.
Lets assume 6-bits. This is 1 part in 64. If I scale the input to the
ADC such that it os 1.0C from 0 to 63 counts then each cunt is 1/64 C
which is about 6 times better then my allowed error of 0.6 C.
My gut-feel is that this is marginal but could work ("work" is defined as
holds temperature within the range) but I'd be happier using 8 bits. Im
pretty sure I can get 8-bits by over sampling and filtering.
I don't know how to analyze this but I'm guessing with n-bits each each
sample has a 1/2 bit error so my I and D terms in the PID controller will
accumulate lots of 1/2 bit errors. I thing I want them "a couple orders
of magnitude" smaller then the allied temperature range.
Of cose one could buy the best ADC on the market. But this is POOR MAN's
project. So he asks, "What is the lowers performance/cost part that will
allow the system to meet its specification?
BTW, a related story. I'm on another couple lists that deal with vacuum
tube audio. We see the same things there people correctlypointing out how
to make something better but the question is always how much better and at
what cost an does it matter. So a fun project was proposed. Set a budget
of $200 to build a tube based stereo Hi Fi amplifier. Who can do the
best. Youhade to publish the BOM with prices and suppliers. Extra points
if you came in under budget. This eliminated all the suggestions to buy
high end hand made transformers from Sweden.
IT turrets out that you see MUCH more interesting designs when you lower
the budget. Anyone can make a high performance system even enough money.
They waste half the cost on useless stuff and the product costs double what
it should and is over complex but is works real, really well. That's
easy. Harder and more interesting is "Can you make something just as good
at 1/2 the price?" Answer is usually Yes. Then you say "what much do you
loose if I set the price to 1/4? The answer is surprisingly little if you
get smart about sourcing parts. Turns out about $180 is the minimum
for pretty decent quality HiFi vacuum tube.
An interesting graph would be Oven Specification vs. Price. What is the
minimum cost for keeping temperature to within 1.0 C, for 0.1C, 0.01 C?
Can you do 1.0C for under $5? or 0.1C for under $10. I bet yes.
I did an exercise a while back to see what is the minimum price and
complexity to build a GPSDO that was good enough only to drive the lab
bench instruments I have. I implements only 1/2 od Lars W's design and
cut his lines of code by about 90%. Turns outhe cost is the XO and about
$10. Compared to my Thunderbolt, performance was not nearly as good but
the ratio of performance over parts cost might be better.
On Tue, Jun 6, 2017 at 2:39 PM, Mark Sims holrum@hotmail.com wrote:
Another thing to watch out for on processor ADCs is their performance near
the supply rails... the AVR ADCs are particularly entertaining below
around 300 mV (with a 5V Vref).
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.
--
Chris Albertson
Redondo Beach, California
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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To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
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Hi,
Indeed. I prefer to have margins and dynamics, as it makes it easier.
Cutting in on that will require more work to ensure it works, hence you
have to work more to save money in order to maintain stable operations.
Cheers,
Magnus
On 06/10/2017 08:50 PM, Bob kb8tq wrote:
Hi
The “limited range” part of it is why the op-amp makes so much sense. If the ADC can “see” +/-10C that’s
way more than will ever be useful. You could make a pretty good case for -5/ +1 C as being way more than
you will ever use. The only real issue with the +1 is ok argument is when you offset the oven off turn to balance
out the temperature coefficient of the oscillator circuit. Yes, one could use a bridge circuit to reduce the
need for that offset. I believe it has been suggested and tried :)
Bob
On Jun 10, 2017, at 2:33 PM, Magnus Danielson magnus@rubidium.dyndns.org wrote:
Hi,
I was about to make this very point myself. The resolution of the ADC needs to be higher than the limit you try to achieve. There is several ways to reason about it, but one is that the system is a bit slugish you want to have higher resolution in order to react of changes before they overshot the limits you want to keep. Another benefit is that you get away from the bang-bang behavior you get when having too few bits.
For an oven you can however cheat some by not requiring linearity in the "too cold" region of temperature. You do want some linearity as you start to come into the right range in order to slow down the heating in order not to do a big overshot.
I have seen a little too much cases where there been too few bits both on ADC and DAC sides. Some of it you can overcome, but it runs into trouble. Get good dynamics, it makes the rest of the design easier.
Cheers,
Magnus
On 06/07/2017 08:32 PM, Bob kb8tq wrote:
Hi
There is a gotcha with the initial assumption: You want the loop to be
quiet at a level well below 0.1C. If it is bouncing around that much,
the second order (rate defendant) tempco of a normal crystal will
become a pretty major issue.
Simple rule of thumb - add at least two bits past whatever the target is.
More or less, if you are after 0.1C and that comes out to 6 bits, you need
eight solid bits to get things to work properly.
Bob
On Jun 7, 2017, at 2:10 PM, Chris Albertson albertson.chris@gmail.com wrote:
One question for the control theory experts.
Assume me goal is to regulate temperer of an aluminum block to within 0.1C,
how good must my ADC be? Is an effective 6-bits good enough?
It seems to me the problem with fewer bits is only quantization noise.
Lets assume 6-bits. This is 1 part in 64. If I scale the input to the
ADC such that it os 1.0C from 0 to 63 counts then each cunt is 1/64 C
which is about 6 times better then my allowed error of 0.6 C.
My gut-feel is that this is marginal but could work ("work" is defined as
holds temperature within the range) but I'd be happier using 8 bits. Im
pretty sure I can get 8-bits by over sampling and filtering.
I don't know how to analyze this but I'm guessing with n-bits each each
sample has a 1/2 bit error so my I and D terms in the PID controller will
accumulate lots of 1/2 bit errors. I thing I want them "a couple orders
of magnitude" smaller then the allied temperature range.
Of cose one could buy the best ADC on the market. But this is POOR MAN's
project. So he asks, "What is the lowers performance/cost part that will
allow the system to meet its specification?
BTW, a related story. I'm on another couple lists that deal with vacuum
tube audio. We see the same things there people correctlypointing out how
to make something better but the question is always how much better and at
what cost an does it matter. So a fun project was proposed. Set a budget
of $200 to build a tube based stereo Hi Fi amplifier. Who can do the
best. Youhade to publish the BOM with prices and suppliers. Extra points
if you came in under budget. This eliminated all the suggestions to buy
high end hand made transformers from Sweden.
IT turrets out that you see MUCH more interesting designs when you lower
the budget. Anyone can make a high performance system even enough money.
They waste half the cost on useless stuff and the product costs double what
it should and is over complex but is works real, really well. That's
easy. Harder and more interesting is "Can you make something just as good
at 1/2 the price?" Answer is usually Yes. Then you say "what much do you
loose if I set the price to 1/4? The answer is surprisingly little if you
get smart about sourcing parts. Turns out about $180 is the minimum
for pretty decent quality HiFi vacuum tube.
An interesting graph would be Oven Specification vs. Price. What is the
minimum cost for keeping temperature to within 1.0 C, for 0.1C, 0.01 C?
Can you do 1.0C for under $5? or 0.1C for under $10. I bet yes.
I did an exercise a while back to see what is the minimum price and
complexity to build a GPSDO that was good enough only to drive the lab
bench instruments I have. I implements only 1/2 od Lars W's design and
cut his lines of code by about 90%. Turns outhe cost is the XO and about
$10. Compared to my Thunderbolt, performance was not nearly as good but
the ratio of performance over parts cost might be better.
On Tue, Jun 6, 2017 at 2:39 PM, Mark Sims holrum@hotmail.com wrote:
Another thing to watch out for on processor ADCs is their performance near
the supply rails... the AVR ADCs are particularly entertaining below
around 300 mV (with a 5V Vref).
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.
--
Chris Albertson
Redondo Beach, California
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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To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
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In message 3897C09A-D76C-474C-8907-9EA25F8C3451@n1k.org, Bob kb8tq writes:
The “limited range” part of it is why the op-amp makes so much
sense. If the ADC can “see” +/-10C that’s way more than will ever be useful.
Most uC's have a pile of mux'ed ADC inputs, so do all of the above:
AI0 = Full range
AI1 = +/- 10C
AI2 = +/- 2C
A big upside to this is that you will not need to invent heuristics
for clamped inputs in your PI(D) controller, something which is a bit
harder than most people realize.
Assuming a 10-bit ADC, the code will look something like this:
double
get_temp(void)
{
T = read_AI2();
if (T > 50 && T < 975) {
T += T2_offset;
T *= T2_scale;
return (T);
}
T = read_AI1();
if (T > 50 && T < 975) {
T += T1_offset;
T *= T1_scale;
return (T);
}
T = read_AI0();
if (T > 50 && T < 975) {
T += T0_offset;
T *= T0_scale;
return (T);
}
abort("You have problems...");
}
--
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.
Hi,
Very good point Poul-Henning, very good point.
The mux is there, we don't need the resolution far out, and with only a
little though code-wise and hardware wise we get the best of resolution
and range where we need it.
Cheers,
Magnus
On 06/10/2017 09:06 PM, Poul-Henning Kamp wrote:
In message 3897C09A-D76C-474C-8907-9EA25F8C3451@n1k.org, Bob kb8tq writes:
The “limited range” part of it is why the op-amp makes so much
sense. If the ADC can “see” +/-10C that’s way more than will ever be useful.
Most uC's have a pile of mux'ed ADC inputs, so do all of the above:
AI0 = Full range
AI1 = +/- 10C
AI2 = +/- 2C
A big upside to this is that you will not need to invent heuristics
for clamped inputs in your PI(D) controller, something which is a bit
harder than most people realize.
Assuming a 10-bit ADC, the code will look something like this:
double
get_temp(void)
{
T = read_AI2();
if (T > 50 && T < 975) {
T += T2_offset;
T *= T2_scale;
return (T);
}
T = read_AI1();
if (T > 50 && T < 975) {
T += T1_offset;
T *= T1_scale;
return (T);
}
T = read_AI0();
if (T > 50 && T < 975) {
T += T0_offset;
T *= T0_scale;
return (T);
}
abort("You have problems...");
}
Hi
At least in my experience, a properly functioning OCXO will rarely (if ever) be outside +/- 1 degree of the
set point when in full PID (integrator enabled) mode. You may well use a custom set of control parameters
for the warmup phase. You might even use a non-PID based control (shut off all power when these things
happen. Then coast to the set point …). I suppose that like many of my posts, the term “fuzzy logic” could
easily be used in more than one way :)
Bob
On Jun 10, 2017, at 3:10 PM, Magnus Danielson magnus@rubidium.dyndns.org wrote:
Hi,
Very good point Poul-Henning, very good point.
The mux is there, we don't need the resolution far out, and with only a little though code-wise and hardware wise we get the best of resolution and range where we need it.
Cheers,
Magnus
On 06/10/2017 09:06 PM, Poul-Henning Kamp wrote:
In message 3897C09A-D76C-474C-8907-9EA25F8C3451@n1k.org, Bob kb8tq writes:
The “limited range” part of it is why the op-amp makes so much
sense. If the ADC can “see” +/-10C that’s way more than will ever be useful.
Most uC's have a pile of mux'ed ADC inputs, so do all of the above:
AI0 = Full range
AI1 = +/- 10C
AI2 = +/- 2C
A big upside to this is that you will not need to invent heuristics
for clamped inputs in your PI(D) controller, something which is a bit
harder than most people realize.
Assuming a 10-bit ADC, the code will look something like this:
double
get_temp(void)
{
T = read_AI2();
if (T > 50 && T < 975) {
T += T2_offset;
T *= T2_scale;
return (T);
}
T = read_AI1();
if (T > 50 && T < 975) {
T += T1_offset;
T *= T1_scale;
return (T);
}
T = read_AI0();
if (T > 50 && T < 975) {
T += T0_offset;
T *= T0_scale;
return (T);
}
abort("You have problems...");
}
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