Calibration of voltage standards
Hi Group,
I have heard of a few different ways to measure one 10V voltage standard
against another 10V voltage standard.
Assume we have two 10V voltage standards. One is calibrated, the other
not only needs to be calibrated, but probably adjusted. For the sake of
simplicity, let's say the two standards are Fluke 732B's.
I think the best way is to connect the two units' (-) terminals
together, then connect a calibrated meter in between the (+) terminals,
and measure the difference. I have also heard that to remove thermal
EMFs, you should use a low-thermal-EMF DPDT switch or a low-thermal-EMF
relay to reverse the connections on the DMM, so you can take the reading
forward and reversed, then split the difference. There is the
possibility to introduce thermal-EMF errors from the switch/relay as
well, so I'm wondering if this is a good idea. This sort of makes sense
to me, but I'm not a metrologist, so I would like to hear what others in
this group think about this.
So, just what is the proper way to accomplish this task?
BTW-- I have an Agilent 34420A and an HP 3458A, which would be the
better instrument for this task?
Best Regards,
Ken Peek
To do a comparison of the sort you are asking about, the
sensitivity of the null meter is much more important than
its ultimate accuracy.
So, neither of your meters is really the right meter to
use for this task. What you want is called a null meter,
and is generally sensitive to the microvolt region.
-Chuck Harris
Ken Peek wrote:
Hi Group,
I have heard of a few different ways to measure one 10V voltage standard
against another 10V voltage standard.
Assume we have two 10V voltage standards. One is calibrated, the other
not only needs to be calibrated, but probably adjusted. For the sake of
simplicity, let's say the two standards are Fluke 732B's.
I think the best way is to connect the two units' (-) terminals
together, then connect a calibrated meter in between the (+) terminals,
and measure the difference. I have also heard that to remove thermal
EMFs, you should use a low-thermal-EMF DPDT switch or a low-thermal-EMF
relay to reverse the connections on the DMM, so you can take the reading
forward and reversed, then split the difference. There is the
possibility to introduce thermal-EMF errors from the switch/relay as
well, so I'm wondering if this is a good idea. This sort of makes sense
to me, but I'm not a metrologist, so I would like to hear what others in
this group think about this.
So, just what is the proper way to accomplish this task?
BTW-- I have an Agilent 34420A and an HP 3458A, which would be the
better instrument for this task?
Best Regards,
Ken Peek
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the 3458A is very well suited as a nullmeter (there e.g. also is an appnote from Fluke on this). I have checked this against other meters such as the e.g. the keithley 155 and 34420A, and for my setup (Fluke 732A, Datron 4910 and others) I have the least problems with noise, common mode issues and so.
The nullmeter method should only be used when the DUT is adjusted. ie., low voltages differnces of a few mV should not be measured, the accuracy is, at least formally, not specified there sufficiently precise.
The second method frequently used and that I am using most of the time (since I do not adjust references, especially the 732A is known to potentially increase its drift thereafter, the 4910 uses digital adjustment, so there it would be ok) is to measure the absolute voltage of both standards with a 3458A and reference the DUT to the standard. This can be done because of the excellent linearity of the 3458A. If I remember correctly the accuracy related to the standard is in the 0,1ppm range at 10V.
Gesendet: Mittwoch, 11. Februar 2015 um 08:36 Uhr
Von: "Chuck Harris" cfharris@erols.com
An: "Discussion of precise voltage measurement" volt-nuts@febo.com
Betreff: Re: [volt-nuts] Calibration of voltage standards
To do a comparison of the sort you are asking about, the
sensitivity of the null meter is much more important than
its ultimate accuracy.
So, neither of your meters is really the right meter to
use for this task. What you want is called a null meter,
and is generally sensitive to the microvolt region.
-Chuck Harris
Ken Peek wrote:
Hi Group,
I have heard of a few different ways to measure one 10V voltage standard
against another 10V voltage standard.
Assume we have two 10V voltage standards. One is calibrated, the other
not only needs to be calibrated, but probably adjusted. For the sake of
simplicity, let's say the two standards are Fluke 732B's.
I think the best way is to connect the two units' (-) terminals
together, then connect a calibrated meter in between the (+) terminals,
and measure the difference. I have also heard that to remove thermal
EMFs, you should use a low-thermal-EMF DPDT switch or a low-thermal-EMF
relay to reverse the connections on the DMM, so you can take the reading
forward and reversed, then split the difference. There is the
possibility to introduce thermal-EMF errors from the switch/relay as
well, so I'm wondering if this is a good idea. This sort of makes sense
to me, but I'm not a metrologist, so I would like to hear what others in
this group think about this.
So, just what is the proper way to accomplish this task?
BTW-- I have an Agilent 34420A and an HP 3458A, which would be the
better instrument for this task?
Best Regards,
Ken Peek
volt-nuts mailing list -- volt-nuts@febo.com
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