Author Topic: DMM for calibrating 20V  (Read 1496 times)

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Offline DoubleDang

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DMM for calibrating 20V
« on: October 05, 2018, 08:31:54 pm »
Hi everyone!

I need to calibrate a particular device to 20V within +/- 2mv,
so as I understand it I'll need a DMM with a combined accuracy of 0.01%, is that right? Or do I need to find something that's an order of magnitude better than that i.e. 0.001%?

I was hopeful to find a handheld DMM for the job (I have to travel with it). However I've looked at the fantastic https://www.eevblog.com/forum/testgear/multimeter-spreadsheet/ and it doesn't appear that handheld meters are acceptable for this. I noticed the Gossen 30M could be possibly used for this purpose but it's discontinued. Am I out of luck for a handheld DMM?

Thanks for your help, please let me know if I'm in the wrong sub-forum :)
 

Offline Andreas

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Re: DMM for calibrating 20V
« Reply #1 on: October 05, 2018, 09:35:19 pm »
Or do I need to find something that's an order of magnitude better than that i.e. 0.001%?

I was hopeful to find a handheld DMM for the job (I have to travel with it).
Hello,

0.001% is typical a 7.5 to 8.5 digit desk multimeter.
But fortunately a factor 3-5 better than your target would give a suitable margin.
with 25-35 ppm in 18-28 deg C environment you can find some 6.5 digit desk multimeters.

But perhaps it is easier to use a handheld precision voltage source and a sensitive "zero voltmeter".
 (or 2 in series since 20V is not that what is typical in use as "process calibrator")
e.g. something like this:
http://www.ianjohnston.com/index.php/onlineshop/handheld-precision-digital-voltage-source-v2-detail

with best regards

Andreas
 
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Offline guenthert

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Re: DMM for calibrating 20V
« Reply #2 on: October 06, 2018, 08:17:41 am »
A cheaper option might be to get two low cost 10V references (e.g. those of voltagestandard.com), put those in series (as Andreas suggested) and measure the difference to the DUT with a handheld DMM.
 
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Online tggzzz

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Re: DMM for calibrating 20V
« Reply #3 on: October 06, 2018, 08:53:21 am »
You may need to consider the output impedance of the source (lower=better) and the input impedance of the meter (higher=better). Apply the standard resistive voltage divider equation to see the effects.

Note that some meters have a very high impedance on the 10V and below ranges, but 10Mohm on higher ranges. Why? Because they use an internal resistive divider to get the DAC input voltage in the 0-10V range.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline try

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Re: DMM for calibrating 20V
« Reply #4 on: October 06, 2018, 10:51:53 am »
Hello DoubleDang,

Hi everyone!

I need to calibrate a particular device to 20V within +/- 2mv,
so as I understand it I'll need a DMM with a combined accuracy of 0.01%, is that right? Or do I need to find something that's an order of magnitude better than that i.e. 0.001%?

No. With an external reference the accuracy of the DMM does not play a role, only short-term stability of the DMM
- as long as it's only about this particular measuring point. :)

Example:
I have a cheap handheld UT61E-Asian DMM - one of those that Dave Jones ("pure opinion") dislikes so much.   

The 20V DC?! figure you need to calibrate fits perfectly into its 22V measurement range right at the upper end as it should be.
It has a resolution of 1mV in the 22V range which seems to be sufficient to achieve that 20V +-2mV goal.

IF you provide 20V (say 20V+-0,2mV) by means of an external reference to the UT61E you just have to keep in mind that value
and trim your device to exactly to that value shown before.

Even when your reference is between 19V and 21V the linearity is not an issue with my UT61E - measured against a 6,5 digit DMM.
Given the exact value of the reference you would simply calculate the error of the UT61E and take it into account when calibrating the device.

The accuracy that you were asking for is only needed when calibrating the reference.

Regards
try



« Last Edit: October 06, 2018, 10:58:12 am by try »
 
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Offline David Hess

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Re: DMM for calibrating 20V
« Reply #5 on: October 07, 2018, 01:54:42 am »
I would use two 10 volt shunt references in series or a reference and gain stage but both will require calibration against a better source.

There may be some suitable process control calibrators but they will be much more expensive.
 
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