Products > Test Equipment

DMM: Measuring AC Volts

(1/2) > >>

On a DMM, there is an OFFSET or NULL button.
I know this button is needed for measuring DC, but is it needed for AC as well? I can't find it in the manual.

I don't know what meter you have, but on my DMMs, there is a 'REL' button (relative) which is mostly used to null the impedance and capacitance of the test leads for low value DUT measurements.
It shouldn't be required for voltage or current measurements (AC or DC).

I always use OFFSET(Fluke), REL(Keithley), and NULL(HP) after shorting out the leads to measure DC, but I haven't used the AC function other than to measure the main, and this was with a  hand held.
When I measure DC Volt, I short both of the leads and press the OFFSET button for accuracy. When I measure the 300 Volt in a freshly NIST Calibrated HP 3457A, it displays 0.29 volt whether I short the leads or not.
If I NULL this when I measure 40V AC, its effect is huge.
Anyways, the calibrator said he doesn't use it. And the manual does not go into specifics of how or when to use this function.

[EDIT] Even though manuals say "The OFFSET feature may be used in all functions." - Fluke 8840A manual page 2-10 &  "You can use the NULL operation on any measurement
function (DC voltage, AC current, and so on)." - HP3457A manual page 2-25. Obviously it shouldn't be used in case of AC because the OFFSET's purpose was to take out the leads's effect.  In case of AC V or I, shorting doesn't shrink the number. The number has something to do with inside the meter. For example offsetting 0.3V to measure 32.5V is huge.

No, you should not null the shorted reading with a true RMS meter. A true RMS converter is non-linear near 0 (which is why ACV is usually only specified from 5-100% of the range). It's designed to be linear within the 5-100% range. If you correct the reading for the shorted leads, it will be off for the higher part of the range. See this Keithley document why this offset is insignificant, and how using REL affects your measurements.

Thank you for the new material to think.
In case of my HP3457A, the offset is 300 counts. So according to the document it will display 0.10005.
The paper says most DMM but I think most Keithley DMM.

One more thing to think is the 100 count in the paper. Is that number happened because they connected the leads? My HP3457 's number is there whenever I choose AC V or I and doesn't change when I put the leads into the terminal and shorted.

Any HP3457A owners to see what number they got at AC V or I setting? my HP3456A has 150 count, my Fluke 8840A and 8842A have 40 to 50 count.


[0] Message Index

[#] Next page

There was an error while thanking
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod