Hey Joe, dear dcac, CDaniel,
Maybe you remember, that I made a strict review on the basic characteristics of the 121GW:
https://www.eevblog.com/forum/testgear/eevblog-121gw-discussion-thread/msg1687298/#msg1687298where I quantitatively verified the specifications of DC/AC voltage and current and Ohm.
My 121GW is from the 2nd batch from Kickstarter, delivered in July 2018 with FW 1.22, now 2.02, and its hardware is still very similar to Joes latest 121GWs.
Very soon I observed and described several problems with the Ohm mode, especially, that negative Ohm values are clipped to zero, so there is a 'hidden' offset (originating from initial calibration), which affects the accuracy of low Ohm measurements, as this offset can't be zeroed.
That clipping effect of Ohm mode is obviously removed latest with FW 2.02, but still present on the capacitance mode.
That explains the big errors you all see with small pF capacitors. This is evident as a threshold effect in capacitance value, i.e. the calibrated zero value plus the actual offset will always be subtracted from the real value. Capacitors smaller than this threshold will be displayed as 0.000pF, and bigger ones will show too small a value.
If I let my 121GW sit for a while in the 10nF range, it will possibly display some pF offset with open leads, and if I then zero the reading, I always get very precise values, even with pF capacitors (e.g. 39pF).
If that obvious (zero) drift is zeroed repeatedly between subsequent readings, then the measurement of the capacitors value is consistently precise, better than the specified 2.5%.
Long measurements on this zero drift make no real sense, I think.
Btw.: I can really check the accuracy by my DE-5000 LCR meter, which is specified down to 0.3% accuracy, depending on range and test frequency.
Interestingly, the actual manual (9/2019) still specifies this mode as:
I loaded FW 1.17, and indeed, the resolution then was 3 digits only, i.e. 10pF resolution in the lowest range.
That also means formally, that +5 digits, or 50 pf variation are allowed.
So do not expect too much of accuracy, because of this misleading specification, or because of the additional digit resolution.
The specification might be updated / improved to the 4 digit resolution, which seems to be quite reasonable, if that threshold problem can be solved.
Also pay attention if you try to re-calibrate the capacitance ranges.
There is no entry for zero value calibration in the small table inside the chapter ZERO OFFSET CALIBRATION.
In the big colored table, though, there is a scrambled entry, 'R1 : ', which might indicate that the first calibration point with open leads sets the offset, and the 2nd one with the nominal reference capacitor sets the gain.
That might explain CDaniels problem, that 0.000 is displayed for a 10nF capacitor, after calibration.
Similar aspects hold for the 50.000 Ohm range, as this has even +20 digit or 20 mOhm specified deviation.
CDaniel criticized in another thread:
'2 - in 50ohm range the last 2 digits are pretty useless . Every time you short the leads you get different reading and fluctuating' At first, I think that you simply measured the varying contact resistance of your probe or short, whatever you have used.
This has nothing to do with the 121GW itself.
Instead, I would say in a general manner, that it is very difficult to make reasonable Ohm measurements in the 1..50 mOhm range with the 2W method only.
The 4W / Kelvin method is required to safely cancel lead and contact resistances.
Additionally, the 121GW uses about 500µA in this range only, i.e. 1 mOhm equals about 0.5µV, so proper cancellation of thermo voltages is also required, i.e. usually the OFFSET COMPENSATION function does this job.
By using solid cables, and proper zeroing (REL), e.g. by use of an appropriate short, one can cancel both, contact resistances and thermo voltage, even on the 121GW.
By this method, and in contrast to CDaniels tests, I get quite accurate and stable readings at 100 mOhm and upper settings on my decade resistance box, and the deviation or fluctuation from nominal are a few mOhms only. Frequent zeroing is also required here.
You can as well measure/detect the variation of the contact resistance of the switches.
So that low resistance resolution can be quite useful, although I would always prefer an appropriate instrument for such measurements, like my 34465A.
Frank