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EEVBlog 121GW Multimeter Firmware Details

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Terry01:

--- Quote from: joeqsmith on November 26, 2018, 06:32:50 am ---
--- Quote ---... gave correct reading but a short time later the reading started to display approx 82 pF instead.
--- End quote ---

If you watch that short clip I made, do you feel I was not waiting long enough?

About three hours now at 27C.  The 121GW  proto still has not budged off 16pF.   Looks like 27C was too low of a setpoint.  The camera, meter and fan are more than what all that insulation will carry away. 

The one problem I have noticed is how far off the meters internal temperature is.  About 10C low.  It may have always been this far off or maybe they changed the thermistor.   The schematics are the same.   There appears to be no way to align it outside of changing R38.  May be nice to have a way to trim it as part of the alignment.

--- End quote ---

I got a 121GW new a few days ago with v1,54 firmware installed and noticed the internal temp is around 2.5C higher than the internal temp in my U1273 when the temp is around 15C. As the temp gets nearer 20C it wanders to being around 3C higher. The 121GW also wanders quite a bit more on start up where as the U1273 is a lot steadier. The 121GW changes from anywhere around .5C to 1C in the first minute after start up.

lowimpedance:
@Joe, I have not watched the video yet but I will and comment then.
When measuring the 100 pf after the unusual drift to the lower value I did not observe a further drift of more than a few LSD. I will check for drift tomorrow with the 100 of from the first start.
I will also pay attention to the measured temperature compared to other meters for any differences from the lab temperature.

lowimpedance:
 Okay this morning I turned the 121 on an noted an initial reading of 0.012 nF with no connected cap. Which is what I had observed previously.
Next I connected the 100 pF and the reading was 0.111 pF then over the next 10 minutes that slowly drifted down to 0.109 pF as the 121 warmed up !. No further significant change observed.
The meter was kept in a temp. controlled lab overnight and is sitting at 21.40 deg. as recorded on a Guildline 9540 PRT temp. meter. The 121 secondary temp. reading was also 21.4 deg.
 Now the interesting bit, with the 100 pF still connected I cycled through the modes on that switch position and now the 100 pF measured as 25 pF and would stay at the value when either
turning on or off or disconnecting or reconnecting the cap.
Leaving the unit off for a short time and then turning back on shows the measured value to be increasing back to the correct value !!. just like something has a residual charge being slowly bled away.
 At re-switch on it showed approx. 68 p and after 10 or so minutes was up to 93 p, and now at 103 pF after 20 plus minutes...... :-DD
Interestingly the display will show 0.000 nF with no connection, when the 121 starts to give wrong measurements.  (mode cycling).
Perhaps the firmware hides all negative readings and just gives all 0  :P.

@Joe , Looking at your video I would say the behaviour was consistent with mine when measuring a 1 nF cap although I did note a change from the initial value down from 1.016 n to 0.977 nF after mode cycling.

OT:, and yay post 1000 !..........only took 7 years  :-DD

CDaniel:
If you have some initial reading when you cycle through modes will clear that ... in my case anyway .
There has to be some kind of automatic zeroing when switching to capacitance , otherwise there is no way to read 0.000nF with or wihout leads ( long , short or how you want ) . If you put the probes close but not twisted together the capacitance is at least 10pF .

So if you leave something connected at start-up will screw-up the auto calibration ... my theory .

Maybe this feature  is not well design and is responsible for what we observe . Why some meters don't have the issue is interesting , the external electromagnetic fields may play a role .

lowimpedance:
Additional info:
 I cycled the modes on the switch position to get the wrong capacitance reading of 24 pF with a 100 pF silver mica capacitor connected, then I left the meter running and waited.
The meter did auto power off but on restarting the meter using a key press the reading was correct at 109 pF. The time that passed was at least 30 minutes.
So all I did was to return after a half hour and turn the mater back on to now show the correct value.

Mode cycling will fully cause the issue to present itself each and every time on my meter.

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