5 mA DC range FAILS again - can anybody replicate?
So i got a replacement 121GW after the problem i had with my first unit (see my posts
I repeated the same tests with a 1 mA DC source, and got essentially the same results, here are photos showing the poor measurement on the 5 mA DC range, but the perfect measurement on the 50 mA range, on my Amprobe AM270 meter, and also using the uCurrent Gold.
Summary: On the 5mA DC current range, the 121GW reads off, and is quite significantly affected by touching the leads. None of these effects are seen on the 50mA range, on the Amprobe meter, nor when using the uCurrent.
Can anybody replicate these findings? Figure out a way to mitigate this problem? Should I consider the 5mA DC current range to be an "extreme" measurement and i'm just expecting too much? I wouldn't think so in a meter in this price range, considering the Amprobe is a significantly lower price class instrument. I kind of think Dave would be giving a Thumbs Down on this performance if he was reviewing it.
Thanks.
--adam
I tested the 5mA DC range of the 121GW, with a precision DC current source, and a 34465A in series.
The 121GW is rock solid, even testing with a strong magnetic 50Hz disturbance by transformer did not show any deviation to the last digit.
I also tested the ac rejection, when I applied a sine wave ac current.
The 121GW in its 5mAdc range shows about +/- 50 counts in very random manner, when I apply 100µAac, the 34465A suppresses this signal about ten times better.
At 10µAac, the 121GW reads about zero.
That means, if you see variations, there must be a strong ac current present in your setup....
Think about it, it's extremely improbable, that two new instrument would be both defective, if nobody else can reconstruct this sensitivity against external noise.
You have set up the experiment both times in the same manner, if I understood you correctly, so you might also have replicated the probably erroneous setup.
It would have been better to modify your setup, before concluding that the 121GW is defective.
I have one idea, what could be your problem:
The DMMCheck PLUS has a small ac/dc switch, which type I assume to be unreliable.
So it could output the 1mA ac current, even if it's set to dc current.
The ac current seems to be a pulsed square wave dc current only, i.e. a 2mA switched unipolar signal, in contrast to the ac voltage, which is a 'true bipolar' square wave.
So the 121GW would see a relatively strong 100Hz ac current, which is not averaged as well as other dual / multiple slope instruments.. and the other DMM could average that to 1mA, as this might be the correct average, but also the rms value (I have to confirm that by the correct math).
So please check with both instruments, whether this supposed dc current gives a reading on the ac current ranges.
There could also be some other failures, like oscillations of the DMMcheck, too high compliance voltage (500mV max!), disturbance from the other DMM, cable or plugs error (the 121GW has segmented current jacks!).
So from remote, it's really difficult to make a correct guess, but for me it's very improbable, that the 121GWs are the culprit.
Frank