055 :: minor calibrations for UNI-T UT61E


Today i took some time to tune up the UNI-T UT61E assisted by the the DMM Check PLUS device. my DMM check plus is not new, but its not very much used, i kind of lost it in a heap of junk in the house and only recently dug it out of its "hole", so it might have gone off its recorded values by some order of ppm, but nonetheless, it is better than having nothing to calibrate against.

and i always knew the UNI-T stock calibration is rather sucky. when put up against the DDM check plus values, all the AC/DC values are at least off by 100 to over 500 counts. which is really a lot.



inside the 61E, there are 3 very important trimmers. VR1, VR2 and VR4. to calibrate DC voltage, you only need to use VR1 which is a multiturn trimmer. When DC volts is calibrated, it also seems to calibrate the milliamperes readings at the same time. so that VR1 will calibrate both DC volts and DC amperes.




for the AC scale, it is somewhat more tricky. VR2 calibrates for the target voltage (in this case what i have is 5volts). But i have gone to adjust this in a very different way. i adjusted first for the 1mA AC milliamperes. once that is done, the AC volts reads slightly over 5 volts, and then i adjusted VR4 to get the 5 volts to its actual value. but thats not the end, disconnect everything, and now you are left with a ghost floating AC voltage, now you need to zero that using VR4 too. VR2 is a very sensitive trim pot, touching it with a metal trimming screwdriver will drive the reading slightly nuts.

after a few rounds of adjusting the VR2-VR4 combo. you will get something like +/- 5 counts from the actual standard, and i reckon that will be enough for this DMM.


 
 

the frequency counter seems to do well on its own. very nice !




the reference resistor is 100.08ohms. so the ohmeter is off by approx -0.1%












The reference resistor in use is 100.08k. so again, it is off by approx -0.13%


according to research by EEVblog forumner Rick Law, there are more intricate calibrations that can be done, but for the extend and possible projected 75ppm possible drift error of the UT61E. i think the above general calibration will do nicely for now. for anything more accurate, i think it will be highly recommended to buy a 7-8 digit scale bench DMM.

 

then i went on to try and calibrate the UT81B. the DC calibration was a breeze. both DC 5volts and 1 milliamps are spot on.


 
 

The AC calibration however, is trash. it appears that there is a huge overshoot in both volts and milliamps.

the ohms, Hz measurements are well within error range (less than 0.2% on average)

in conclusion, UT81B is utter rubbish. UT61E however is quite usable and more so after calibrating. or maybe i should make a SINE wave based volts reference.




i guess these will be the last CHINA DMM i will buy. the next few measurement equipments will either be really high end or DIY (hopefully)

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