Calibration of a BK Precision 2709B multimeter

[Lefuneste]

Hi guys!
While building a decade resistance network, I have realized that my BK Precision 2709B multimeter lacks quite a bit of precision in the kOhm range (above 600 Ohms, 700 Ohms are displayed as 0.696 kOhms for instance). Below 600 Ohms I am spot on. I've opened it, and have found 3 tiny pots marked V1 V2 and V3. Obviously, the manufacturer of the device does not provide any documentation as how to calibrate the thing. 

Does anyone out there have any idea what these pots are, or what procedure should be used to calibrate this meter in V, A and Ohms? For instance, if someone knows how to work these pots on a similar BK precision DMM, I think I could adapt the procedure to this specific model. I believe there should be some similarities between these DMM. Any insight would be highly appreciated!! Thanks!! 

[dr_p]

How do you know it's not reading correctly? What kind of 700ohm resistor are you measuring? Do you have other DMMs that show different values? What model? Are they calibrated?

[Lefuneste]

Ok this is an excellent question (I wanted to address it in my initial post actually...).

Here's the reasons why I know it's not reading correctly.
-I measure a decade resistance box I have made myself. All resistors within a given group have the same value (within 1% specifications but actually are better than that as I selected them manually to match).

-I can measure each resistor individually. For instance, on the 100 Ohms set of resistors, I can measure each resistor (while in the Ohm range) and each one is spot on 100 Ohms (+- probes and traces residual resistance).

-When measuring them in series (as they should be used in such a device), I get these exact measurements from 1 up to 600 Ohms. As I have determined my residual resistance from probes and traces to be 0.56 Ohms, I can get 2 decimal points in precision : for instance I can almost exactly read from 1.56 Ohms to 600.56 Ohms for any value I select on the device.

-But above 600 Ohms, the DMM switches automatically to the kOhms range. And then where I should ideally read 0.700 kOhms, I read 0.696 kOhms. It's not such a big difference, but it's quite noticeable. I am quite certain that this DMM can be more accurate.

Even if we exclude any absolute precision here (ie the DMM could actually be wrong even in the Ohms range), one could say that the precision of the DMM changes quite dramatically when it changes it's range from Ohms to kOhms. Hence the need to recalibrate it.

Apart from that I have confirmed the problem with my other DMMs (Brymen and UEI).

[Lefuneste]

So basically, my question is to try understanding what these VR1 VR2 and VR3 pots are supposed to be, on this BK precision 2709B DMM. 

[ketil b]

So according to the data sheet for your meter the error on that range is 1.2% of reading + 5 counts and the error in your measurement is 0.57% which is well inside its speck. I would not try to calibrate a meter that is in speck it might end up in a mess. If you need more accurate see if you can borrow a better meter from some one, your local hacker space is a good bet.

Thanks

Ketil

[Lefuneste]

Thanks for the reply guys. Yeah Ok I guess it is too risky to mess around with these pots, without the proper documentation on how to calibrate the DMM... Damn it...

[Lefuneste]

Hi guys. For the benefit of the community, I've attached the reply I had from BK Precision when asking for the calibration procedure for the 2709B multimeter. To my great surprise they did send it to me and it is a very simple procedure indeed. It comes down to 2 pots. It may be useful to others, for the moment I don't have a precision source for the 390mV, but the procedure might work to calibrate the DMM at higher voltages too, I don't see why it wouldn't...

Bazinga !!