Electronics > Metrology

Calibration on common multimeters?

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MBY:
I was inspired by this thread: https://www.eevblog.com/forum/index.php?topic=3020.0

Many medium to high range handheld DMMs today used closed case calibration. The calibration is not done via trimpots but by feeding a reference voltage/current/resistance/freq/etc to the meter when put in "calibration mode". This is demanding for the hobbyist in that you cannot calibrate to an arbitrary voltage as you need fixed stable voltages at, say, 19.000 or 39.000 VDC, eg.

I have plenty of top quality, stable instrument to do calibration and and lots of chepo, mid range and high end DMMs. My problem? I don't know how to enter calibration mode (if even present).

I thought that this thread could be a list of known methods to enter cal mode in common DMMs. This procedure are typical (and for good reasons) left out of the user manual and service manuals are seldom to be found for lower brands of DMMs.

So, a bunch of meters that I know calibration can be done on, but no other clues.

If you know some DMMs calibration procedure, especially if it is a "soft" closed case cal, please share with us!

More specific requests: BK Precision 2709B, Amprobe/Meterman 37XR-A (I'm sure that at least the 2709B has a calibration procedure and no adjustable components on PCB, so it must be a software one). Do anyone know?

(I have emailed a bunch of manufacturers but no luck so far).

Edit: I lied about the BK 2709B - it really have at least three really small pots on the PCB, named VR1-3. Anyhow, without the cal procedure I don't dare to adjust them.

Edit: The list (only on entry so far :( ):
37XR-A: 37XR Calibration Procedure (soft + HW adjust, no closed case cal)

Edit: Changed topic name to better reflect that the thread is intended to list calibration procedures in general. Sorry, I was not very clear in my first post.

alm:

--- Quote from: MBY on December 10, 2011, 12:14:10 am ---This is demanding for the hobbyist in that you cannot calibrate to an arbitrary voltage as you need fixed stable voltages at, say, 19.000 or 39.000 VDC, eg.

--- End quote ---
Some (high-end) meters allow you to specify the voltage you feed it, so you can calibrate it to 13.783V.


--- Quote from: olsenn on December 09, 2011, 06:20:19 pm ---I have plenty of top quality, stable instrument to do calibration and and lots of chepo, mid range and high end DMMs. My problem? I don't know how to enter calibration mode (if even present).

--- End quote ---
The common method used to be a recessed button or switch. The use of a calibration seal requires some sort of mechanical button under the seal, but I'm not sure if the cheapies even use a calibration seal? The only cheap DMMs I've seen use old-fashioned trimmers.

IanB:
Let me add the Protek 6500 to the list. This has a calibration seal on the back (actually a removable sticker) and under it a recessed button to press. However, if one presses the calibration button, the question becomes what happens next? Could one totally screw up the meter by pressing it and not having instructions on how to exit calibration mode without saving changes?

EEVblog:
I had to re-cal my Gossen Energy recently when a firmware update trashed the cal values >:(
I have some of it on video, I needed to use USB cable and a terminal program, and the commands and procedure that Gossen sent me.
Pretty annoying.
It appears as though the DC calibration (which is all I've done so far, to 10V) set the resistance as well.

Dave.

alm:
With models not using a computer interface, it often involves setting the meter to the function you want to adjust, and then use some of the buttons to cycle through the various ranges. The actual buttons, order and expected input should be in the calibration manual, unless you have some free time and don't mind corrupting the current cal constants.

Most of the cheap meters are unlikely to ever see a cal lab since the cal costs exceeds the costs of the meter, so I doubt that they spend much time documenting it.


--- Quote from: EEVblog on December 10, 2011, 12:46:24 am ---It appears as though the DC calibration (which is all I've done so far, to 10V) set the resistance as well.

--- End quote ---
I'm surprised, how do they calibrate the internal current source with just a voltage source? Even artifact calibration usually requires a voltage reference and a reference resistor.

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