Usually professional tools like this are pretty expensive, but if you are lucky they reach their end of life cycle in big production companies and then they swamp the second hand market.
I bought the HP 3478A at the right time for about 100$.
There is a big problem with this multimeter, the calibration chip is battery buffered.
If you had bad luck you buy one which has lost it's calibration data and then operates way out of spec.
Recently the 6,5 digit workhorses like the Agilent 34401A are becoming more and more affordable.
I could not resist and bought one at a ham radio flee market.
Unfortunately it is hard to test such precise devices and a new calibration would cost twice as much as the device itself.
So I needed a cheap high precision voltage source to test the meter.
I found one from Geller Labs, which is based on the AD587LQ (cerdip!) or AD587LN from Analog Devices. Since they also sell bare pcbs I populated mine using a way cheaper (but also more temp. drifting) version the AD587KNZ.
To get reproduceable results it is advisable to let the meter and voltage source run for at least half an hour.
Btw. I ignore the least significant digit on a display or when important I round it.
If you round the least significant digit all displays would have produced the perfect result of 10V !
That is why I can live with a more drifting version of the Analog chip and drifting it does unfortunately.
I would like to test a calibrated version with the cerdip chip, but only for curiosity.
Bbtw. 8.5 digit meters are becoming more affordable,....
|Svr board with Agilent U1272A multimeter|
|Agilent 34401 6,5 digit multimeter|
|HP 3478A 5,5 digit multimeter|
Geller Labs Svr transfer reference