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Old 1st Feb 2016, 7:02 pm   #17
G0HZU_JMR
Dekatron
 
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
Default Re: A precision voltage calibrator

Quote:
I've worked in many RF labs. myself during my pre-retirement years - as have you. (From your location, it seem likely that you and I have worked in the same establishment). So you and I both know that it is usual to not only find the very best test gear in such places but that such test gear is always religiously re-calibrated on regular intervals and to very high standards. And that costs lots of money. In this thread, we are talking about a typical home-based D-I-Y enthusiast's workshop - and with a very limited budget: expensive external re-calibration services are out of the question for many of us.
I think you are missing the point I was trying to make. At work we have a sub class of test equipment that is marked up as 'for indication only' and this includes various scopes, sig gens, spectrum analysers, counters and DMMs.

This test gear is 'too good to throw away' but also not considered worthy of a yearly calibration. In my experience this test gear maintains a reasonable accuracy for many years.

Here at home I have several old DMMs that were all bought very cheaply a very long time ago and they all still read 10.00V on the same DC test source. I don't think I've adjusted any of them in well over 15 years. In the case of my old Maplin Gold DMM it may be much, much longer than 15 years.

I'd also argue that if someone does a 'lot' of modern electronics work they will often be measuring 12V and 5V regulators and if the DVM was out by 0.2V at 10V then I think it would raise suspicion if the spread in readings over time across various 12V regs typically centred on 12.3V rather than 12V.

Quote:
Dekatron: I've never had any problems even with some of my 20 year old DMMs on the DC ranges
That's my experience too. Even a cheapy DMM I bought at a rally as a factory second for about £1 still reads 10.00V after over 15 years.

If I had a DMM that had drifted by 0.25V at 10V then I'd almost certainly bin it rather than attempt to fix it by merely readjusting the internal reference.

I tried measuring 10V with my two old DSO scopes here and they both managed to read it within 0.02V when I selected mean voltage on a suitably sensitive range. One scope is 20 years old and is an ex works scope that hasn't been used (let alone calibrated) for many years and the other is my Tek TDS2012. I've had this scope for over 10 years and it has never been away for any kind of adjustment or calibration. It only has a 3 digit display for voltage so I used a 9.8V test signal for this scope and it measured 9.81V. The old HP scope measured 9.98V for a 10V test source.

By varying the Y control I could add another +/- 0.03V on the scope reading but that still isn't bad for a scope.
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Regards, Jeremy G0HZU

Last edited by G0HZU_JMR; 1st Feb 2016 at 7:28 pm.
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