DMM that measure temperature

[Anonymous]

Posted by Muzzer on 22/07/2016 12:59:13:

Yes, that's a bit more realistic. Apart from the resolution, you have to allow for the tolerances, ageing and temperature coefficients of the reference, ADC, various potchains etc – and the actual calibration itself. And that's before you think about noise.

And also before checking integral and differential non-linearity and ENOB.

As far as I'm aware most DVMs use a form of dual slope converter, so the concept of number of bits doesn't come into it. Instead the overall accuracy is controlled by the accuracy of the comparison voltage reference and the stability and resolution of the clock.

Andrew

[Neil WyattModerator @neilwyatt]

Posted by Andrew Johnston on 22/07/2016 14:59:43:

Posted by Muzzer on 22/07/2016 12:59:13:

Yes, that's a bit more realistic. Apart from the resolution, you have to allow for the tolerances, ageing and temperature coefficients of the reference, ADC, various potchains etc – and the actual calibration itself. And that's before you think about noise.

And also before checking integral and differential non-linearity and ENOB.

As far as I'm aware most DVMs use a form of dual slope converter, so the concept of number of bits doesn't come into it. Instead the overall accuracy is controlled by the accuracy of the comparison voltage reference and the stability and resolution of the clock.

Andrew

Probably true, it's cheap, doesn't need complex logic and has excellent linearity.

[SillyOldDufferModerator @sillyoldduffer]

Posted by David lawrence 3 on 22/07/2016 14:38:40:

… as we thought the Mk 8 avo was a bit old. …

 

I had a good look at my Avo8 and am going to make the dangerous claim that I have the oldest. Can anyone beat 1955?

Although the meter has Mk II red and black terminals, it's an original Avo8 (no 500VDC range) and the serial number suggests it was made in February 1955. In addition to the usual markings it has an Admiralty Pattern Number (10S/16411) and Government Broad Arrow.

Still in GWO. Antiques Roadshow here I come!

Dave

 

 

 

Edited By SillyOldDuffer on 22/07/2016 16:07:50

[AjohnwParticipant @ajohnw51620]

Referemces are not anywhere near as much of a problem as they used to be. These things are used all over the place these days with varying drift problems – not as easy to use as the wiki sort of implies

**LINK**

Dual slope relies on short term stability of a capacitor and the accuracy of the clock rate which can easily be very high. I'm not sure that they are generally used in dvm's now. I believe this article is correct but there may well be other arrangements

**LINK**

The accuracy problems are largely down to the resistive ladder used in the DAC plus add ons for various ranges.

Dual slope had the advantage that it could be arranged to filter ripple. It measures voltage via charge. As most things use switching supplies that's not such an advantage as it was and could be a disadvantage anyway.

Ageing – there has been a lot of electronics around in a number of areas for a long long time. Often when things like TV's fail it's because something has been stressed too far to save cost. Some companies have spent a lot of money trying to make chips and stuff like that fail due to age.

John

–

[john fletcher 1Participant @johnfletcher1]

To SillyoldDuffer. If you venture onto Vintage Radio forum you will find out your AVO is a relatively new one, plenty of pre 1939 one available and plenty made during WW 11. They were made for all the armed forces. My AVO test Bridge was made in February 1944 and still going strong. To identify the date of manufacture look on the top right of the scale and you will see the code, in my case 2-44.John

[SillyOldDufferModerator @sillyoldduffer]

Thanks John. Another family heirloom turns out to be worth about twenty quid…

[Gordon WParticipant @gordonw]

Went to a car boot last week, first for a couple of years. One chap had an old Avo in a wood box with some other bits, he claimed it was a special for testing batteries on submarines WW11. He wouldn't let me look closelyat it and wanted £175, so I passed. The meter itself was sort of pear shaped in plan and marked Avo.

[Stuart BridgerParticipant @stuartbridger82290]

My first proper job, post apprenticeship was in the "Electronics Maintenance" shop at Brooklands (BAe Weybridge) One of our tasks was repairing Avo's, there were many 100's in use across the site. Any that failed calibration or got damaged came to us. The worst job was setting up the overload cut-out. This was a fiendish mechanical system, that relied on the acceleration of the moving coil needle to trip the spring loaded cut-out button. They had to be carefully adjusted to ensure that they didn't nuisance trip, or not trip at all, damaging the meter. I still have a Pan Climatic Model 8X, which was the spec supplied to the the forces, but haven't used it in over 20 years. My Fluke 77 DMM gets used for everything I need, but I don't do stuff that needs to be able to see a trend, for which you cannot beat an analogue meter. The Avo 7's and early MK8's were very well made, solid as a rock, unless badly abused. The newer 8 Mk V (if my memory is correct) was horrible, internally it used a new technology of a flexible PCB that warapped round the inside, instead of the point to point wring of the earlier models. Unreliable and very dififcult to work on.

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