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DMM fans - noisy friends?
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Amaruk:
Are you fans of DMMs with fans? Ideally I prefer my instruments to be quiet but I fully understand that fans are often required in order to manage a good thermal balance in a high-precision instrument.

I have two bench DMMs, a Siglent SDM3045X which does not have an internal fan, and a Keysight 34461A which indeed does have an internal fan. The latter is a 6.5 digit DMM where as the former is only a 4.5 digit DMM. Today I was curious to find out how much these two DMMs drift over time after they are powered up. I know that you are supposed to turn them on at least one hour or so before you use them for maximum accuracy. But how important (or not) is it to do that? Since I am developing my own custom instrument control software that has data logging capabilities I decided to run a little experiment with two the DMMs and a 5V voltage reference. The results were very interesting…

The experiment: I powered up the DMMs for 5 minutes before starting the drift test. This was to allow the DMMs time to fully connect to the network as all data acquisition is done over LAN. Both DMMs were then connected in parallel to the 5V voltage reference (AD584) and the test started. During the test my software measured the voltages from the DMMs (5 sample averages) every 30 seconds for about two hours. I then computed the voltage drift over time in PPM (parts-per-million) based on the first reported value from each DMM.

The results are shown in the images below. It should be noted that during the test the ambient temperature in the lab was held constant within 1F (0.5C).

While I did expect to see drift in the results, I did not realize that the difference between the two DMMs was going to be this big. Very interesting.


TiN:
What is has to do with fans? :)
Amaruk:
That is a very good point. I just assumed that the fan played a role in this - together with the voltage reference used. But I don't know what the cause of the drift is. As for voltage references, the one I use in my test (AD584) is not very expensive and from this test it looks very stable right from the start. So, what do you guys think is the reason some DMMs drift less than others?
TiN:
It is a complex question and all parts, like physical, circuit and component design are important contributors into DMM stability, warmup time and other performance parameters. Some circuits working better and more stable then others. Many instruments with fans are more stable than ones without fans, and the opposite. You are comparing two instruments of very different goals and performance specifications. Higher sensitivity instruments, say 8.5d meters would often need weeks and months of time to get stable to their best design capabilities. This is why you often see requirement never to turn off equipment in calibration labs, even when not in use at the moment. Same reason why you have high-end frequency standards always on  :)
bdunham7:

--- Quote from: Amaruk on February 06, 2022, 01:16:23 am ---Are you fans of DMMs with fans? Ideally I prefer my instruments to be quiet but I fully understand that fans are often required in order to manage a good thermal balance in a high-precision instrument.

--- End quote ---

Fans may cause a DMM to reach thermal equilibrium sooner and also reduce internal temperature gradients.  This may improve performance or they may be used to allow for less-expensive components to suffice for a given spec.  This doesn't really have anything to do with the differences between your two meters because they are in entirely different performance categories.

I somehow don't have any DMMs with fans.  Of the two that I have that are comparable with your 34461A, one (8846A) has less warmup drift and one (34401A) has more.  The 34401A has a long warmup time, it doesn't totally stabilize until 3 hours or so--and it looks like your 34461A isn't much quicker, unless there is some other cause for the small drift you have observed.  You'd have to leave them for a longer time--like 24 hours--to see how much they wander around even after fully warmed up.
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