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DMM fans - noisy friends?
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LaurentR:

--- Quote from: Rydda on February 07, 2022, 02:29:17 am ---To answer the OPs original question: I really, really hate instruments with noisy fans. Especially instruments that according to the dissipated power should be able to manage without a fan.  I know that I (with ambient noise level next to none in my lab space) am probably part of a small minority of customers. It seems that manufacturers ( and reviewers on YT ) all base their noise level judgements on industrial environments with a large amount of ambient noise.

--- End quote ---

Same here. My bench is in a very quiet location and I hate noisy equipment.

I haven't dared fan-swapping the 3446x but maybe I should. The sound level is not super high but the whine is really annoying.

I did fan-swap the 33522B function gen (less worried about losing accuracy). That one is louder than the 3446X and the fan is hard to access.

I also got a Instek PEL-3031E electronic load last year. It has a decently large fan which is thermally control, but even at "idle", it sounds like a jet engine. Fortunately, they use a common off-the-shelf fan and I managed to find a quieter model in the same series. It's also very easy to replace.

I had fan-swapped all my Rigol gear when I still had it.

The only recent piece of TE I have that's appropriately quiet is the E36313A supply. It is basically silent at idle and not too objectionable at full power. They still managed to screw up the "off" mode where the supply periodically wakes up and the fan spins for a few seconds...Not a real annoyance but you don't expect anything to wake up and turn the fan on when it's off.

The most annoying part is that there is rarely an indication of noise levels in the datasheets and technical support usually has no more information.
guenthert:
     Fans might be necessary for ATE which might end up in racks exposed to uncomfortable temperatures.  For bench instruments they are silly.  I don't buy the 'reaching thermal equilibrium quicker' argument.  For that to happen the fan needs to be temperature controlled (with a well adjusted PID controller to avoid oscillations), which sadly (and incomprehensibly) they typically are not.

     There doesn't seem to be much rhyme and rhythm to which DMMs have a fan either.  First generation of HP3456A had them, later revisions not (I'm not aware of other major changes between the revisions).  The popular HP34401A doesn't have one, yet the successors (including 34410A) do.  Top end HP3458A has one,  yet the top end models of Datron, Fluke achieve comparable precision (not speed though) without.  And even the 8.5 digit Datron 1271, markedly intended for industrial ATE, doesn't have one.

 :-//
bdunham7:

--- Quote from: guenthert on March 28, 2022, 10:54:00 pm ---For bench instruments they are silly.  I don't buy the 'reaching thermal equilibrium quicker' argument.  For that to happen the fan needs to be temperature controlled (with a well adjusted PID controller to avoid oscillations), which sadly (and incomprehensibly) they typically are not.

--- End quote ---

The issue is internal temperature gradients, not the overall internal temperature.  And for that, a fixed-speed fan does achieve equilibrium much faster.  This mainly allows the manufacturer to get away with some less expensive parts.  The Keithley engineer that used to be here in the DMM6500 thread explained that.  If not for fans, any DMM in the sub-50ppm arena needs some very carefully selected and trimmed resistor networks and that gets expensive.  Otherwise you just have to tolerate a 3-hour or so warmup for best accuracy.
Kleinstein:
The time to reach thermal equilibrium depends on the heat capacity and thermal resistance to the environment. The fan lowers that heat resistance and thus speeds up the time to reach equilibrium. In addition the overall temperature rise is usually somewhat smaller and thus a smaller step to start with.

Another point is that without a fan the air exchange in / out of the case can be fluctuating, e.g. driven by drafts in the room or pressure from someone opening or closing doors or wind gusts. This gives extra temperature fluktutions a fan can reduce.

A temperature regulated fan has the problem that the speed of the fan also effects the temerature gradients. So ideally one would need calibration depending on the fan speed. To speed up the warm up, the option would be to start the fan only with a delay, like after some 10 minutes. In principle simple, but could be confusing for the user.

The need for a fan also depends on the power level in the instrument: ideally a DMM would be relatively low power (e.g. 5 W)  and could get away without a fan. A somewhat longer warm up is usually not that important, as one rarely needs the best accuracy. Than a longer power on may be needed anyway, not just for the temperature, but also for other effects (stress from thermal mismatch and humidity effects) a fan can not speed up much.
guenthert:
      Well, yes, for voltage dividers temperature gradient is relevant rather than the temperature per se, but that's not the case for other components, e.g. shunt resistors.

      I also don't think air speed itself needs to be taken into account when calibrating.  Presumably the fan controller would adjust air speed so that a target temperature within the chassis is maintained, regardless of heat generation and ambient temperature, i.e. for warmer ambient air, more air will be pulled which should then also equalize the heat being carried away from components (same for more humid air).

     Clearly a (controlled) fan would not completely eliminate warm-up time.  The fan ought to start immediately at high speed in order to i) overcome friction and ii) assure that a mixed air reaches the sensor(s) and (perhaps quickly) slow to the appropriate speed, but never stop (so that the sensors aren't isolated).

     The chassis-internal target temperature of a controlled-fan equipped device (necessarily) would be higher than any expected ambient temperature, which might accelerate drift or aging of components and hence be considered undesirable.  Further the control-loop will cause some fluctuation in the chassis-internal temperature depending on load (e.g. due to high-speed measurements or high-current across a shunt resistor) which might be worse then using a high, steady air flow when ambient temperature is stable and within specified range.  I wonder if it has been investigated whether that's the case or the engineers just chose the simple, cheap solution ...
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