[time-nuts] Replacement fan in SR620

Magnus Danielson magnus at rubidium.dyndns.org
Sun Feb 2 19:31:01 UTC 2014


On 02/02/14 20:16, Charles Steinmetz wrote:
> Magnus wrote:
>
>>> My point was, the thermistor is never "happy."  It always wants more
>>> cooling.  So it spins the fan up to full speed and is still too hot to
>>> reach equlibrium.
>>
>> Which only means that the thermistor setup is shifted and needs to be
>> adjusted to achieve the goal.
>>      *   *   *
>>> So, the question is, do we just replace fans when
>>> they go bad and live with the poor thermal design, or do we try to
>>> improve the thermal design?  If we want to improve the thermal design,
>>> the methods available to us are:
>>
>> I think a sub-set of these are needed, but you also need to include
>> (v) change the balance-point for the thermistor stabilization.
>
> Well, sure, you can raise the setpoint until the fan slows down and
> regulates the temperature, but then the regulated temperature would be
> too high.  In other words, if the fan cannot keep things cool enough by
> running at full speed, raising the set-point so the fan runs slower is
> not the right answer even if it does result in some approximation to
> isothermy.  [Note that adjusting the setpoint *down* will have no effect
> -- the fan already cannot cool the instrument enough to reach equilibrium.]

I'm not discussing it as a single measure, but rather a thing you can 
look at only after you got air-flow working much better.

>> I think the single biggest problem is too little air inlet, which
>> forces the fan to run in stupidly high speed without actually do much,
>> as the air input has too low cross-section and with several small
>> holes you need to create a large pressure difference in order to
>> achieve the air flow wanted. Also, those holes help to create noise as
>> they are not shaped to avoid turbulence. Best way is just achieve
>> large enough cross-section.
>
> I have experimented with cardboard top covers that have various holes in
> them.  My conclusion is that even with more air inlet area, you still
> want more cfm of air flow (i.e., a stronger fan).  You also need to
> adjust the airflow to the various sections of the interior to keep
> everything sufficiently cool.  As it is, there would be way too much
> temperature variation from one part of the interior to another even if
> you did reach a regulated equilibrium for the thermistor location
> (wherever you put the thermistor).

I agree. You need both, but just tossing in a stronger fan isn't going 
to cut it either.

Cheers,
Magnus



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