[time-nuts] Replacement fan in SR620

[Charles Steinmetz]

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 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).

Best regards,

Charles