[time-nuts] Replacement fan in SR620

[Charles Steinmetz]

Magnus wrote:

>If the heat sources where well coupled to the air-flow, which they 
>are not, and the flow-path as low air-flow resistance, which it also 
>doesn't have, requires the fan to work at high rate to get any air 
>move, and to get the thermistor happy.

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.

>You can mount a fan to create a steady flow on the right side, and 
>that way cool off much of the heat there.

You mean an internal fan, I take it.  That still doesn't solve the 
problem of too little air moving through the box to lower the 
internal temperature to the target value in normal room ambient temperatures.

>Well, we *do* care about a stable internal temperature, since it 
>will also shift calibration factors, so the stabler the internal 
>temperature is and hence various shifts which is being compensate, 
>the more accurate it becomes.

Right.  But unless the thermal design has been optimized (and it has 
not, on the SR620), you have to choose which part of the interior you 
want to be regulated to a constant temperature.  The very worst 
possible choice is to put the thermistor in the exhaust stream of the 
fan (where SRS put it).  It needs to be somewhere inside the box, and 
where you put it determines what part of the interior is 
regulated.  Of course, all of this assumes that you use a fan that 
moves enough air to actually reach equilibrium before it gets to full 
speed.  The stock fan doesn't, so NO place inside the box is 
regulated to a constant temperature, except in very cold ambient temperatures.

>>Perhaps SRS did not intend to regulate the interior temperature of the
>>SR620 -- maybe they just wanted it to warm up faster (if you did away
>>with the thermistor and had the fan run full speed whenever the counter
>>was on, it would presumably take longer to warm up).
>
>Maybe, would make kind of sense, on the other hand, they could have 
>achieve both quick heat-up and stable but lower temperature and 
>quieter if they wanted.

Yes, they could have.  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) use a fan that moves 
more air, so the location of the thermistor is actually regulated to 
a constant temperature in normal ambient temperatures (not just in 
very cold ambient temperatures); (ii) relocate the thermistor to the 
most temperature-critical area inside the instrument; (iii) make 
additional air inlet holes, strategically placed to evenly cool the 
various "zones" of the interior; and (iv) add air baffles inside the 
box to evenly cool the various zones of the interior.

>It would indeed be interesting. The Papst 624 seems quite capable 
>little critter and there seem to be some magic to the 624 number 
>matching, which doesn't seem to be accidental.

No magic.  "624" just means that it is a 60x60mm fan that runs on 
24v.  Most fan manufacturers make a dozen or more fans with all 
different current draws, air movement, noise, number of fan blades, 
bearings, etc., etc., all with "624" in the part number (or whatever 
code that manufacturer uses for "60x60 at 24v" -- 0624, 2406, 
etc.).  Papst makes 624HH, 624N, 624/2H3P, 624H, 624M, 624/2HH, 624L, 
624J, 624F, etc.

Best regards,

Charles