On 02/02/14 18:47, Charles Steinmetz wrote:
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.
Which only means that the thermistor setup is shifted and needs to be
adjusted to achieve the goal.
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.
No, I mean an external fan.
The box have far to little air intake, as you have already pointed out,
and I agree fully.
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.
I agree. There is many things which needs to be fixed to get there,
there is no single silver-bullet to solve it all.
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.
I think a sub-set of these are needed, but you also need to include (v)
change the balance-point for the thermistor stabilization.
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.
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@24v" -- 0624, 2406, etc.). Papst makes
624HH, 624N, 624/2H3P, 624H, 624M, 624/2HH, 624L, 624J, 624F, etc.
Ah, makes sense. It was a long time I looked at fan details myself.
Cheers,
Magnus
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