[time-nuts] Replacement fan in SR620

[Tom Knox]

A little off topic but It seems many instruments (the SR620 and 53132A included) would work best with an internal fan. (A closed system, not exchanging outside air). Possibly with some sort of internal/external heat sink if needed. Or in high power situations outside air would flow through  a hollow heatsink (again not exchanging outside and internal air). If nothing else this would keep dust out. Since fan filters can really restrict air-flow and without a filter dust on circuit boards can act as on thermal insulator leading to overheating while conducting static.
Thomas Knox



> Date: Sun, 2 Feb 2014 19:17:03 +0100
> From: magnus at rubidium.dyndns.org
> To: time-nuts at febo.com
> Subject: Re: [time-nuts] Replacement fan in SR620
> 
> 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 at 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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