Will Matney wrote:
>
>The pressure on the outside is the low pressure and the pressure in the
>chassis is the high. With the blower off, that's the low pressure or
>room pressure where the water is settled. The difference between low
>and high shows the back pressure. The low side though, I don't
>understand what your saying? I always checked them the same as the set
>ups shown in the handbooks showing the same setup using the tubing. Are
>you counting the pressure above the chassis as the low pressure? It
>should be the same as the outside pressure unless the cabinet is not
>ventilated well enough. On most Alpha photos I've seen the tubes
>chimney runs right to the top and out the vents. You could check it
>that way but in reality it should be the same, especially if it set up
>that way. I doubt that perf metal will hold back the outgoing air flow
>very much. Now if above the chassis wasn't ventillated very good to the
>outside where it caused a restriction to flow, then yes, the above
>chassis would be used I
> would think.
>
Pressure can be tricky to measure at low levels. It is like trying to
measure voltage with an insensitive meter, and no clear idea where you
should apply the two probes. You can measure "something"... but you
don't really know what it means.
As Will points out, the pressure that you measure depends on far too
many different variables: the shape of the enclosure, exactly where you
locate the probe, how the end of the tube is oriented relative to the
air flow, and what you count as "the low side".
But it doesn't matter anyway. What actually provides the cooling is the
flow rate of air. A pressure measurement is only an indirect way of
measuring the volume flow rate through a particular tube and socket - so
why not go straight to what you really need to know?
A volume flow measurement is SO EASY!
Ten years ago, K6GT described a very simple way of measuring volume flow
rate, by timing how long it takes to inflate a thin plastic garbage bag.
It is WAY easier and more meaningful than trying to measure pressure.
Original reference: AB6YL (now K6GT), QST 5/96.
A quick search of the AMPS archives for "plastic bag" yields:
/archives//html/Amps/1998-03/msg00171.html
/archives//html/Amps/1998-03/msg00172.html
/archives//html/Amps/1998-03/msg00430.html
/archives//html/Amps/2001-10/msg00117.html
To see these messages, click the link to the AMPS home page (at the
bottom of every posted message) and then click "Searchable archives".
>What most have been doing with the high speed blowers is cutting the
>speed back with a rheostat. Then if the temperature gets so hot, a
>thermostat shorts out the rheostat and full voltage is applied to the
>motor. In his case, my guess he has a fan already slow enough, and I'm
>afraid may be too slow.
>
Cutting back the rpm from 3000 to 1000 would be pretty disastrous. Both
the pressure and the volume flow rate would reduce by a large factor.
Are you sure they have sent you the correct motor?
--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek
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