>a while back somebody (I believe it was John) suggested a small electrical
>driven compressor as an alternative to a vacuum pump or venturi. He
>especially recommended the UNMP50 from KNF. Their brushless type (for
>permanent operation) costs $ 213.00. The pump will make 2.7 l per min at 4
>Hg. I'm ready to order one from KNF, but I'm somewhat reluctant. Has anybody
>tried such a pump yet?
Is this for normal operations of vacuum instruments? I used
to work for a company that sold STANDBY electrically driven
vacuum pumps . . . they take a LOT of snort . . . like
125 to 300 watts. Got that much extra available full time
from your alternator? . . .
Found this on the internet:
(3) Vacuum loads may be calculated as follows:
(a) Gyroscopic instruments require optimum value of airflow to
produce their rated rotor speed. For instance, a bank and pitch
indicator requires approximately 2.30 cubic feet per minute for its
operation
. . . 2.3 cu feet per minute is about 65 liters per minute
and a resistance or pressure drop of 4.00 inches Hg. Therefore, operating
an instrument requiring 4.00 inches Hg from oneventuri would be marginal.
Similarly, the directional gyro indicator consumes approximately 1.30 cubic
feet per minute and a pressure drop of 4.00 in Hg. . . . It should be noted
that the negative pressure air source must not only deliver the optimum value
of vacuum to the instruments, but must also have sufficient volume capacity
to accommodate the total flow requirements of the various instruments which
it serves.
. . . adding 1.3 cu-ft/min (37 more liters/min) brings the total
up to more than 3.6 cu/ft or 100 liters per minute at 4 in-hg or
better.
Looking at KNF's website, the only pumps capable of this
performance have to plug in the wall. It taks a LOT of
suck to run a vacuum instrument. This may be why those little
engine driven pumps in airplanes are so hard pressed to run
for very long. . . .
Bob . . .
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( Knowing about a thing is different than )
( understanding it. One can know a lot )
( and still understand nothing. )
( C.F. Kettering )
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http://www.aeroelectric.com
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