metalresearcher
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Bike pump valves suitable for manual vacuum pump ?
I bought a hand-operated vacuum pump with a transparent platic bell on a flea market for $5.
It contained a bicycle tube valve but I could evacuate it to less than 70mbar absolute (1 psi for the Americans).
Recenty I found an old bicycle pump of which I reversed the piston (to enable that the action stroke is sucking rather than pumping up) and attached a
plastic hose. I got old bike tubes from the local bike repair shop in which I could fit new valves from the bike shop (photo 1). I got a $1 brass hse
barb which I lathed off on the other side on which I soldered the brass valve tible which I sawrd off the discarded bike tubes. The resulgt in photo
2. Then Iattached a hose to it and the valve pointing towards the bike pump and on the other end of the gauge I attached a hose which was
attached to a stoppered test tube (photo 3, test tube not shown).
Then I evacuated the test tube but could get no lower pressure than 0.3 bar (ambient pressure -0.7 bar). What am I doing wrong ?
Or are these kinds of valves untuitable for vacuum applications ?
UPDATE: Here a test with evacuating a 250ml Florence flask. Again not under 0.25..0.3 bar :-(
[Edited on 2011-4-15 by metalresearcher]
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Hades_Foundation
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I think you've got two problems, at least if I understand the setup correctly.
You have dead space (don't know if that's the right term) between the pump and the valve . If that space (in the plastic tube) is for example 20 cc,
and the displacement volume of your pump is 250 cc, then the lowest pressure you can obtain would be 20 / 270 = 0.074 bar.
But I think the bigger problem is the valve from the bike tube.
http://en.wikipedia.org/wiki/Schrader_valve
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The valve core is a poppet valve assisted by a spring. |
I assume that's the case with your valve. The pressure in your test tube or whatever you got on the other side has to overcome the pressure of that
spring before the valve will open. If that pressure is 0.3 bar, that's the lowest value you'll be able to get.
You need a valve without a spring:
http://en.wikipedia.org/wiki/Presta_valve
Yours is probably http://en.wikipedia.org/wiki/Poppet_valve
maybe it's possible to remove the spring? Not sure if it would still seal completely though..
hope this helps.
[Edited on 18-4-2011 by Hades_Foundation]
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metalresearcher
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Here I did a new test without a bike valve so I have to close the valve manually at every strike. But I got 0.2 bar this time and sometimes I got 0.1
bar
<iframe sandbox width="420" height="315" src="http://www.youtube.com/embed/KhNs30-y8Fc" frameborder="0" allowfullscreen></iframe>
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neptunium
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how low are you trying to go ?
volumetric pumps have limitations their are good as primary pump, that fight against the atmospher. if you could hook up 2 pumps like this you could
reach a little lower but sooner or later you will have to change the method of vacuum .
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metalresearcher
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I try to go under 0.1 bar or even 0.05 bar.
I managed once 0.08 bar (water boiled @ 40C) but could not reproduce it again with same setup even when shortening the hose between pump barb and
valve to < 10cm (now it is 80cm).
@Np: You mean two pumps in series ?
[Edited on 2012-1-8 by metalresearcher]
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neptunium
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yes in series will get a better vacuum . you should be able to get to about 10 torr(sorry old school...0.02bar) with 2 pumps in series...i got to 0.08
with a fridge compressor before no problems
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Texium
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Thread Moved
20-11-2023 at 12:03 |
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