Hot bench CFM rise

Orifice Style bench discussions
ivanhoew
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Re: Hot bench CFM rise

Post by ivanhoew »

big blower ftw :D
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tt911er
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Re: Hot bench CFM rise

Post by tt911er »

Test pressure is automatickly held. But after long highflow session ( pitot tubing 3x9 point), + hot wheather conditions I noticed 5-7 % cfm rise. I couldn't duplicate the issue clearly enough but I think it increased the CFM figures on both intake and exhaust mode. Could that be that air temp was that much higher that temp correction would be needed?

Juhani
Tony
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Re: Hot bench CFM rise

Post by Tony »

If its a PTS orifice syle bench, no temperature correction will be needed.

The reason being, the exact same air flows through the measurement orifice and what you are testing.
So temperature and humidity effects will be the same on both measurement orifice and the item under test.
What you are really doing is comparing the ratio between the two pressure drops, and relating that to the original orifice plate calibration figure.

That is what makes the orifice bench so repeatable, you don't need to add any corrections to the measurement.
You can test the same port, day, night, summer or winter, and it will always read the same when tested against the same measurement orifice.
Also known as the infamous "Warpspeed" on some other Forums.
SSR
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Re: Hot bench CFM rise

Post by SSR »

Hot air travels faster than cold air, discuss.....
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jfholm
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Re: Hot bench CFM rise

Post by jfholm »

SSR wrote:Hot air travels faster than cold air, discuss.....
Also altitude makes a difference
SSR
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Re: Hot bench CFM rise

Post by SSR »

jfholm wrote:
SSR wrote:Hot air travels faster than cold air, discuss.....
Also altitude makes a difference
Yes but I doubt he's taking his 'bench down to the cellar to gain CFM. ;)

I was deliberately not saying too much, because whilst I know hot air is faster I'm unsure if it's relevant here. Worth considering or is it more probable that there has to be much more of a difference in temp to see a CFM change?
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tt911er
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Re: Hot bench CFM rise

Post by tt911er »

Thanks for your replies!

What else it could be if we leave leake (most propably reason) out of the question?
What can go wrong with testpressure autohold ?

Juhani
Brucepts
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Re: Hot bench CFM rise

Post by Brucepts »

Is the test pressure moving after a long run or just the CFM?

I would look for a leak around the internal orifice plate of orifice plate board inside
Bruce

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tt911er
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Re: Hot bench CFM rise

Post by tt911er »

Thanks Bruce,

Just the CFM. Test pressure stays nicely in 28.
Orifice sealing is one thing that I have to check.
Thanks.

Juhani
SWR
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Re: Hot bench CFM rise

Post by SWR »

If it is a big orifice and you are testing at 30ºC instead of say 18ºC, the change of air viscosity is enough to account for that.
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