Hot bench CFM rise

ivanhoew · Post by **ivanhoew** » Mon Sep 01, 2014 9:17 am

big blower ftw

tt911er · Post by **tt911er** » Thu Sep 04, 2014 3:30 am

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 · Post by **Tony** » Thu Sep 04, 2014 4:12 am

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.

SSR · Post by **SSR** » Sat Sep 06, 2014 7:05 am

Hot air travels faster than cold air, discuss.....

jfholm · Post by **jfholm** » Sat Sep 06, 2014 12:37 pm

SSR wrote:Hot air travels faster than cold air, discuss.....

Also altitude makes a difference

SSR · Post by **SSR** » Sat Sep 06, 2014 1:16 pm

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?

tt911er · Post by **tt911er** » Thu Sep 18, 2014 6:21 am

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

Post by **Brucepts** » Thu Sep 18, 2014 6:30 am

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

tt911er · Post by **tt911er** » Thu Sep 18, 2014 11:01 am

Thanks Bruce,

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

Juhani

SWR · Post by **SWR** » Mon Dec 21, 2015 6:45 am

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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