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 Hot bench CFM rise 
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Joined: Wed Feb 16, 2011 5:12 am
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Location: oxford ,england .
big blower ftw :D

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Mon Sep 01, 2014 9:17 am
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Joined: Wed Jun 12, 2013 2:19 am
Posts: 42
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


Thu Sep 04, 2014 3:30 am
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Joined: Wed Mar 03, 2010 3:40 pm
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Location: Melbourne, Australia
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.

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Thu Sep 04, 2014 4:12 am
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Joined: Sat Feb 27, 2010 4:56 pm
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Hot air travels faster than cold air, discuss.....

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Sat Sep 06, 2014 7:05 am
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Joined: Fri Feb 19, 2010 7:36 pm
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Location: Grantsville, Utah 45 min west of Salt Lake City
SSR wrote:
Hot air travels faster than cold air, discuss.....


Also altitude makes a difference


Sat Sep 06, 2014 12:37 pm
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Joined: Sat Feb 27, 2010 4:56 pm
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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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Sat Sep 06, 2014 1:16 pm
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Joined: Wed Jun 12, 2013 2:19 am
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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


Thu Sep 18, 2014 6:21 am
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Joined: Fri Jan 08, 2010 3:35 pm
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Location: Pennsylvania
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

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Thu Sep 18, 2014 6:30 am
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Joined: Wed Jun 12, 2013 2:19 am
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Thanks Bruce,

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

Juhani


Thu Sep 18, 2014 11:01 am
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Joined: Sun Feb 28, 2010 10:00 am
Posts: 62
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.


Mon Dec 21, 2015 6:45 am
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