Air Pressure Test

[leitmotif]

Hello all;
I am trying to design a pressure tester with RSLogix 500 to detect a air pressure leak in metal tubing. The leak rate cannot be more than 100th of a psi per 4 seconds at a pressured chamber of 60, 40 and 100psi.
Willie

0.01 psi per 40 = 2.5 e-3 or .025%

.01 psi per 100 = 1 e-4 or .001%

Sure sounds like a tight (too tight ??) tolerance to me. I recognize they want to do the test quickly especially if testing each and every unit which is why the 4 second criteria.

What are you building these for
-- DOD nuclear weapons
-- biohazard labs?

Instead of measuring pressure drop can you change the spec to flow ie a leakrate of .1ml per minute or second etc at X,Y and Z pressure? This way you can use a flow meter both visual and electronic if you wish.

Dan Bentler

 

[rdrast]

I'm with Alaric on this one. Pressure drop is not an accurate way to measure leaks, but a helium leak detector will find any leak that exists, depending on your sensitivity requirements.

Another good point about a helium check system, is that with a manual probe, you can identify the exact location of the leak.

 

[MartB]

We recently built a leak tester with very tight tolerances using a mass flowmeter from Omega (http://www.omega.com/toc_asp/subsectionSC.asp?subsection=D05&book=Green)

I don't have the exact spec for the machine to hand right now, but I know the customer was very pleased with it and the last time I spoke with them, they said they were very impressed with the reliability and consistency.

Might be worth you taking a look anyway.

 

[BudW]

Leak Testing

My $0.02 worth.

Are rejected parts repaired or scraped? Do you need to find the leak area? Is the reject limit a customer specification or government mandate?

FYI: A pressure drop of 0.010 PSI over 4.0 seconds is about 1.0 cc/m (Cubic Centimeters per Minute) when testing a 100.0 cubic centimeter vessel.

Like many other posts here one way to test is Helium spectrometer. Helium spectrometer can find very small leak rates and with the use of a manual probe you may also find the leak point. Helium spectrometer does have the drawback of cost of equipment, you need to supply Helium and may want to invest in a Helium recovery system.

The lower limit for most Pressure Decay, Pressure Differential, Laminar Flow and Mass Flow testing is usually 1.0 cc/m. Your test specifications are close to this limit, but very much doable using pressure testing. Using your test specifications you will not be able to find the leak area by feeling with your hand but you could use a soapy water solution (Snoop is a brand name liquid leak detector) to find leaks in common leak areas. Caution, using lots of liquid leak detector will gum up your test fixture and promote rust of ferrous parts. You could also dunk you fixture under water only upon a reject to find the leak area.

http://www.swagelock.com/search/find_products_results.aspx

We used a Emerson Rosemount 3051C delta pressure transducer (0 to 0.1 in H2O) in leak test systems for years. We used this transducer in both pressure decay and Laminar flow style testing. This transducer also has built in temperature compensation (will not help during large temp swings like large by doors opened on a cool winter day and such). This transducer also can handle large pressure differential. Some delta P transducers fail if only slightly over pressurized by a few in H2O.

http://www.emersonprocess.com/home/

No matter what test method you use you may want to add a pressure switch or an inexpensive pressure transducer to the side of tube opposite of your fill point. Ensure that there is no occlusion (blockage) in the tube preventing gas flow through the tube.

FYI: I worked on a feasibility study in the late 1980’s. The study was to find the size of a hole that would allow gasoline to leak. The study used Unleaded Gasoline, filtered down to 2 micron, Viscosity = .426cP @ 80 deg F and Density = Specific gravity of 0.734. We pressurized the gasoline against an orifice to 100 psig, kept the temperature at 80 deg F. and the opposite side of the orifice was left open to atmosphere. We use a microscope to find an orifice that would allow the gasoline to just slightly wet the atmosphere side of the orifice. We found that a 7.5micron orifice would not leak any gasoline and a 10.0 micron orifice would allow the wetting of the atmosphere side of the orifice. This 10.0 micron orifice was then leak tested using air at different pressures. The 10.0 micron orifice flowed 1.4 cc/m @ 10 psig, 2.09 @ 20, 2.75 @ 30, 4.0 @ 50 and 6.61 cc/m @ 90psig.