How are Leak Rates Pressure Dependent

Hydrogen Leak Detector

Pressure and Your Leak Rate- Helium Leak Detection

 Pressure and leaks go together. More leaks cause you stress and for that Dr. Phil may be the answer. But what about test gas pressure in your test object.

 Will not being concerned about pressure in your test object give you leaky parts that fail in the field? The answer is yes and that should be cause concern or an increase in pressure of the Dr. Phil variety.

When visiting production leak testing customers, everyone seems to have a good idea of their leak rate. I visit a lot of customers sniffing for helium and/or hydrogen. At these visits I ask them their leak rate specification and are commonly told “ Our leak rate is 0.10 oz/yr of R410A”. But in reality, what they should have said was “My leak rate specification is 0.10 oz/yr at 400 psi”.

 It makes total sense that as you increase the pressure,in the test object, the leak rate increases. Most of the time in sniffing the tracer gas, the pressure is kept to a maximum of 150 psi as a safety concern. But many times, the operating pressure of the unit is up to 400 psi. As you increase pressure, the volume of test gas used increases. With the cost of helium rising there is a temptation to reduce the test pressure to save money. So how much difference can pressure make?

For sniffing the leak rate equation is based on “Poiseuille’s” equation for viscous flow where Q is the flow or leak rate:


Q = D4 (P12 - P22)




Q= Leak rate in atm. cc/sec.

D= Leak Diameter in cm.

n= Gas viscosity in bar sec.

L= Length of leak in cm.

P1= Test pressure on one side of leak (bar).

P2= Test pressure on other side of leak (bar).  


Looking at the equation, you see that pressure is squared, thus a product in operation at 400 psi (27.2 bar) compared to testing at 150 psi (10.2 bar) is quite a difference.

 Taking 27.2- 10.2 = 17 bar squared -= 289 times greater leak

 So the product operating at 400 psi, that leak shown at 150 psi is now 289 times larger.

 Moral of the story is that if your product passes a 0.10 oz/yr leak test at 150 psi, in the field running at higher pressures, the real product operating leak rate is much larger than the low pressure test leak rate.

Schoonover Inc. ( ) represents companies for helium leak detectors and helium leak testing systems. For further questions on your application contact us at 800-331-2808 or at

Value of a Polycold

Value of The Polycold on a Vacuum Coating System 
How is a Polycold similar to a shot blocker in the NCAA Basketball Tournament


One principal of high vacuum that is easily understood is the concept of water vapor in a vacuum chamber. If you live where I live in the deep south part of the USA, humidity or water vapor in the air is easily understood. Obviously, this water laden air is also in the vacuum coating chamber, lining the walls and surfaces of the vacuum chamber. 

When a chamber is pumped to a pressure of 10-3 torr or below, most of the chamber’s volume gas has been pumped away and water vapor becomes the predominant gas within the chamber. At this point the reduction in pressure is controller by the rate of water being desorbed from the surfaces inside the vacuum chamber and that water molecule finding its way to the inlet port of the high vacuum and being pumped out of the chamber. More than likely, a water molecule desorbing will simply find its way to another surface inside the vacuum chamber to again re-sorb on the surface.

So where is the basketball shot blocker reference? 

Imagine a three point shooter in basketball. The best player makes 40% of his shots and he is highly skilled. Compare that to a water molecule in a vacuum chamber that is not highly skilled at all to find the inlet to the "basket" in a vacuum system or better known as the inlet to the high vacuum pump. So what we need is better skilled water molecules able to land in the inlet? Well yes but scientifically not practical.

The Polycold coil inside the vacuum chamber acts as the shot blocker. With the coil cryogenically cooled, any water molecule moving off the surface has a much better chance to hit the Polycold coil than the inlet of the pump. The more surface area of the coil, the better chance to be "blocked" and trapped by the cryogenic Polycold coil. Once trapped on the Polycold coil that water molecule is no longer "in play" inside the chamber and stays on the coil until the next regen cycle.

Installing a Polycold in your high vacuum system pays great benefit in reducing the high vacuum pump down time of the chamber by up to half. This can result in extra coating runs per day and thus more $$ profit.

Schoonover can work with you to provide a turn key solution that includes, rebuilt Polycold, coil design, coil fabrication and installation.  Utilizing a rebuilt Polycold instead of new provides a long-term "good as new" solution with a total price (Polycold, coil and install) usually at or less the cost of a new Polycold. So more bang for your vacuum pumping buck. Visit us at and contact us for more information on this cost saving addition to your coating system.