Leak Testing Compensation and Calibration


What is Comp & Cal?

In leak testing you’ll frequently come across the expression “compensation and calibration “or “comp and cal”. This article explains leak testing compensation and calibration in detail including why it’s important, how it’s performed and what challenges you can expect to encounter during the process.

The Adiabatic Effect

One of the challenges in leak testing using air to fill or pressurize a part is the “adiabatic effect”. An adiabatic effect is one in which heat is not transferred into or out of a system yet a change in temperature occurs. The change is due to work being done on the system’s surroundings. A good example is that of a piston compressing a gas in a cylinder. Did you notice how your bicycle tire pump gets hot when you inflate the tire?

The adiabatic effect occurs when the air pressure in a volume is increased. Remember the calculation (P1xV1)/T1 = (P2xV2)/T2. As the pressure is increased, the number of molecular collisions also increases because the distance between the molecules decreases. The collisions create friction and the result is heat. We learn early in life that heating the air in a closed vessel increases the pressure. When the pressure ceases to increase or the work stops the air molecules slow down and the number of collisions decreases. This in turns leads to cooling and a drop in pressure. If we start to measure the leak rate immediately when we stop filling the part, the small drop in pressure from the reduced molecular activity will resemble a leak and we might get a false result.

Since our objective in leak testing is to complete the test as fast as we can without losing accuracy, we must compensate for the pressure loss observed as the part cools after adiabatic heating. To do this we teach the leak tester to recognize the small pressure drop for what it is and to add it back to the leak rate calculation as “zero”. The amount of compensation required is called the compensation value or “comp” value and it is specific to the part being tested and the pressure being applied. For use later in this explanation let’s say for example that our comp value is -0.08 psi.

Now let’s turn our attention to the “calibration” in compensation and calibration.

“Calibration” of the leak test.

Let’s begin by making it clear that in this context, calibration is not used to describe a scientific calibration of the leak tester. In the context of compensation and calibration, we are referring to the “calibration” of the leak test. The pressure decay leak tester uses a sensor to measure the pressure drop – it does not measure the leak rate. However, most customers want to see the result of the leak test displayed as a leak rate because the engineer who designed the part declared a leak rate as part of the specification.

In order to perform the necessary calculation to convert the pressure drop to a leak rate we must teach the leak tester how a certain pressure drop relates to a leak rate. This is just like teaching the leak tester to recognize the pressure drop seen in a good part due to the adiabatic effect. To do this we require a Leak Master, a certified device that has a known leak rate at a certain pressure. To perform the calibration the leak master is inserted into the circuit connected to a known good part. If the Leak Master is certified to be, say, 4cc/minute at 60 psi and we observe a pressure drop of 0.2 psi in one minute we now know that a leak rate of 4cc /min = a pressure drop of 0.2psi.

Putting the two together, if our comp value is -0.08 psi and our cal value is -0.2 psi and the leak tester measures a total pressure drop of 0.28 psi then the leak rate is 4cc/min.

Uson can help!

This is the principle of compensation and calibration and it’s clear to see that it is a very valuable concept. If you have questions regarding compensation and calibration the team at Uson is here to help.  Contact us via the website or call us directly at +1.281.671.2000.

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