Building Relationships That Work

## Creating A Master Part

Creating a Master Part for Leak Test Calibration Purposes

The major key to a successful pressure decay (or differential pressure decay) leak test is a proper calibration. This process begins by first creating a known-good part. Let’s start with the basics.

Pressure decay (PD):

1. Definition = The measurement of part pressure loss over a specific amount of time.
2. PD is Volume Dependent. Parts with differing volumes, pressurized to the same pressure, will vary in pressure loss, even if they are leaking with the same flow rate. So consider a basketball and soccer ball pressurized to the same pressure, leaking at the same flow rate- say 20 standard cubic centimeters per minute.  Which ball will show the greater amount of pressure loss? You are correct if you said the soccer ball. It has a smaller volume, so it will show a greater pressure loss than the basketball leaking at the same flow rate. So in conclusion, calibration to a master part that has the same volume as the part to be tested is critical to an accurate and repeatable PD leak test.
3. Temperature Matters – We are essentially measuring pressure change in the part and converting this pressure loss/change into a leak rate for accept/reject purposes.  Anything that affects the pressure in the part during test will affect the repeatability of the test.  Temperature change can have a significant impact on the pressure measurement so a stable environment is also an important consideration for repeatable leak testing.
4. Part Volume Matters – The larger the part the longer the cycle time – we all get that.  The material make up of the part also matters – plastic versus cast iron as an example.  If a bottle expands and contracts during a test – what will it change? The pressure inside the bottle of course, so keep this in mind when establishing timers and calibrating.  The larger the part the more difficult it is to differentiate from a good part to a bad part with pressure decay leak testing.  You can’t fight Mother Nature – in the end physics always wins!

When we calibrate (press the Autocal Button), we use a known-good part to establish the zero set point (1st Test) and the known-good part combined with a calibrated leak standard to determine the reject set point (2nd Test).  Those become the upper and lower control limits that differentiate a good part from a reject part.

How in the do we assure we have a known-good part?

• Dunk Testing – The part pressurized with air and submerged under water, observing for escaping bubbles.
• Snoop – Again pressurize the part with air and then use snoop or soapy water to inspect for leaks while under pressure.
• Mass Flow – If you have an instrument capable of measuring Mass Flow just pressurize the part and press hold.  Observe the readings to determine if the part is leak free (this will not work for leak rates under 3 sccm).

We have a known-good or master part.  Now what?  Here are the steps to calibration:

1. Place the master part into the leak test fixture.
2. Insure that the Leak Standard being used for calibration matches the leak standard information in the instrument.
3. Actuate the clamps & seals.
4. Press the Autocal Button to begin calibration.
5. Ideally we want the performance factor to be above 40 – the higher the better the calibration and therefore the more repeatable the test will be.

Now that you have a master part it is important to clearly differentiate it and protect it from damage.  We especially don’t want it to be shipped to down stream processes or to the customer…although we do know it is a good part!!