Is Temperature Change Affecting Your Leak Test Data?

While there are several factors that can affect leak test data, temperature is less obvious to customers, not because they don’t understand the impact temperature can have, but because they may not immediately notice it as a consideration. When you work in the same area day-in and day-out, you can become desensitized to small environmental changes and contacting the experts can mean the difference between hours or days to find a solution to the problem. This article outlines how Uson helped a customer identify temperature change as the reason behind data variability and an increase in false rejects through lots of leak testing expertise and a little detective work.

A Little Background

A long time customer contacted us to help them locate the source of variability in their leak test data and an increase in false rejects. This customer assembles fuel rails with fuel injectors that are manually inserted. Once the injectors are inserted the assembly is tested for leaks. Naturally the customer was frustrated and concerned that there was no obvious cause.

The customer’s leak testing system includes a Qualitek mR with a QDIFF differential pressure sensor. The test specification is 77psig with a reject threshold of 0.003psig. Let’s look at it a different way. The reject level is just 0.004%! It’s a very small percentage but nevertheless it is within the capability of the instrument. At this point you might be inclined to think increasing the limit would be an easy fix, however, our customer has to work to a specification and nobody needs high pressure fuel spraying over a hot engine.

Gathering Clues From The Ideal Gas Law To Help Solve The Problem

So, what’s going on here? Let’s go back to school and dig out our old physics notes. In there somewhere we are bound to find the Ideal Gas Law:

Pressure x Volume/Temperature = Constant.

Using the Ideal Gas Law to calculate the pressure drop due to leakage we assume that the temperature and volume are constant, and usually that’s OK. However, when we visited the customer to figure out what was happening with the leak test system a slight draft was noticeable. Could this draft create enough of a temperature change to affect the pressure change such that it became greater than the reject limit?

If the volume is fixed then a temperature change of just 0.004% would be enough to cause a false reject. That’s a temperature change of just 0.018 ℉. Relocating the system to a draft-free area and adding a protective screen. Not hard to imagine that a slight draft could influence such a change is it? Moving the system to a draft- free area and putting a screen around it was the proposed solution.

Leak Test Specification & “What if ?”scenarios

There was something else we needed to consider before moving forward with the proposed solution. What happens if the system is moved but the false rejects keep coming?

To mitigate this possible outcome we dive into the assembly process. When inserting the injectors into the fuel rail they are pushed into O-ring ports. Could the test pressure cause the O-rings to move and change the volume? The volume is 200cm3. How much volume increase would affect the test result and create a false reject?

Just 0.008cm3 is the answer.

Clearly this is a very challenging application and it highlights the importance of leak test specification when designing a system, as well as the importance of asking “what if?” when considering any application. What if the temperature changes? What if the part volume is slightly variable? What if the test itself changes the conditions? Oh, and don’t get me started on adiabatic effects…

Uson can help

The above situation showcases what we do best. Of course we sell some of the best leak testing equipment available, but it’s our expertise and years of experience helping customers through unique challenges and complex applications that sets Uson apart from the competition.

Contact us and let’s find the right leak test equipment and solution for your application.