I think I have said this before; medical devices can present some of the greatest challenges in leak testing. That’s kind of ironic in a way, since they are often among the smallest things we are asked to test. The sheer variety is mind boggling. You can easily be forgiven for wondering where they all go with a visit to just one manufacturer.
Because they perform vital often life saving functions, it’s no surprise that testing them correctly is vital. If even one bad part reaches a patient the consequences can be catastrophic. This leads us to the first challenge:-
One hundred percent inspection
In some industries although the quality engineer seeks perfection there can exist a compromise between inspection and throughput by using statistical sampling methods. In the medical device industry perfection and production objectives must be met without compromise. Therefore fast, one hundred percent inspection is expected and it demands multi channel leak testers as well as accurate and reliable fixturing.
Everybody knows that medical device manufacturing is a highly regulated industry. It’s for the safety and peace of mind of patients, not just in the USA, but around the world. No one would argue that’s not a good thing. However, it brings with it ever increasing requirements for capturing and storing data. Some devices are serial numbered, chipped, or bar-coded and that brings us to the next challenge:
Although much of modern leak testing was born out of rocket science, it’s not rocket science as they say. We’ve come a long way since automated leak testing started to become established, Nevertheless, it still sounds so easy – just fill something with air or gas and monitor the pressure change or detect the escaping gas with some sort of gas detector. So what could possibly go wrong?
Well, as with most things there’s a downside risk. Let’s take a look at some common pitfalls for the unwary.
- • Using unfiltered air. Unless the air that is used to pressurize the part is clean and dry, there exists the almost certainty that the leak tester will become damaged either eventually or extremely quickly. Some people might be surprised how dirty the air in factories can be before we even begin to notice the taste or irritation. I remember visiting a wheel bearing plant where literally everything touched was covered in a film of oil. I’ve seen leak testers literally dripping with oil when they arrived at the service department. One instrument maker mounted PCBs vertically so that oil mist dripped off and did not pool on the boards.
Metal particles in the air or other trash can also destroy sensitive transducers and precision valves.
Fluid handling is a subject worth having its own specialized category of engineers, so it would be foolish to try and discuss it in detail in a short affair like this blog. However, we can’t simply ignore its importance as it relates to leak detection.
As leak detection specialists we are concerned with dynamic viscosity, the resistance to flow when an external force is applied.
In very simple terms viscosity is a measure of attachment to the neighbouring molecules. The net result is that a more viscous liquid or gas will pass through a hole more slowly than a less viscous one.
Units of Measure
To complicate things, there are different units of measurement for different fields of study. For leak testing geeks our unit of measure is the PASCAL seconds (Pa.s) which may also be expressed as the Newton second per square metre (N s/m2) or as the kilogram per metre second (kg/(ms)). You’ll also come across the poise (P) and centipoise (cP)
1 cP = 10-2 P = 10-3 Pa s
The viscosity of a liquid or gas is determined by the strength of attraction between molecules. The following equation may be used to calculate the ratio between a liquid and gas:
Note: This assumes laminar flow characteristics, which should be verified in practice.
In last week’s blog we discussed the plight of a fuel rail manufacturer trying to work with a very small reject limit. I received several questions about the wisdom of setting up such a test in the first place and so this week I am going to elaborate on leak rate specification.Here are some things to consider when creating a leak test specification.
Is it fit for purpose?
Over the history of modern leak testing, collectively the industry has solved probably tens of thousands of leak testing applications, even more, and a sort of unofficial playbook has emerged for pressure decay leak testing and other methods. Your supplier’s application engineering group is aware of these specifications in one form or another. Here are some examples:
Clearly there is a benefit in selecting a specification that reflects the actual usage conditions. This is where it’s possible that things can become a little difficult. Of course I am referring to the catch all demand that “it must not leak!”. For the record, once again, everything leaks, it’s just a matter of degree. If we corner ourselves by insisting on the tightest specification possible we are inviting trouble and unnecessary expense.
It was the smallest thing but it was a big deal. The customer is assembling fuel rails. The fuel injectors are manually inserted and then the assembly is tested for leaks.
Bob (not his real name) is highly frustrated with the variability of his leak detection data and downright upset with getting a whole lot of false rejects.
Bob’s leak detection 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 say just change the limit up a bit and everything will be fine. The trouble is Bob has to work to a specification and nobody needs high pressure fuel spraying over a hot engine.
The Devil in the DetailOn the face of it this looks like a straightforward leak testing application. The part is made of rigid plastic, it’s not too big and the leak rate specification is comparable to that of other fuel system components. The purpose of the carbon canister or
In the last two blogs we have discussed “Comp & Cal procedures and how to avoid ten common mistakes. If you read those posts you’ll recall that the compensation feature allows us to add back the pressure change that occurs when a good part is tested. The pressure change is caused by the adiabatic effect and using the compensation feature lets us shorten the test cycle because we don’t have to wait for the system to settle down before we start the test phase.
In this post I’m going to introduce another type of compensation – it’s the ability to automatically adjust the leak tester compensation value that we created in the Comp and Cal procedure. Now hold on, I can hear you say we just took great care to identify the correct compensation value and now you’re going to start changing it already? Why would we want to do that?
The answer is pretty clear when you think about why we compensated in the first place. It was because the temperature change caused by the adiabatic effect increased the pressure and created what would appear as a leak as the temperature cooled and the pressure dropped.
- 1. Never perform Comp & Cal procedure without first verifying that you are using the correct master part. This sounds obvious but it’s easy to pick up the wrong part and it happens often.
- 2. Do not run Comp & Cal without making sure that you have the correct Leak Master or calibrated leak on hand. This is a very common mistake, so beware.
- look the same and it’s easy to get them mixed up especially when the pressure is on to get the production line back up and running.