“We are what we repeatedly
not an act but a habit.”
I came across this quotation recently and it quickly struck me how it relates to quality assurance. And so, with it being the beginning of a new year, this post is rather more
“Uh-Oh, what could this possibly be about?” you might be wondering. There can’t be any such thing as “zero-time”.
Well, strictly speaking you are right, but there’s a neglected feature in the Optima leak tester that we couldn’t think of a better name for.
The Uson Optima vT is designed to be accurate and reliable, but at the same time FAST. This is why it has multiple high-resolution sensor inputs that are sampled simultaneously. The Optima vT can make accept/reject decisions in a short period of time. Most applications require more than a simple accept/reject decision though and it is usually also necessary to log the sensor data that led to the decision.
Stay with me while I attempt to explain how this handy little feature works and how it just might take your leak testing to another level………
A long time ago, Uson came up with the concept of breaking the leak test into multiple “steps” that are named and numbered. Not only does this simplify the test program, it allows customization, and provides the option of logging a result for each step. This is where zero-time steps are handy.
In this post I thought we might try to put hole sizes,leaks, and leak rates in perspective.
Much continues to be written about determining realistic leak specifications and the chestnut of the “no leakage permitted” type of specification is still commonly encountered.
Looking through some old training material I thought
Probably the simplest type of leak test is still the dunk test wherein a part is connected to an air supply and held underwater while the operator looks for bubbles.
Of course the disadvantages of this method are well known and technology has introduced non-destructive and more objective tests. As
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.
In the business of leak detection it’s often easy to create names for things and then those names stick around for a while and eventually become a permanent fixture of our specialized vocabulary.
Great examples of this can be found hiding in plain sight with the manuals and application notes