Automation 101:  Vacuum Leak Test to the Rescue

Especially in medical device, automotive component and industrial equipment markets, customers rely on parts manufactured to be leak-free in countless ways. From emergency response equipment to everyday electronics, leak-free components must perform without fail. In turn, testing these components is the most critical manufacturing step in their production, catching failures before they are sold to users who will depend on them. A vacuum test is one of several most common industrial leak testing methods available, and one that is readily deployable in an automated fashion for manufacturers of all sizes.  Here’s a detailed look at vacuum leak testing technology, its role in fabrication and available automation approaches below.    

What Is a Vacuum Leak Test in Industrial Manufacturing? 

Air-tight products serve a multitude of important roles across a wide range of industries and markets. From emergency response equipment to everyday children’s toys, we can find countless products designed to contain contents or resist infiltration from outside materials with a reasonable degree of certainty that they will remain leak-free. Let’s take a quick look at examples of products in different market sectors that require some level of leak-free construction:

From the short list above, you might immediately recognize that not all of these items could be considered completely leak-free, and you’d be right! This term – leak free – sets the stage for our discussion about leak testing, presenting us with a phrase that seems simple enough, but in practice can be highly technical.    

Simply put, there is essentially no such thing as entirely leak-free when it comes to fabricated parts and products, nor would it be practical to expect as much. With enough pressure, heat, speed or force, particles of decreasing size can pass through any common material (plastic, metal, glass or even advanced polymers). While a product might be leak-free from atmospheric air at normal ambient pressure and temperature, it could still take on water if you submerged it deep enough that the increasing outside pressure became too great for the product to withstand. Even more subtly, a product might be leak-free to certain liquids but not to certain gasses under the exact same conditions, based on the difference in liquid and gas molecule sizes. So, in practice, we need a technical specification to define the details of allowable leak conditions in any given product’s design.  

A leak specification will typically cover the required leak condition details in normal use, as well as a testing procedure by which to confirm this specification on parts after they’re manufactured. For example, a leak specification may include:

• Leak media, which defines if the product is to withstand solid, water, air, gas or a combination of these medias.
• Allowable leak rate may be a single or multiple measurement, such as 5cc/minute, per media type. 
• Applicable conditions outline the expected conditions when the allowable leak rate is experienced, such as ambient environmental temperature and pressure, as well as media temperature and pressure.
• Test method specifies a testing process designed to confirm all above specifications, and the allowable pass-fail results. (More on this below.)

Vacuum Leak Testing Methodology 

Several leak testing methods used in industrial fabrication today include pressure decay, vacuum decay and mass flow measurement. These tests use precision instrumentation to subject a part or assembly to highly tuned conditions, measuring extremely small changes in these conditions that may indicate a transfer of media into or out of the test part. Since we’re looking here at vacuum leak testing, let’s dive further into this testing method.  

Vacuum testing relies on a pressure gradient to be created across a part, with one pressure being established inside of the part being tested, and a different pressure being established outside of the part. Vacuum technology is then used to compel this pressure gradient across the part, and extremely precise instruments are used to detect and measure any flow that may occur between these areas. Depending on the part being tested, vacuum testing can be conducted either into or out of  the part.

• With in-flow vacuum testing, a vacuum is pulled on the interior of the part, which evacuates all media (such as air) from inside of the part, and a higher pressure of test media (such as helium) is created outside of the part. Any leaks that occur will pull helium from outside to inside of the part. This helium transfer can be measured by the vacuum instruments monitoring the part’s interior.  
• With out-flow vacuum testing, a vacuum is pulled on the exterior of the part, which evacuates all media (such as air again) from outside of the part, and a higher pressure of test media (again, helium) is pumped into the part’s interior. Any leaks that occur will pull helium from inside to outside of the part. This helium transfer can be measured by precision instruments monitoring the part’s exterior.

In-flow testing is slightly more common for industrial and commercial parts, evaluating conditions that would compromise a sensitive component’s interior space intentionally designed to be free of foreign contaminants (such as implantable medical devices). In cases where containing a part’s contents under changing exterior conditions is critical, such as for aerospace or submersible marine parts, out-flow testing is the preferred method. Part designers and engineers need to consider all possible operating conditions for each part and determine if one or both test types are applicable.  

Regardless of test direction, a vacuum leak test is most commonly conducted in one of two test types, vacuum decay or vacuum mass flow:

• The vacuum decay method consists of pulling a vacuum on a test chamber, in which the test part resides and is sealed. The vacuum level in the chamber is monitored for any increase in pressure (or decrease of vacuum) over a fixed period of time. If the chamber’s vacuum level decreases, a leak through the part that added air into the test chamber would be suspected. For parts where a certain amount of leakage is acceptable, the difference between initial and final vacuum readings is used to determine pass/fail.  
• The vacuum mass flow test is used for parts that are either quite large or cannot structurally withstand static pressure or vacuum. In this test, a flow of air or inert gas is sent through the test part. The amount of gas sent to the part is precisely measured against the amount of gas that escapes the part, compared to the allowable leakage rate. This difference is used to determine pass/fail.  

How to Automate Vacuum Leak Testing  

In the simplest form, vacuum testing is really no more than creating a pressure gradient from inside to outside of a part and observing for changes in that gradient, which could be performed with handheld suction pumps and pressure gauges from a hardware store. In industrial manufacturing however, typically an automated test station or inline machine is desired in order to provide high-speed, repeatable, documented test batteries on parts before they are considered complete. Such systems can be provided in many configurations and scales: 

• Manual vacuum testing is usually delivered as a benchtop system such as AMS’s LT-201 Leak Test System. A manual vacuum test station consists of a vacuum pump and a precision gas measurement instrument. The part to be tested is set on the bench, open ends sealed, and the vacuum pump connected to a test fitting ported to the part’s interior. The vacuum pump is used in either the vacuum decay or vacuum mass flow method to conduct its test, and test values are then reviewed to determine if the part passes or fails. (The below automation levels can also be conducted in either vacuum decay or vacuum mass flow).    
• Semi-automated vacuum testing is provided by more advanced vacuum leak test systems. AMS’s LT-401, for instance, offers semi-automated, feature-rich test processes that allow for higher volumes, faster test cycles, robust safety features and integrated quality control benefits. Semi-automated systems accept parts placed by operators into their test fixtures and take it from there. Poka-Yoke functions assure that the part is assembled correctly, actuated fixture mandibles orient and seal the part ends, and a PLC-driven test sequence is executed once or across multiple cycles. A part is deemed to pass or fail via the system’s automatic computational comparison of test media values from start to finish, and the results are documented in regulatory-compliant, local or networked QA data storage systems.   
• Fully automated vacuum testing occurs within batch or continuous manufacturing lines, without operator part handling. Envision the core of the above semi-automated system placed in line with other production equipment, using robotics or conveyance mechanisms to handle the part, while all other testing functions remain the same. Acceptable parts are then mechanically conveyed downstream to packaging, while failing parts are rejected to accumulating tables awaiting rework or repair. Fully automated systems offer the highest volumes and least amount of human labor, and given their cost, usually are only selected by the largest manufacturers.  

Choosing the Right Leak Test Operations

What do you want to know about leak testing? We’re happy to discuss the right application for your project. Contact us today.