Leak testing is a very important process for part quality control. If you invest in the equipment and the time for leak testing, you should also strive for the highest reliability in your leak testing results.
Maintaining good background levels of tracer gas is very important in gaining reliable results. The smallest detectable leak rate in leak testing strongly depends on the background concentration of tracer gas. Although the leak detectors only detect changes of tracer gas concentration, higher background concentrations also tend to show higher absolute fluctuations. Leak testing for small leaks in high background levels of the gas you would like to detect is like trying to hear the slightest whisper while listening to high volume rock music.
Below are the precautions to take to make sure your background does not rise to undesired levels and negatively influence the reliability of your leak testing.
Parts need to be evacuated and filled with tracer gas before leak testing. This tracer gas needs to be vented or reclaimed from the part after leak testing. To connect the filling or reclaiming equipment, many customers use quick coupling connectors. By design, these connectors release a small amount of tracer gas during disconnecting (a small volume inside the connector stores some tracer gas which is released to the open on disconnecting). These small amounts may add up during the day and cause your background to rise. Ideally, the filling and reclaiming process should be separated from the actual leak testing station so that the released gas is not getting to the actual test station.
However, in many automotive applications the parts are filled and vented within the test station for high throughput (with the connectors staying connected throughout the entire leak testing process). In this case, it is important to have good ventilation in your testing area. Helium / hydrogen do not fly to the ceiling of your leak testing area like a filled balloon would do. Tracer gas forms clouds which move around. As both tracer gases still have the tendency to move up, it is recommended to supply fresh air from the bottom and put the exhaust at the top of the test station.
After leak testing, the part needs to be disconnected from the gas supply lines. If you simply open the connector after the test, the helium filling will be vented to the testing environment, due to the overpressure in the part. This will cause some clouds of tracer gas contaminating the testing area. There are two options:
Reclaim the tracer gas from the part before opening the connectors. This will also help you save helium, as the reclaimed gas can be reused for consequent testing cycles and will decrease your helium cost. This is particularly recommended if using pure tracer gas.
If you already use diluted tracer gas and you do not want to reclaim the tracer gas, make sure the gas being released from the part is vented to the outside of the building and not anywhere near the air compressor inlet.
To fill and reclaim tracer gas, connectors, valves, pressure regulators and / or supply lines are needed. Some gas valves leak to the outside by default, but they have a connection to suck away this leaked gas. All of these may develop leaks over time which may cause a constant contribution to your tracer gas background. So, it is a good idea to take a sniffer leak detector and check these spots for leaks regularly and if there are leaks, repair them. Please note: Gas lines must be metal sealed.
A gross leak in your part may release huge amounts of tracer gas if left undetected. Hence, it is good practice to do a gross leak test during the filling process. Parts should be evacuated before filling (to allow for complete filling with tracer gas). After evacuation and before filling with tracer gas, make sure to watch the evacuation pressure for a couple of seconds. If the evacuation pressure rises during this holding time, the part has a gross leak. Do not fill the part with tracer gas in this case to avoid unnecessary release of tracer gas. Many filling stations have included this function as a standard procedure already. Check if you use this function in your process.
Also, you may watch the leak rate signal of your leak detector and stop the process immediately after the trigger is exceeded and remove the leaky part from your leak testing area. Do not wait for the signal to reach its maximum.
Please contact us if you have problems with your background of tracer gas. INFICON experts will help you to find the source(s) of your elevated background and show you how to avoid this.