How to Select and Implement a Leak Detector Part 2
Response time to a helium signal:
The response time of a leak detector is the time it takes to sense a helium signal and measure 63% of the total leak rate value. This is often misunderstood and confused by its electronic response time. There could be a significant difference between the two. Usually the electronic response time is very short – around 0.5 to 1 second. The leak detector response time can be much greater and depending upon, helium pumping speed of the leak detector and the size of the test volume under vacuum and the conductance of the connecting hose to leak test, for instance.
Figure 5 shows the case of a helium leak detector used without any auxiliary pump. The response time to a helium signal of a leak detector is dependent upon the helium pumping (S), and the internal volume of the part (V).
Most of the time, the volume is dictated by the application (part internal size or test chamber dead volume) and although it can be reduced by using inserts, yet, many times it can remain significantly high. As a general rule, the smaller the volume, the faster the cycle time.
As stated above, the helium conductance connecting the leak detector to the part to be tested can have a significant impact on the leak detector response time as shown on figure 6.
Although a helium leak detector has a specific helium pumping speed at its inlet, the response time will depend on the effective helium pumping speed at the inlet of the part hence the importance of selecting the right vacuum line. Taking the conductance examples mentioned previously a ¼” and a 1” internal diameter, 3 ft long vacuum line, let’s calculate the effective helium pumping speed at the end of two vacuum lines when connected to a leak detector with a helium pumping speed at its inlet of 2.5 l/sec.
1) CHe = 9.5 E-2 l/sec => SCHE = CHe x SHe = 9.5 E-2 x 2.5 = 9.1E-2 l/sec
CHe + SHe 9.5 E-2 + 2.5
2) CHe = 6 l/sec => SCHE = CHe x SHe = 6 x 2.5 = 1.8 l/sec
CHe + SHe 6 + 2.5
The above example (1) shows clearly the huge impact on the helium pumping speed when the wrong vacuum line is used. The starting helium pumping speed from the leak detector was 2.5 l/sec. but the effective helium pumping speed at the inlet of the part is down to 9.1E-2 l/sec. In short, the wrong choice vacuum line would penalize the leak detector performance, increasing significantly the helium response time and slowing down the total test cycle.
In example (2) the vacuum line choice is much better, although there is still a drop of about 25% in helium pumping speed.
As a general rule, always use a vacuum line that matches the size of the leak detector inlet flange, and use the shorter length possible.
As a first conclusion, the faster the pump down time and the helium signal response from the leak detector, the shorter the cycle time.
Test Fixture and Part Handling:
Once the right leak detector for the application has been selected, one would think that we are done, not so. The implementation of a leak detector requires some attention as well. Quite often, little attention is given to the tooling, fixture, the handling of the parts to be leak tested, the helium management, and the test environment. These are just as important as the selecting the right detector for the application. How good is it to have the fastest car on earth and use it on dirt road? (It would be unthinkable to buy a fast sports car only to drive it on a rough dirt road.)
Having the right tooling and fixture is very critical when leak testing any component. A well designed tooling and fixture will guarantee proper sealing of the part to be tested. It will also prevent any helium background issues. It will avoid virtual leak that could be another source of helium background (see figure: 7).
Experience shows that too many times vacuum grease is excessively used to help sealing parts on a inadequate fixture. Vacuum grease traps helium, increasing the helium background and making the leak test process slower and more difficult because the leak detector will have to “fight” the helium trap in the grease
Alcohol is also occasionally used to seal parts on a rubber seal. This practice is not recommended either as alcohol vapor can damage some of the key components of a leak detector.
When designing a test chamber to leak test pressurized parts, the dead volume of the chamber should be kept to a minimum. As seen previously, the smaller the volume, the faster the pump down time and the faster the response time.
Parts should be clean and dry (as much as possible) before being leak tested. If not, the pump down time will be slowed down due to the presence of water vapor which is difficult to pump out. Water vapor can also be condensed on the inside walls of a test chamber or the part making it even more difficult to reach the desired vacuum level. When pulling vacuum inside a part, gas molecules are removed and an energy exchange takes place causing the temperature of the part to decrease. Depending upon the amount of water present and the speed of the energy exchange, water vapor condensed on the walls of the part may freeze during the pump down and potentially clog small leaks. Ensuring the parts to be tested are dry and clean will reduce this risk.
The handling of the test parts can be critical for some application. A helium leak detector is capable of measuring leaks over a range of over 10 (from 0.1 atm.cc/sc (very large leaks) down to 10-12 Atm.cc/sec (very small leaks)). Under a pressure differential of one atmosphere, those leak rates are equivalent to a range of hole sizes from approximately 50 to 0.5 µm. The handling of the parts is therefore critical, as contamination such as a finger print could clog a leak of just a few microns in diameter.
Although only 2 to 4% of the total worldwide helium production is used for leak testing, helium supply is often limited and its price has been steadily increasing over the past decade. There are solutions to address this but this would be a topic for another article. Nevertheless, proper application of the helium to the test will minimize waste, ensure a reliable and repeatable test, help the leak detector function without helium background contamination. For example, when leak testing a part using the spraying technique (measuring “inleakage”), only a very small amount of helium should be used. To detect a leak of 1E-7 atm.cc/sec for instance, a couple of small bubbles of helium is more than adequate. Spraying more helium is a total waste of money. Further, the more helium sprayed in a room where a leak detector is used, the higher the helium background. After each test, the inlet of the leak detector will vent using ambient air “polluted” with helium. This increases the helium concentration in the leak detector and slows down the test cycle. The same can happen when a part is pressurized with helium (versus spraying) and the helium is not properly vented from the test area. Managing the amount of helium in the test area is critical to optimum performance. Other techniques such as venting the leak detector with nitrogen and providing ventilation to the test are can also help address helium background issues.
Leak testing parts as fast as possible with a high level of accuracy and repeatability is every engineer’s wish. Selecting the right leak detector instrument is the first step. Implementation of the instrument with a well designed fixture and good leak testing practice is a critical second step. Only the combination of both will ensure optimum performance and desired end results.