Leak testers used in a factory must be robust, safe, accurate and repeatable. Some modern leak testers are also being asked to gather data for statistical analysis and to make decisions based on selected parameters.
A new tester, the H2000, from Sensistor Technologies Inc. (N. Billerica, MA) uses the hydrogen method to fulfill all of these needs. It detects the leak, finds its location and determines the leak rate. The H2000 can be used in automatic and manual testing operations, and can be built so that it will act as a go/no-go station.
"The hydrogen method has been used since the 1980s," says Dave Morris, general manager of Sensistor Technologies. "What is different today, compared to earlier versions, is that we have stabilized a lot more on the electronic side, as most technologies have, so we can get better repeatability."
Helium gas is one such gas that has been used effectively in many applications. However, says Morris, it must be used in tandem with a mass spectrometer, which is expensive to purchase and maintain, and is sensitive to environmental factors that could be found on a factory floor. It requires pumps and vacuums that may be vulnerable to dust and may result in higher maintenance costs. Helium is also a non-regenerative resource, and is more expensive than hydrogen, he says.
The solution was to develop a stable detector with a sensor that was extremely responsive to hydrogen in low concentrations. In 1983 Sensistor Technologies co-founders, Dr. Claes Nylander and Fredrik Enquist, cracked the problem by designing a sensor that was specific and sensitive to hydrogen at 2.5 by 108 cubic centimeters per second (cc/sec) at atmospheric conditions. Likewise, they developed software and electronics to provide a stable and linear output signal for a display.
Diluted hydrogen is now considered a green-labeled gas; it is environmentally safe/nonflammable as per ISO 10156. It also meets with ISO 14000 for companies required to use renewable substances. It is the lightest and most viscous of gases and fills test objects quickly, which means that larger parts can be inspected.
The H2000 can be used as an automatic system or in a manual operation using the H50 hand probe. The hand probe is used to detect a leak location. It emits an audible signal when it senses hydrogen molecules and this tone grows in pitch as it nears the leak.
In a production line, a chamber test is used whereby the part is uploaded into the chamber, which is then flooded with hydrogen. The part is either ruled good, meaning that it has either no leaks or is within the leak limit, or bad, meaning that it is out of tolerance. Component size and leak size may affect cycle times, but in general these tests can be completed in 1 to 30 seconds.
"In an automatic system, a chamber test would be conducted. In that case, you are looking for the total leakage. You are not trying to locate the leak, you are just asking the question, ‘does it leak?' and ‘what is the overall leakage of the part?' and that you can do by putting the part into the chamber."
The H2000 requires no pumps or vacuums, which Morris says eliminates the need for any "real" maintenance. "The detector requires nothing more than changing a sensor every 2 or 3 years and that can be done in less than a minute," he adds.
Many users are choosing to integrate pressure-decay/mass flow with hydrogen test systems, says Morris. "An example of this might be a car engine assembly," he adds. "The manufacturer wants to know if the whole product is tight or not. If it fails the test, operators must locate where the leak is so it can be fixed on line. One way of doing that is to combine the pressure-decay/mass flow test and the hydrogen method test. If the part is tested using the pressure decay method, the user can backfill it with the hydrogen nitrogen mixture and then use a probe to go around suspect points to find the leak position."
The hydrogen sensor is a unique type of gas sensor, which is sensitive to hydrogen and totally insensitive to almost all other substances. While other hydrogen gas sensors (thermal, catalytic) used in hydrogen gas alarm equipment are normally sensitive to a lot of other gases and vapors, the Sensistor sensor in not affected by common interference such as methane, propane, gasoline vapors, water vapors or organic solvent vapors.
The sensor consists of an integrated circuit containing a MOS-FET amplifier. Instead of an electrical input to the amplifier, there is a thin film of metal hydride, which is permeable to hydrogen atoms only. Hydrogen molecules (each consisting of two hydrogen atoms) split into atoms when hitting the metal surface, and are selectively absorbed by the film. The absorbed hydrogen atoms give rise to a small voltage, which is amplified and transferred to the detector instrument. The instrument carefully controls the temperature of the sensor chip and interprets the sensor signals. The sensor voltage is not a simple function of the gas concentration. Therefore there is some signal processing in the instrument to compensate for background, drift and other factors, and to determine the hydrogen concentration in a gas sample.
Source: Edited from Hydrogen Sensor Working Principles by Sensistor Technologies.