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TIP OF THE MONTH 01

QUESTION: Which test gas produces the highest detection sensitivity in a sniffing test?

TIP OF THE MONTH 01

ANSWER: in practice, helium and hydrogen have the same detection sensitivity. Why is this the case?

REASONING: For detectors which are used to detect trace gases in a leak test where the gases are not exhausted, the detection limit is determined by the concentration, or more precisely, by the background level of the test gas in the surrounding air.

In an ideal case, the air at the location of the testing shows the natural concentration of the test gas. For helium, the natural concentration of the gas is 5 ppm (1ppm = 1/1000000 = 0.0001%), for hydrogen, the concentration is 0.5 ppm. This distinction leads to the supposition that one can measure to a much higher degree of sensitivity with hydrogen, because a background signal ten times lower is to be expected.

However, because of the flammability of hydrogen, it is necessary to work with forming gas, which is inert. Forming gas is a nitrogen mixture with only 5% hydrogen. Through the use of forming gas, as opposed to pure hydrogen, the sensitivity is decreased by a factor of 20. This means that it should be possible to measure leak rates using helium with a degree of sensitivity twice as high as that of hydrogen.

Another issue concerns the viscosity of the forming gas, which is 10% lower than the viscosity of helium. As a result more forming gas flows through a leak of a certain geometry than does helium.

On the other hand, the gas flowing into the helium sniffing probe produces a pumping effect. Depending on the geometry of the test object, more of the test gas flowing out can be analysed as would be the case with a hydrogen sniffer. A hydrogen sniffing probe does not normally suck in or aspirate any gas; it can however be optionally equipped with an aspiration device.

During actual use, the leak rate to be detected is normally several orders of magnitude larger than the detection limit of the leak detector. This means that the theoretical factor of 2 resulting from our observations of the test gases (hydrogen and helium) is in practice irrelevant for leak detection. In fact, there are numerous other factors which influence the test result more strongly than the theoretical differences between the text gases. These factors include:

  • leaky valves on the test gas canister or container
  • leaks from regulators
  • fluctuating primary pressures as the container empties
  • other sources of gas

As a general rule, the constancy of the background signal is the determining factor in the detection limit. A constant background can be easily eliminated by a zero suppression of the leakage testing device, whereby the detection sensitivity is increased. However, variations are then intensified to the same degree and can lead to aberrant display indications.

Therefore, as a rule of thumb, a detection limit in the area of several 10-7mbar, l, s-1 expected, regardless of whether helium or forming gas is used. Lower-quoted values are therefore most likely advertising stunts rather than serious technical data. With a helium calibration leak at a leakage rate of 1.10-7 mbar . l . s-1 , it is easy to differentiate between fact and fiction.

Glossary

Forming gas: forming gas is a collective name for less-active gas mixtures consisting of nitrogen N2 and hydrogen H2, where the ignition limit in comparison to pure hydrogen is massively reduced or completely eliminated. Forming gas is used in the working of metals at heat, for example in soldering, welding (join protection), milling, pressing and annealing.

The hydrogen contained in the gas works reductively on metal oxides and prevents oxidation from occurring. Forming gases are non-toxic, but at a hydrogen composition of less than 5.5% they are non-flammable.

Sniffing test: In a sniffing test, the test object is filled with test gas. The test gas (for example hydrogen or helium) flows out through leaks and is detected by a sniffing probe, either through manual use or using a pre-installed probe. This probe feeds into a detector. The leading types of detector are sector mass spectrometers or semiconductor sensors.

Detection limit: The detection limit indicates the value of a measuring system, up to which a measurement (of gas) can be reliably detected with the measuring system. The reliability of the detection is defined by a statistically specified so-called confidence interval. In circumstances where the measurement values are under or over the detection limit, they are designated as immeasurable within the scope of the measurement technique.

In practice, this means that it is the user who must decide for themselves, which difference or which quotients between the measured signal and the test gas background are to be determined as adequate or satisfactory in order to obtain a clearly-defined result of ‘good’ or ‘bad’ in the test. A very conservative value is when the measured leakage rate signal is greater than the background leakage rate by a factor of 10. Sophisticated measuring equipment or the use of zero suppression can help to obtain a reliable ‘good’ or ‘bad’ test result even in case of a bad signal-to-noise ratio.

Zero suppression: Using zero suppression will place a zero value on the display screen of a measuring device. This means that a real, measured background leakage rate is shown as value zero. The value of the background leakage rate can therefore be anything.

Zero suppression is used in test gas leak detection most often when there is a very high concentration of the test gas in the immediate environment. A zero suppression is used with small leakage signals and a simultaneously high test gas background to obtain a reliable representation of the zero value and to increase the display resolution - so for example in Sniffing test leakage detection or in highly sensitive measuring at the detection limits of detectors.


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