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Why Use Helium for Leak Testing?

You use helium in your production leak testing and you are concerned about helium prices and supply interruptions. Your gas supplier might have even put you on rations or told you that you have to cut your consumption by 10% per year. What are your options?

Your options depend upon your goals. If your goal is to reduce helium consumption then you have options such as helium recovery or blending down your helium to lower concentrations. If your goal is to become independent of helium then your challenge is to find an alternate leak testing method.

Let’s first review why helium leak testing has become the method of choice for the majority of high sensitivity leak testing applications.

Perhaps the biggest factor that makes helium an ideal tracer gas for leak testing is its low atmospheric concentration (5 ppm). This low concentration allows designers to configure testing equipment that can detect leaks below 10-9 atmcc/sec in vacuum applications and below 1 x 10-6 atmcc/sec in sniffing or atmospheric applications. Though the instruments can detect below these leak rate values, residual background helium tracer gas in a system creates a noise in production environments that prevents practical detection to lower values.

Another reason helium has found favor in production leak testing is due to the development of mass spectrometer instruments that can detect very small concentrations of helium. The first helium mass spectrometers were developed in the Manhattan Project during World War II for the unprecedented leak-tightness requirements needed by the uranium enrichment plants. The technology has  significantly matured since then, resulting in robust and very sensitive instruments. Most helium leak detectors use a magnetic sector mass spectrometer to filter out unwanted gases and focus just on the signal from trace amounts of helium. Helium’s atomic mass of 4 makes it relatively easy to detect with minimal interference from other gas species; however, other gas species, such as argon, which might be a candidate tracer gas for leak testing, will appear at peaks 20 and 40 in a mass spectrometer. There are many potential gas species and their fragments that can appear on or near these argon peaks making it difficult to separate argon from other contaminant species. Argon also has a much higher concentration in the atmosphere at 9,300 parts per million, resulting in much higher residual background signals, ultimately affecting the ability to detect small leaks with argon tracer gas.

Helium’s small molecular size makes it an ideal gas for traveling through small, molecular flow leaks. The rate at which helium will travel through these leaks is higher compared to most gases. On the other hand, helium’s relatively high gas viscosity causes it to travel slower compared to many gases in viscous flow leaks. Viscous flow leaks are larger than molecular flow leaks.

These are just a few of the key reasons helium mass spectrometer leak detection has become the high sensitivity leak testing method of choice.

In our next post we will discuss options for reducing helium consumption.