Two Minute Tips  

Accelerometer Hermeticity

Peter Eitnier | Senior Application Engineer, Wilcoxon Sensing Technologies

Most industrial sensor manufacturers’ specifications indicate that their sensors are hermetically sealed, and some go as far as to apply IP ratings to their vibration sensors, even though an electrical connection still needs to be made.  At its core, hermeticity refers to a seal or closure being complete and airtight.  There are different levels of hermeticity, though, depending on the quality of the seal, and the only way to measure is through a leak test, usually performed with helium.

Epoxy-sealed sensors are generally not considered hermetic, but sensors that are welded can undergo a helium leak test (HLT) to qualify their sealing.  An HLT is performed by placing the units in a chamber and pressurizing it with helium.  Typical pressures are 90-120 psi.  After exposure, the units are removed and placed in a test chamber.  A vacuum is drawn, and the remaining escaping gasses are sent to a mass spectrometer tuned to detect helium.  The HLT equipment can detect leak rates as low as 1×10-9 cc/sec, but those with leak rates above approximately 1×10-3 cc/sec will saturate the detector.  Wilcoxon identifies these units as “gross leakers” and not to be considered hermetically sealed, yet establishes even lower leakage rates for standard product, giving us the highest hermetic seal rating in the industry.

Technology Leakage Rate (cc/sec)
Discrete semiconductors 1×10-9
Wilcoxon general-purpose industrial accelerometers 1×10-8
RF modules 1×10-7
Wilcoxon specialty sensors 1×10-6
ICs microcircuits 1×10-6
Gross leaker 1×10-3

There are various benefits of providing a hermetic seal this trustworthy.  One is that having been subjected to pressure testing of 120 psi, the sensors are rated to withstand 100 psi of pressure from submersion.  Because helium particles are tiny compared to water molecules, passing an HLT ensures that the accelerometer will survive underwater submersion indefinitely, with the appropriate connector, of course.  Additionally, a hermetic seal minimizes the constant exchange of atmosphere inside and outside of the sensor that is quickened by cyclical temperature variations, extending the expected lifetime to 20 years or more.

Ensuring your vibration sensors have a true hermetic seal is vital not only to the viability of the sensor itself but to the safety of those workers who trust the data being outputted and build maintenance programs around it.  A long-term mean-time-before-failure (MTBF) is essential to minimizing hardware costs and maximizing the lifetime of your machinery, something directly affected by a sensor’s hermetic rating.  If your sensors are being sealed as the manufacturer claims they are, it’s just one less thing you’ll have to worry about!

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About the Author

Peter Eitnier Senior Application Engineer, Wilcoxon Sensing Technologies

Peter Eitnier is a Senior Application Engineer at Wilcoxon Sensing Technologies in Frederick, Maryland. He holds an ISO CAT II certification as a vibration analyst and a degree in Mechanical Engineering from the University of Maryland. Since joining Wilcoxon in 2012, he has specialized in providing technical expertise to customers in a variety of applications, as well as supporting sales and marketing efforts and product development.