HERMETIC SEALING AND TEST METHODS Loosely defined as a “metal to metal weld, braze, solder or metal to glass fusion,” hermetic sealing is a vague (andoften abused) term. Even a porous weld would qualify as hermetic even though its ability to prevent contaminationwould be negligible. Epoxy sealed sensors are not considered hermetic. In order to quantify and compare levels of hermetic sealing, we can use an example leakage scale (see table below) where the leakage rate of helium is stated in cc/second at 1atmosphere of pressure. Wilcoxon industrial sensors have the highest hermetic industrial seal rating in the industry. Wilcoxon uses theHelium Leak Test (HLT) to qualify the sealing of all sensor designs and maintains a continuous monitoring program to insure the proper sealing of all hermetically rated industrial accelerometers.

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Leakage RateExampleTime for 1 ccSuitable Test (cc/sec)to Leak (@1 atm.)Method 10
–4 Poorly Built Accelerometers2.78 hoursBubble Test10
–6 Beverage Cans11.57 daysHLT10
–7 Vacuum Process Systems3.86 monthsHLT10
–8 Typical Wilcoxon Industrial Accelerometers3.22 yearsHLT10
–9 Pacemaker32 yearsHLT Bubble Testing (simplified description) A bubble test is performed by subjecting units to be tested to a pressurized atmosphere. 50 to 100 Pounds per Square Inch (PSI) wouldbe typical of the pressure used. The units are exposed, typically, for thirty minutes to one hour. At the end of the pressure soak theunits are submerged under water and observed to see if bubbles form on the surface, bubbles are continuously formed and break free,or a stream of bubbles emanate from the unit under test. The lowest leak rates detectable by bubble testing are around 1 x 10
-5 cc/second. Helium Leak Test (simplified description) HLT is performed by placing units to be tested in a chamber and then pressurizing that container with Helium. Pressures of 90 to 120PSI are typical of the exposure for standard leak tests. Wilcoxon uses 120 PSI. After an appropriate time of exposure, the units areremoved and, one at a time, placed in a test chamber. A vacuum is drawn and, when the vacuum is low enough, the remainingescaping gasses are sent to a mass spectrometer tuned to detect Helium. The HLT equipment can detect leak rates to 1 x 10
-9 andbelow. Units with leak rates above about 1 x10
-3 will saturate the detector and are usually considered “gross leakers.” A “gross leaker”is not considered hermetic, even though it is welded and may look sealed! Wilcoxon Quality All accelerometers rated as “hermetic” on the data sheet are qualified using HLT. Wilcoxon uses the results of HLT in settingparameters for welding equipment to insure consistent hermetic qualified welds. Since all hermetically rated accelerometer designshave been subjected to pressure testing of 120 PSI, they are rated to withstand 100 PSI of pressure from submersion. Since Heliummolecules are very tiny compared to water molecules, passing an HLT insures surviving underwater submersion for an indefiniteperiod of time. OTHER ENVIRONMENTAL CONSIDERATIONS INCLUDE SHOCK OVERLOAD PROTECTION, RADIO FREQUENCY (RF)INTERFERENCE SUSCEPTIBILITY AND INTERMODULATION OR WASHOVER DISTORTION.

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Sensors used with a magnet in a walkaround program need to incorporate shock protection to protect against the very highacceleration levels associated with “snapping” the magnet onto equipment. Because tests at Wilcoxon have shown shock levelsdue to magnet mounting can reach several thousand g’s, a protection level of 5,000 g was built into all industrial accelerometers.High levels of RF signals in the area of an accelerometer have potential to introduce spurious signals which can be mistaken for machineryvibration. To protect against these false signals, Wilcoxon uses an internal shield within the sensor and employs the highest level RFIprotection circuitry in the industry. (Refer to Junction boxes for more information).Very high frequency signals can generate “washover” in the usable band of an accelerometer. Washover shows up as false signals thatcould be misinterpreted as vibration data. Also known as intermodulation, generation of these signals is common in high noise environments. Many Wilcoxon industrial sensors protect against washover distortion with the use of special filtering techniques.
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WILCOXON RESEARCH, INC. • 1-800-945-2696 • TEL 301-330-8811 • FAX 301-330-8873 • EMAIL sensors@wilcoxon.com • WEB www.wilcoxon.com

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