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POINTS TO CONSIDER There are many factors to consider when select- ing an accelerometer for testing purposes:
Mass/size – Mass loading is the effect of adding mass to a test structure and in doing so changing the natural frequency. This must be minimised. Sensitivity – IEPE accelerometers are fixed sensitivity and have a fixed measurement range, due to their nominal output of 5-7VDC. Anything above this will cause an overload of the onboard electronics. Frequency range – All accelerometers have a flat frequency response range over which the sensitivity does not vary by more than ± five per cent. ensure this range matches the test range. Operating temperature – Ensure the test conditions are within the operational range of the accelerometer. Connector orientation – Top or side entry connectors are available, using the correct option will reduce stress on the cable connec- tion, this is critical to ensure no data loss. Mounting method – This is a huge topic and an area where significant issues can be caused: • Stud mount – gives excellent high frequency
responses and is widely considered the best solution if you can drill a hole in your test item. • Adhesive mount – use as little adhesive as
possible, too much and you will dampen the vibration response and cause errors in your data. Cyanoacrylate is commonly used. Be careful when removing them. Do not knock them off. Twist them off with a spanner or use the teardrop tool provided. • Petro Wax – great for low frequency modal
testing or when you want to move accelerome- ters often. Use as little wax as possible, only use at normal ambient temperatures and be aware of the frequency limitations.
CALIBRATION DJB recommend annual calibration. If you are testing at high or low temperature, consider having your accelerome- ters calibrated at your test temperature. Ensure your calibration frequency range covers your testing frequency range so you can prove accuracy. Ensure your accelerometers are checked for cross-axis during your calibration. This is very important and a feature which is best controlled by DJB’s patented unique Konic Shear design.
CABLES Low Noise or Non-Low Noise? That is the question (see DJB’s Youtube page for a practical demo). To control triboelectric noise caused by cable
movement, all charge accelerometers must be used with low noise cables, otherwise data accuracy will be com- promised. IEPE accelerometers
can be used with almost any type of cable including non- low noise types. These are some of
the key points but there is a lot more detail that should be considered for each of these. If you would like to know more or see how accelerometers work and are built consider attending one of DJB’s full training courses at their UK factory and remember ‘If in doubt ask!’. DJB engineers are available to guide you on the correct choice for your test.
DJB Instruments UK T: 01638 712288
www.djbinstruments.com
MEASUREMENT & SENSORS DIRECTORY | 2017-2018 7
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