Guidance in Selecting the Correct Accelerometer

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Guide to selecting the correct accelerometer

Power Supply
The most commonly used is the 2 wire piezo-electric accelerometer with an integrated circuit that is driven from 18 to 28V DC with a constant current of 2 to 10mA. This is typical of the PZS range of accelerometers and the power is provided by the online vibration monitors (Sentry, Aegis, DN2601, Plantscan etc.) or from the portable data collector/analyser. More common with older designs is a 3 or 4 wire device where the power supply connection is separate from the output signal. Of this type there are normally 2 types, 12V DC supply or 24V DC supply. The 4-20mA loop powered accelerometers are 2 wire with a 16-32V DC supply required.

 

Output Signal
The output sensitivity of the PZS range of accelerometers is 100mV/g and this is suitable for most applications and is generally considered to be the “industry standard”. The PZST4 accelerometer is available with the normal 100mV/g output for vibration together with a 10mV/°C output for surface temperature.

In rare applications where high levels of g are expected (above 70g Peak), lower sensitivity 10mV/g accelerometers are available. A higher output of 500mV/g is available for applications where the amplitude of the vibration is very low, this is often the case with Hydro Power Plant where slow rotation is also an issue. Sensonics can manufactures a version of the PZS accelerometer ideal for slow rotating machines with 500mV/g output sensitivity and a frequency response down to 0.6 Hz/36 RPM (3dB). For seismic monitoring (H2) or structural vibration (H2) the 4V/g SP4 is used down to 0.1Hz (3dB).

With 10µA/g PZA4 shock response accelerometer is used for early detection of problems with rolling element bearings.

The PZDC provides a direct 4-20mA output for connection to a PLC/DCS/BMS system. A dual output option is available providing both 4-20mA and 100mV/g signals for vibration.

Frequency Response
This is entirely dependant upon the speed & type of machine to be monitored. ISO 10816 provides details but the 0.4Hz to 11kHz (3dB) of the PZS3 accelerometer covers the common applications. The lighter/smaller structure of the PZP1 accelerometer offers 0.5Hz to 20kHz (3dB).

Harsh Environments
Most of Sensonics' accelerometers are manufactured in stainless steel and sealed to IP66 or IP67. Submersible accelerometers sealed to IP68 up to 10BAR are available for underwater applications with 316 grade stainless steel or duplex alloy used for sub-sea.

Intrinsically safe accelerometers with ATEX approval for use in areas with potentially explosive atmospheres is an option with the PZS3, PZS4 or PZS5.

Standard operating temperatures are 80/120/140°C depending upon options selected whilst higher temperature options are available at 150°C for the PZA6, 240°C for PZHT or PZP1 and 450°C for the PZEHT. The PZP1 accelerometer also has a radiation resistant option available for use in a nuclear radiation environment.

An internal shield such as a secondary internally isolated case is utilised on the PZS3, PZS4 & PZS5 to provide extra protection against high levels of electromagnetic interference and thermal shock.

Connectivity
Connection options vary from range to range but the options include integral cable with plain cable, armoured cable, armoured high temperature cable or armoured flexible conduit. The integral connector option with 2 or 3-pin MIL, M12, BNC/TNC or Microdot. Top exit connection is lower cost although side exit is more convenient.

Mounting
Lower cost and easiest to install is the single point mounting where the surface should be drilled, tapped and spot-faced where necessary to provide a flat mounting surface. If it is inappropriate to drill and tap, a glue-on stud may be used with high strength epoxy to bond it to the machine and thus providing a suitable means to attach the accelerometer. Many threads are available with the most common being ¼” UNF or M8, the Quickfit (3/4 turn to lock) is preferred by some. 3-point mount accelerometers are used for the most rugged of requirements on larger machines where large flat areas are available to mount. Isolation mounting pads or studs must be used when mounting accelerometers on alternators/generators or where ground loops exist from poor earthing.
 
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