Choosing an accelerometer

Accelerometer selection: important factors

 
With the variety of vibration sensors available, choosing the right one for your application can be difficult. You’ll need to determine the characteristics of the machine and the operating environment as well as the sensor specifications. There are several factors that need to be taken into consideration before you proceed to checkout.

Overall vibration level

Before investing in a sensor, you should familiarize yourself with the vibration levels of the machine. What are the typical vibration levels, and what are the highest levels? At what frequencies do the faults you want to detect occur? Answering these questions will help you figure out the sensitivity and acceleration range best suited to your application.

For most rotating machinery, with overall vibration levels up to 10 g rms, a standard 100 mV/g accelerometer will capture the data you need. For these sensors, an 80 g range is typical and adequate for most applications. It’s important that the highest vibration levels of the machine do not exceed the acceleration range of the sensor, because high amplitude vibrations can cause sensor overload. This leads to signal clipping and distortion over the entire operating frequency of the sensor.

Frequency range

If the frequency range of interest isn’t already known, you can usually determine it by evaluating data from similar systems or applications or by placing a test sensor at several locations on the machine. You’ll also need to know the fault frequencies you want to be able to detect. Most industrial machinery vibrations have signal components of interest at frequencies much higher than running speed; exactly which frequencies these will be depends on the type, number and placement of the component parts of the machine, the specific faults you want to detect, and other factors. After determining the frequency range of interest, make sure the sensor’s bandwidth can capture the high frequency requirement. If the frequency is higher than what the sensor can measure, the signal becomes corrupted and readings are unreliable.

There’s a tradeoff between sensitivity and frequency range – high sensitivity accelerometers are better for measuring lower frequencies, and low sensitivity accelerometers capture higher frequencies.

  Sensor sensitivity vs. frequency response

Consider using high-frequency, low-sensitivity sensors for monitoring bearings for roller element defects, measuring gear mesh fault frequencies, or other high frequency applications. On slow-speed machinery, low-frequency (0.1 to 10 Hz), high-sensitivity sensors are preferable for monitoring overall machine vibration. However, 100 mV/g accelerometers are a good, cost-effective option for most applications.

Temperature range

Standard industrial accelerometers with built-in electronics can withstand temperatures up to 120°C (248°F). Any higher and you run the risk of damaging the sensor’s electronic components, which will result in unreliable data and shorter sensor lifespan.

If the ambient temperature exceeds 120°C, a high temperature sensor designed to operate up to 150°C (302°F) without loss of performance should be used instead. Determine if any thermal transients are present near the installation point, because these can cause metal case expansion and interfere with measurement accuracy. Above 150°C, you’ll generally need a charge mode accelerometer without internal electronic circuitry and a charge amplifier.

Additionally, check sensor specifications for sensitivity variation with temperature to make sure it’s acceptable for your measurement requirement. Operating temperatures close to the upper limit of the sensor’s range may not outright damage the electronic components but can certainly affect measurement accuracy.

Environmental factors

There are environmental factors other than temperature to take into account. What are the humidity levels of the area where the sensor will be installed? Are any dusts, gases, liquids, or corrosive chemicals present that could contaminate or damage the sensor?

The hermetic seal standard on modern accelerometers helps prevent intrusion of contaminants or moisture. If there are flammable contaminants (dusts, vapors or gases, fibers) present where the sensor will operate, make sure the sensor you choose has the appropriate hazardous location certification. (Read more about hazardous area sensors here.)

Check for any nearby sources of electromagnetic or radio frequency interference (EMI or RFI, respectively) or electrostatic discharge (ESD). The accelerometer should be EMI/RFI shielded and built to withstand ESD up to 20 kV.

Mounting locations

The space available to mount the sensor must be taken into consideration. If there’s less space around the machine, a side exit sensor can be a better choice to allow enough room for a cable run. Dual-output or triaxial accelerometers take multiple measurements from a single point and can be a good option where space is limited.

The short version

Figure out the overall vibration level, what fault frequencies you need to be able to measure, and any special environmental conditions or certification requirements.

Use our sensor selection checklist to help you keep track of the information you'll need to choose the best accelerometer. And, of course, you can always contact our technical services team for assistance.