Davidson Measurement Tech Tips

Vibration Sensor Characteristics

Looking at vibration sensors and the precautions that need to be taken, especially for those carrying out condition monitoring surveys and resulting predictive maintenance activities.

Condition monitoring is based on the philosophy of taking measurements on a regular basis and then comparing them over a period of time, which maybe weeks, months if not years. As a result it is essential to use a sensor that is reliable and repeatable over time. When the aim is to protect a piece of machinery, which can cost anything from a few dollars for a bearing to hundreds of millions of dollars, the last thing you need is an unreliable sensor. So what are the key factors?

Accuracy - though a very much talked about factor with sensors when it comes to condition monitoring it is not an essential characteristic. Sure, it has to be within certain limits for the benchmark reading, however, once that is taken then accuracy does not come into the equation; the need is for repeatability and reliability.

Repeatability - once we start taking measurements from the benchmark, we need full confidence that the sensor has not changed its sensitivity as a result of drift within either its element or any built-in electronics. Predictive maintenance is based on changes from one reading to the next, so if the sensor characteristics do not change then you know any change in output is caused by change in the vibration of the structure, the very thing that tells us about the health of the machine.

Temperature drift - also a critical factor for any change in temperature could be seen on the output, thus it may look as though the vibration of the machine has improved or become worse when in fact it was as a result of a temperature change. Precautions can be taken to minimise temperature effects, eg placing a rubber boot over the sensor or keeping it in an area with constant temperature airflow.