Nine Questions and Nine Answers for Industrial Vibration Sensor Selection

　　Selecting an accelerometer for predictive maintenance is a daunting task, even for experienced engineers. This process can usually be filtered into nine questions. Find out the answers to the following nine questions, apply them according to your actual situation, and you will be able to find the vibration monitoring solution.

　　Question 1: What are you trying to measure? This may seem obvious, but think twice. What are you really trying to measure? In other words, what are you trying to do? What are you hoping to get? What are you going to do with the data? Accelerometers monitor vibration and provide raw vibration data, while vibration transmitters provide root mean square (RMS) )value. It is useful to analyze raw vibration data because it contains information on all vibration signals, true peak amplitude and vibration frequency. Because the RMS total or peak value is a continuous 4-20 mA signal, it is very useful in eg PLC, DCS, SCADA systems and PI control systems. Some applications use both signals at the same time. By identifying the various signals required by the application, the search can be narrowed down considerably. Also, are you measuring vibration using acceleration or velocity or displacement? Have you considered that some industrial sensors can output vibration and temperature at the same time? Later, some field applications, such as vertical pumps, are good for monitoring more than one shaft vibration. Does your field application require single, dual or triaxial measurement?

　　Question 2: How big is the amplitude? The amplitude or range of the vibration being measured determines which range of sensors to use. Typical accelerometer sensitivity is 100 mV/g for standard applications (50g range) and 500 mV/g for low frequency or low amplitude applications (10g range). 4-20 mA transmitters for general industrial applications typically use a range of 0-1 in/s or 0-2 in/s.

　　Question 3: What is the vibration frequency? The physical structure and dynamical system respond differently to different excitation frequencies. Vibration sensors are no different. Piezoelectric materials behave like high-pass filters, so even good piezoelectric sensors have a low frequency limit of about 0.2Hz. The sensor acts as a single-degree-of-freedom dynamic system with a natural resonant frequency. The signal is greatly amplified at the natural resonant frequency, resulting in a significant change in sensitivity, possibly overrange. Most industrial accelerometers have single or dual RC filters to offset the excited resonant frequencies. It is critical to choose the frequency range available to the sensor that includes the frequencies you are interested in.

　　Question 4: What is the ambient temperature? For ICP accelerometers and 4-20mA transmitters, extremely high ambient temperatures pose a threat to the electronics inside. Charge-mode accelerometers can be used in very high ambient temperatures and have no built-in electronics, instead using a remote charge amplifier. The charging mode acceleration sensor is equipped with an integrated hard-wired cable, which can be used in environments where temperatures exceed 260°C, such as gas turbine vibration monitoring.

　　Question 5: Will it be immersed in liquid? Industrial accelerometers with integral polyurethane cable can be immersed in liquid installation. For high pressure applications, good sensors are pressure tested for one hour. Fully submerged applications require an integral cable. Integral cables are also required in applications that are sprayed rather than completely submerged, such as machine tool cutting fluids.

　　Question 6: Will there be exposure to potentially harmful chemicals or debris? Industrial accelerometers can be constructed from corrosion and chemical resistant stainless steel. In environments with hazardous chemicals, the sensor considers the use of teflon corrosion-resistant connecting cables. It is highly recommended to check the chemical compatibility chart for any suspected chemical. For environments exposed to chips, one-piece armored cable provides good protection.

　　Question 7: Do you need ejector, offset, compact links? End sensors all need to be installed in the available space of the device. The shape of the sensor has little effect on its performance, but needs to be considered for safe installation and maintenance operations in the field. A small accelerometer with a cage nut design that can be held in any orientation, but is handy when it comes with an integrated cable.

　　Question 8: Are you using a high-accuracy or low-cost sensor? There are two main differences between low-cost and high-accuracy accelerometers. First, the units of accuracy are usually fully scaled, which means that sensitivity response measurements are plotted over the available frequency range. Low-cost accelerometers are single-point calibrated, making sensitivity measurements at only one frequency. Second, high-precision accelerometers have tight tolerances in certain specifications such as sensitivity and frequency range.

　　For example, a high-accuracy accelerometer has a nominal sensitivity of 100 mV/g ± 5% (95 mV/g to 105 mV/g), while a low-cost accelerometer has a nominal sensitivity of 100 mV/g ± 10% (90 mV/g to 110mV/g). Customers can set the sensor's calibrated sensitivity in the data acquisition system, so that low-cost sensors can also provide accurate, repeatable data. As for frequency, high-accuracy accelerometers are typically 5% off, while low-cost sensors can provide a frequency range of 3 dB. Nonetheless, a low-cost sensor can provide excellent frequency response.

　　Question 9: Do you need a special certification code? Both CSA and ATEX certified accelerometers and 4-20 mA transmitters can be used in hazardous areas. Compare sensor certifications to make sure it meets your needs. The answers to nine questions can greatly narrow your search for applied solutions. Bear in mind that the combined answers may be mutually exclusive, that is, solutions that satisfy each criterion do not exist. For example, certain models for use in hazardous areas may not have ATEX certification. Additionally, specialized field applications may have other considerations.