BY BRÜEL & KJÆR
Piezoelectric Materials and Temperature
Typical general-purpose accelerometers can tolerate temperatures up to about 250°C.
At higher temperatures, the piezoelectric ceramic will begin to depolarize so that the sensitivity will be permanently altered. Such an accelerometer may still be used after recalibration if the depolarization is not too severe.
For temperatures ranging from -196°C up to 482°C, accelerometers with special piezoelectric materials are available.
All piezoelectric materials are temperature dependent so that any change in the ambient temperature will result in a change in the sensitivity of the accelerometer. For this reason, all B&K accelerometers are delivered with a typical sensitivity versus temperature curve so that measured levels can be corrected for the change in accelerometer sensitivity when measuring at temperatures significantly higher or lower than 20°C.
Piezoelectric accelerometers also exhibit a varying output when subjected to small temperature fluctuations, called temperature transients, in the measuring environment. This is normally only a problem where the very low level or low-frequency vibrations are being measured. Modern shear-type accelerometers have a very low sensitivity to temperature transients while compression types can have 100 or more times higher outputs.
When accelerometers are to be fixed to surfaces with higher temperatures than 250°C, a heat sink, and mica washer can be inserted between the base and the measuring surface. With surface temperatures of 350 to 400°C, the accelerometer base can be held below 250°C by this method. A stream of cooling air can provide additional assistance.
Accelerometer Cable Noise
Since piezoelectric accelerometers have a high output impedance, problems can sometimes arise with noise signals induced in the connecting cable. These disturbances can result from ground loops, triboelectric noise, or electromagnetic noise.LEARN MORE
Currents sometimes flow in the shield of accelerometer cables because the accelerometer and measuring equipment is earthed separately. The ground loop can be broken by electrically isolating the accelerometer base from the mounting surface using an isolating stud and mica washer or isolation adaptors. In modern instruments so-called floating inputs can be used if the voltages are not too high.
Tribo-electric noise is often induced into the accelerometer cable by the mechanical motion of the cable itself. It originates from local capacity and charge changes due to dynamic bending, compression, and tension of the layers making up the cable. This problem is avoided by using a proper graphited accelerometer cable and taping or gluing it down as close to the accelerometer as possible. For CCLD accelerometers this is normally not a significant problem.
The noise from electromagnetic interference is induced in the accelerometer cable when it lies in the vicinity of running machinery. A double-shielded cable helps in this respect, but in severe cases, a balanced accelerometer and differential preamplifier should be used.
Environmental Factors Affecting Accelerometers
When an accelerometer is mounted on a surface that is undergoing strain variations, the output will be generated as a result of the strain being transmitted to the sensing element. Accelerometers are designed with thick, stiff bases to minimize this effect: DeltaShear® types have a particularly low base strain sensitivity because the sensing element is mounted on a center post rather than directly to the accelerometer base.
Most B&K accelerometers (NOT CCLD) can be used under gamma radiation doses of 100 Gy/h (10kRad/h) up to accumulated doses of 20 kGy (2 MRad) without significant change in characteristics. Certain accelerometers can be used in heavy radiation with accumulated doses of more than 1 MGy (100 MRad).
The magnetic sensitivity of piezoelectric accelerometers are very low, normally less than 0,1 to 2,5 m/s2 per T (0,01 to 0,25 m/s2 per kGauss) in the least favorable orientation of the accelerometer in the magnetic field.
B&K accelerometers are sealed, either by epoxy bonding or welding to ensure reliable operation in humid environments. For short-duration use in liquids, or where heavy condensation is likely, Teflon sealed accelerometer cables are recommended. The accelerometer connector should also be sealed with an acid-free room temperature vulcanizing silicon rubber or mastic. Industrial accelerometers with integral cables should be used for permanent use in humid or wet areas.
The materials used in the construction of all Brüel & Kjær accelerometers have a high resistance to most of the corrosive agents encountered in the industry. The main components are titanium and stainless steel.
The noise levels present in machinery are normally not sufficiently high to cause any significant error in vibration measurements. Normally, the acoustically induced vibration in the structure on which the accelerometer is mounted is far greater than the airborne excitation.
Piezoelectric accelerometers are sensitive to vibrations acting in directions other than coinciding with their main axis. In the transverse plane, perpendicular to the main axis, the sensitivity is less than 3 to 5% of the main axis sensitivity (typically < 2%). As the transverse resonant frequency normally lies at about 1/3 of the main axis resonant frequency, it should be considered where high levels of transverse vibration are present.