3.1 Degradation in sensor performance can occur due to dropping, mechanical shock while mounted on the test structure, temperature cycles, and so forth. It is necessary and desirable to have a simple measurement procedure that will check the consistency of sensor response, while holding all other variables constant.
3.2 While test blocks of many different kinds have been used for this purpose for many years, an acrylic polymer rod offers the best all-around combination of suitable acoustic properties, practical convenience, ease of procurement, and low cost.
3.3 Because the acoustic properties of the acrylic rod are known to depend on temperature, this practice requires that the rod, sensors, and couplant be stabilized at the same working temperature, prior to application of the practice.
3.4 Attention should be paid to storage conditions for the acrylic polymer rod. For example, it should not be left in a freezing or hot environment overnight, unless it is given time for temperature stabilization before use.
3.5 Properly applied and with proper record keeping, this practice can be used in many ways, such as:
3.5.1 To determine when a sensor is no longer suitable for use.
3.5.2 To check sensors that have been exposed to high-risk conditions such as dropping, overheating, and so forth.
3.5.3 To get an early warning of sensor degradation over time.
3.5.4 To obtain matched sets of sensors and preamplifiers.
3.5.5 To verify sensors quickly but accurately in the field, and to assist troubleshooting when a channel does not pass a performance check.
