Acoustic Emission Sensors:
Their Selection and Calibration

AE Sensor Design:
The piezoelectric effect, first demonstrated by the Curie brothers in 1880, occurs when pressure applied on a crystal with unit cells that exhibit no center of symmetry will develop an electric field, or conversely, a mechanical deformation will occur at the application of an electric field. The design of PAC's sensors (transducers) is structured upon the optimization of several piezoelectric parameters coupled with our know-how of delicate final assembly.

The make-up of a typical AE sensor along with its piezoelectric element, in this case a crystal (Figure 1). Characteristic evaluation of the piezoelectric element, is dependent on the accurate measurements of density, size, clamped capacitance and low field dissipation facto. We inspect and control all relevant parameters of our piezoelectric elements (along with ascertaining their impedances and critical frequencies per IEEE standards) because we manufacture them. At PAC, our sensors are designed by AE engineering specialists.

PAC sensors are either single-ended (S) or differential (D) in construction. The single-ended design employs a single crystal to provide high sensitivity and omni-directional response, regardless of orientation to the AE excitation. The differential designs provide common mode rejection of unwanted signals in environments of high electromagnetic interference. The output of a differential sensor is processed by a differential amplifier to provide 24dB common-mode rejection.

In our sensor manufacturing facilities, quality materials and workmanship combine to produce reliable, high-performance sensors. These sensors are handcrafted by skilled and experienced assemblers who consistently produce superior units. The sensor housing and integral electronics are designed to eliminate RFI/EMI and microphonic interference.

AE Sensor Calibration:
Calibration, traceable to the U.S. National Institute of Standards and Technology (NIST), formerly NBS, is the important final element of the manufacturing process. A sensor's response during a test can be predicted by its frequency response, provided that an appropriate calibration method is used. Recognizing this fact, NIST developed the surface wave calibration method. This procedure subjects the sensor to a surface wave similar to that of an actual AE event.

PAC calibration uses either the NIST Transient Surface Wave Calibration (ASTM E1106-86, Standard Method for Primary Calibration of AE Sensors) or the White Noise Continuous Sweep method(ASTM E976-84, Standard Guide for Determining the Reproducibility of AE Sensor Response), otherwise known as the Face-to-Face Technique.

All PAC sensors come with a certificate displaying characteristics and method of calibration.  The Transient Surface Wave Technique is ideal for AE burst

applications. Sensitivity is expressed in voltage output per vertical velocity vs. frequency. The Face-to-face Technique is based on voltage output per unit of pressure input and is recommended for continuous AE monitoring applications. The frequency response is used to select the proper filters for the preamplifiers and the analysis instrumentation.

Mounting a Sensor:
An essential requirement in mounting a sensor is sufficient acoustic coupling between the sensor face and the structure surface. First, make sure that the sensor's surface is smooth and clean, allowing for maximum couplant adhesion. Application of a couplant layer should be thin, so it can fill gaps caused by surface roughness and eliminate air gaps to ensure good acoustic transmission. The sensor should be held firmly to the testing surface at all times. Commonly used couplants are vacuum greases, water-soluble glycols, slovent-soluble resins, and proprietary ultrasonic couplants.

In addition to coupling, the sensor must always be stationary. One way to achieve this is to use glue, which can also serve as a couplant. Before using a glue, consider the ease and technique of removal, since not all sensors can withstand a large removal force between the housing and the mounting face (wear plate). To prevent attenuation, avoid air bubbles and thick glue layers. Another way to help a sensor stay stationary, is to use a holding device such as tape, elastic bands, springs, magnetic hold-downs, and other special fixtures. Caution: Make sure that any mechanical mount used does not make electrical contact between the sensor case and the structure.

Other sensor mounting precautions lie in whether the mounting surface is either very hot or very cold. Typical couplants may be unstable on hot surfaces or may freeze solid and debond on cold surfaces. A metal rod which conducts the acoustic signal for the hot or cold structure to the sensor at an ambient temperature, called a waveguide, is a common solution.

Summary:
PAC sensors have been custom designed for use on various structures and accommodate almost all of the varying conditions presented in AE testing. As applications for AE testing expand, PAC designers develop new sensors to meet newly-identified consumer needs. Whatever the application, PAC acoustic emission sensors are there listening!