In order to generate an electrical output from a pressure input, that pressure must first be converted into a proportional displacement or strain. This strain is then transmitted to an electrical transduction element which generates the required signal. Thus, most transducers are comprised of two main components, one mechanical, such as the diaphragm, and one electrical, such as, the quartz crystal or the semiconductor bridge.
Quartz is the heart of piezoelectric pressure transducers. Its characteristics of long-term stability, high rigidity and strength, wide measuring range and wide temperature range make it the ideal sensing element for dynamic pressure transducers.
While quartz based pressure transducers are ideally suited for measuring dynamic events, they cannot perform truly static measurements. For static measurements, highly insulated materials must be used in the transducer cables and connectors to insure a maximum discharge time constant and optimal operation of the charge amplifier (i.e. minimal drift). Since quartz has a very high insulation resistance (>1013 ohms), short-term static pressure measurements are more feasible than with any other piezoelectric material. Quartz based piezoelectric systems can routinely measure large pressures for minutes and perhaps even hours. Low level pressures can be measured "statically" for much shorter intervals.
Piezoelectric pressure transducers are also referred to as "gage" or "relative" because they produce an output only when they sense a change in pressure. Since most measurements originate at atmospheric pressure, this point is often referred to as "zero" pressure when using piezoelectric systems. Other types of pressure measurement are absolute or differential. Absolute pressure measurements are made with respect to vacuum, while differential pressure is measured between two different points.
Quartz pressure transducers consist of three basic parts: the transducer housing, the quartz sensing element and the diaphragm for transferring the pressure to the element. One diaphragm type is the welded sheet metal version for minimal thermal shock error and minimum mass (and maximum frequency response). In addition, higher sensitivity can be achieved by increasing the diaphragm size or increasing its effective area.
Direct thermocouple measurements can be made during cutting. The measurement system is shown in the following figure. The rig was first run without cutting, and the reading on the millivoltmeter resulting from the rubbing action of the constant wire on the work piece was recorded. This reading was subsequently subtracted from the readings taken while cutting was in progress. With this method, the temperature at selected points around the end face of the tubular work piece were measured and then used to calculate the proportion of the shear zone heat conducted into the work piece.
Radiation Methods. When the tool-workpiece area can be observed directly, camera and film sensitive to infrared radiation can be used to determine temperature distributions. In this technique, a furnace of known temperature distribution was photographed simultaneously with the cutting operation using an infrared-sensitive plate, enabling the optical density of the plate to be calibrated against temperature.