Piezoelectric Pressure Transducers,
Design & Use

PRESSURE MEASUREMENTS
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 and one electrical. In Kistler pressure transducers, the mechanical element is the diaphragm, and the electrical element is the quartz crystal or the semiconductor bridge. This section will deal with quartz piezoelectric pressure transducers.

Quartz is the heart of Kistler 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. Pressure measurements with ranges up to 150,000 psi, temperatures up to 350°C, rise times of 1 µsec and resonant frequencies up to 500 kHz are all possible with Kistler piezoelectric pressure transducers.

While quartz-based pressure transducers are ideally suited for measuring dynamic events, they cannot perform truly static measurements. Although the electrical charge delivered under a static load can be registered, it cannot be stored for an indefinite period of time. 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. For this reason Kistler piezoelectric pressure transducers are often described as being "quasistatic."

PIEZOTRON^® pressure transducers use the same quartz sensing element as standard (charge output) piezoelectric units and also include a miniature, built-in charge-to-voltage converter for low impedance voltage output. This allows the usage of general purpose cable in environments where moisture or contamination would be detrimental to the high insulation resistance required with high impedance transducers. PIEZOTRON pressure transducers generally have time constants long enough for calibration, but usually not long enough for static measurements. For this reason they are usually restricted to dynamic applications.

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.

PRESSURE TRANSDUCER DESIGN
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.

Pressure transducers are tailored for a particular application by using a specific housing and diaphragm type together with a suitable technique for preloading the quartz element. This packaging technique distributes the load from the transducer's diaphragm to the preloaded element and ultimately determines its measuring range. High pressure transducers can incorporate a high degree of load shunting around the measuring element which could increase their size and range at the expense of (decreased) sensitivity.

Diaphragm Types
One diaphragm type used by Kistler is the welded sheet metal version for minimal thermal shock error and minimum mass (and maximum frequency response). Another type is the rugged one piece machined diaphragm for applications with high pressure and high cycle rates (i.e. hydraulics, fuel injection, ballistics or combustion engine pressures).

Higher sensitivity can be achieved by increasing the diaphragm size or increasing its effective area.

Mounting Considerations
Installation of a pressure transducer into its mounting bore causes some deformation of the transducer body, threads, diaphragm and element assembly, thereby introducing measurement errors. All Kistler pressure transducers are designed so that the magnitude of the mounting torque has little effect on the measured signal. Generally, high pressure units require large mounting torques (for prop sealing) and hence have a greater potential for measuring errors

The Antistrain design is a patented Kistler technique for minimizing mounting torque sensitivity in (high) pressure transducer installations. This design provides mechanical separation between the mounting threads and the measuring element (see figure below). The measuring element is isolated from the force flow by the annular gap. Consequently, changes in the force flow due to mounting deformation cannot transmit themselves to the measuring element. This design is incorporated into several Kistler pressure transducers for measuring engine combustion and high pressure ballistics.

Antistrain Design

Acceleration Sensitivity
In a pressure transducer, acceleration sensitivity is caused by the parts of the quartz element, sleeve and diaphragm acting as a seismic mass. This error signal is generally very small but can be further reduced by using acceleration compensation. This compensation consists of a tuned acceleration system connected in series, with opposite polarity, to the pressure measuring system. Many Kistler pressure transducers incorporate this feature into their designs.

Temperature Effects
There are two ways that temperature variations effect a transducer's output. One is the Temperature Coefficient of Sensitivity which is defined as a change in sensitivity at different (constant) operating temperatures. The other is the susceptibility of the transducer to temperature shocks or transients.

Ideally a piezoelectric transducer should be operated in an environment with constant temperature. Although quartz itself produces no output change with varying temperatures, the metallic preload acting on the quartz element will expand and contract with the temperature fluctuations. This produces an output which is temperature and not pressure dependent.

There are a number of ways to reduce this temperature dependency in pressure transducers, One method is to water-cool the transducer. Some Kistler pressure transducers have built-in water-cooling tubes and others can be mounted into separate cooling adapters. This method increases a transducer's operating temperature range and is also very effective for maintaining zero stability in long-term measurements. However, water-cooling (by itself) has limited success in reducing the effects of rapid temperature transients. Recessed passage mounting and/or coating the transducer's diaphragm with grease or RTV silicone are methods commonly used to suppress transient temperature effects. Some Kistler ballistic and engine pressure transducers incorporate diaphragms with built-in thermal shields to minimize thermal transient effects.

Another Kistler performance advantage comes from the patented Polystable element. This element utilizes a unique quartz cut for reduced susceptibility to temperature variations and also extends the operating temperature range to 350°C (compared to 260°C for standard units). Kistler pressure transducers with Polystable elements are routinely mounted flush (without water-cooling) in an engine to measure combustion chamber pressures. Some of these units also include water-cooling and/or integrated thermal shields to enhance accuracy.

Linearity and Hysteresis
Two important characteristics which help determine a transducer's overall performance are linearity and hysteresis. Check the glossary for the complete definitions of these terms.

Kistler's design and assembly techniques result in outstanding linearity and hysteresis which are valid over the transducer's entire measuring range. Many Kistler units are provided with 1 % and/or 10% partial range calibration, for which the linearity and hysteresis specifications are the same as the full-scale values. This allows the transducer to be used in pressure ranges which are much less than their full-scale rating.