Piezoelectric pressure sensors use a specialized material to create a small voltage when mechanical stress is applied to it. In this blog we will explain the principles behind the Piezoelectric pressure sensor, review its specific attributes and then discuss how it is used in vacuum gauge technology and industry applications.
A Piezoelectric pressure sensor functions on the principle that when mechanical stress is applied to a piezoelectric crystal, an electric potential is generated which is directly proportional to the pressure applied. A vacuum gauge that uses a piezoelectric pressure sensor typically houses the sensor in the diaphragm. This provides good linearity for vacuum transducers as the output signal correlates to the applied pressure. This signal is then used to produce an output voltage that is converted to a pressure measurement. Piezoelectric sensors are rugged and often used for measuring dynamic pressure. Because Piezoelectric sensors have a high sensitivity to dynamic changes in pressure, they are well-suited to the measurement of small changes, even in very high-pressure environments. Although they have a high sensitivity to dynamic pressure that can also be used to measure static pressures.
Piezoelectric pressure sensors are typically used for measuring atmospheric pressure but can be paired with additional vacuum, pressure sensor technologies to create a wide-range vacuum gauge capable of measurement from vacuum to atmosphere. In some cases, a wide-range vacuum gauge can be further expanded upon by adding a 3rd technology to provide a compact solution capable of wide-range measurement from atmospheric pressure to ultra-high vacuum.
In vacuum technology, pressure measurement is accomplished using either “Direct” or “Indirect” methods. Direct gauges are so-called because they directly measure the force imparted on a surface. Based on the formula: P = F /A (pressure (P) equals force (F) per unit area (A), the gauge directly measures the pressure. Some examples of direct gauges include: bourdon gauges, capacitance manometers and Piezoelectric gauges.
Bourdon Gauge
One of the primary benefits of a direct vacuum gauge is the ability to make accurate measurements regardless of gas type. For example, if the system has 20 Torr of argon, helium, methane, or air, a direct measurement gauge will read the same pressure. Because of this attribute, direct gauges are referred to as “gas composition independent”. These are helpful to see the operating pressure being used during the process.
Indirect gauges do not “directly” measure the force associated with the gas in the chamber. Rather, these gauges measure a property associated with the gas. For example, thermocouple vacuum gauges measure the thermal conductivity of the gas, which is a function of the pressure. Another example of an indirect gauge is the pirani vacuum gauge which measures the pressure-dependent thermal conductivity of the gas in a vacuum using a heated element, such as a wire or thin-film membrane. The heated element is part of a resistance bridge. The temperature, and thus the resistance of the heated element in the vacuum, changes as the pressure changes. By measuring the electrical behavior of the bridge, the pressure in the vacuum can be determined. Consequently, thermocouple and Pirani vacuum gauges can both be called indirect gauges.
Thermocouple Gauge Pirani Gauge
Because the HVG-2020A measures from 0.1 Torr to 1000 Torr, it is well-suited to vacuum pressure impregnation (VPI) applications. Vacuum pressure impregnation (VPI) is an important application for applying insulating materials, as well as producing void-free castings. A typical example is the encapsulation of windings in electric motors. If an insulating resin is simply “painted” on a winding, the result will be a network of uninsulated voids between the winding layers. Applying pressure may force some insulation into the voids, but the subsequent release of that pressure will cause the trapped gas to expand again, causing voids to reappear.
These voids in the insulation can lead to motor failure due to movement of the wiring during operation. Also, in high-voltage applications, these air-filled gaps can serve as sites for corona discharge formation, leading to losses in efficiency and resulting in further weakening of the dielectric.
The proper method to prepare windings, and other potted devices for impregnation, is to begin by applying vacuum.
The HVG-2020A is easy to install, can be configured with an optional touchscreen display to offer a choice of data views, and provides both analog and digital output for process control integration.
Analog I/O: The HVG-2020A has a 9-pin D-sub connection on top of the gauge that allows an analog output signal to be measured amongst other features. The selected linear analog output signal is proportional to the full-scale range of the sensor (1000 Torr).
All of these digital communication options (with the exception of TTL) enable PC connection and allow monitoring and viewing of data remotely using our free Microsoft® Windows®-based software. This software has many useful features including data logging and customization / configuration of the vacuum gauge. Digital communication is also used to change the analog output, adjust Hi and Lo setpoint values, stream pressure readings, or change pressure units (among many other functions), when the HVG-2020A is not configured with the optional display.
Touchscreen Display: The most powerful feature of the HVG-2020A is the optional touchscreen display which allows monitoring of pressure measurements in a variety of combinations and graphic representations while operating. The display is powered off the vacuum gauge power supply (no additional power supply needed) and is especially useful for installations in which a remote display would be inconvenient. Five different display modes (shown left to right below) include: Pressure, Pressure and Temperature, Setpoint, Bar Graph, and Pressure over Time. Note that the pressure measurement is always displayed in each mode.
The touchscreen’s Menu Button allows the user to cycle through a selection of submenus to change the screen orientation (should the gauge be mounted in a position other than vertical), zero the gauge (only performed if the system pressure is known to be well below 0.1 Torr), view device information (serial number and firmware level), change the analog output, select RS232 or RS485 and a number of baud rates, and restore the vacuum gauge’s configuration to factory default settings. The straight-forward arrangement of measurements and easy to read display, lets you “see clearly”, similar to 20/20 vision!
Rough Vacuum Monitoring