INTRODUCTION
Pressure gauges should be selected considering the media and ambient operating conditions. Gauge selection should take into consideration the corrosive environment in which it is to operate. The media being measured must be compatible with the wetted parts of the pressure instrument. Improper application can damage the gauge, causing failure or personal injury and property damage. Diaphragm seals (also called gauge isolators) can be added to the system to protect the gauge from corrosive attack, and prevent viscous or dirty media from clogging Bourdon tube pressure gauges.
There are several common types of mechanical pressure measurement devices including bellows, Bourdon tube, capsule element and diaphragm element gauges. Bourdon tubes are circular shaped tubes with an oval cross-section. The pressure of the media acts on the inside of this tube resulting in the oval cross-section becoming almost round. Because of the curvature of the tube ring, the Bourdon tube bends when tension occurs. The end of the tube, which is not fixed, moves thus being a measurement of the pressure. Bourdon tubes with a number of superimposed coils of the same diameter (helical coils) are used for measuring high pressures.
Capsule element gauges consist of two circular shaped, convoluted membranes sealed around their circumference. The pressure acts on the inside of the capsule and a pointer indicates the generated stroke movement. Pressure gauges with capsule elements are more suitable for gaseous media and relatively low pressures.
Diaphragm element gauges combine both a chemical seal and a pressure gauge into one unit. Diaphragm elements are circular shaped, convoluted membranes that are either clamped around the rim between two flanges or welded in place. The measured media exerts a force on the diaphragm. A metal pushrod welded to the top of the diaphragm transmits the deflection of the diaphragm to the linkage. The linkage, in turn, translates the lateral motion of the push rod into a rotational motion of the pointer.
Sensors, detectors and transducers covers a wide category of devices used to monitor, measure, test, record, yze and/or display data as generated due to changes in a measured norm. Major sensor and sensor switch categories include acceleration and vibration, acoustic, ytical, density and specific gravity, electrical and electromagnetic, encoders and resolvers, environmental, flow, force, gas, humidity and moisture, level, linear and orientation position, pressure, proximity or presence, rotary position, temperature, tension, tilt, torque, velocity, viscosity, and weather sensors.
Pressure sensors are devices that read changes in pressure, and relay this data to recorders or switches. They are commonly used in safety devices such as safety mats, edges and bumpers to actuate shut-off switches. Other pressure sensors including transducers, elements, indicators, gauges (bellows, bourdon tubes, capsule elements and diaphragm elements) and controllers.
1. PIEZO ELECTRIC PRESSURE SENSOR
Piezoelectric Pressure Sensors measure dynamic pressures. They are generally not suited for static pressure measurements. Dynamic pressure measurements including turbulence, blast, ballistics and engine combustion under varying conditions may require sensors with special capabilities. Fast response, ruggedness, high stiffness, extended ranges, and the ability to also measure quasi-static pressures.
SENSOR CONSTRUCTION AND WORKING
Piezoelectric pressure sensors are available in various shapes and thread configurations to allow suitable mounting for various types of pressure measurements. Quartz s are used in most sensors to ensure stable, repeatable operation. The quartz s are usually preloaded in the housings to ensure good linearity. Tourmaline, another stable naturally piezoelectric , is used in some sensors where volumetric sensitivity is required. When the is stressed, a charge is generated. This high-impedance output must be routed through a special low-noise cable to an impedance-converting amplifier, such as a laboratory charge amplifier or source follower. High insulation resistance must be maintained in the cables and connections. The primary function of the charge or voltage amplifier is to convert the high-impedance output to a usable low-impedance voltage signal for recording purposes. Laboratory charge amplifiers provide added versatility for signal normalization, ranging, and filtering. charge amplifiers have additional input adjustments for quasi-static measurements, static calibration, and drift-free dynamic operation. Miniature in-line amplifiers are generally of fixed range and frequency.
Charge mode quartz pressure sensors may be used at higher temperatures, since the temperature limitation is determined by the temperature limit of the s rather than built-in electronics. When considering the use of charge mode systems, remember that the output from the s is a high impedance charge. The internal components of the pressure sensor and the external electrical connector maintain a very high (typically 10 e13 ohm) insulation resistance. Consequently, any connectors, cables, or amplifiers used must also have a very high insulation resistance to maintain signal integrity.
Environmental contaminants on the connector, such as moisture, dirt, oil, or grease contribute to reduced insulation, resulting in signal drift and inconsistent results. Use of special low-noise cable is required with charge mode pressure sensors. Standard, two-wire, or coaxial cable, when flexed, generates a charge between the conductors. This is referred to as triboelectric noise and cannot be distinguished from the sensor's charge output. Low-noise cables have a special graphite lubricant between the dielectric and the braided shield, which minimizes the triboelectric effect and improves the quality of the sensor's charge output signal.
2. VARIABLE RELUCTANCE PRESSURE TRANSDUCER
Features · High Sensitivity to Low Pressures · Ranges as Low as 0.1 In H2O Full Scale · Changeable Sensing Diaphragms · Rugged Construction · Gas or Liquid Media · Fast Dynamic Response
Variable Reluctance Pressure Sensing Technology : The ability to measure very low pressures accurately, to provide changeable transducer ranges, high frequency response and rugged durability all depend on manufacturing a pressure transducer with no linkages or other connections to the sensing element. Here is how it is done. A variable reluctance pressure transducer is perhaps best described as an inductive half-bridge, and consists of a pressure sensing diaphragm and two coils. The coils are wired in series and are mounted so their axes are normal to the plane of the diaphragm. Clamped ly between the coil housings, the diaphragm is free to move in response to differential pressure. The coils are supplied with an AC excitation, typically 5 Vrms at 3 or 5 Khz. The coils are matched so that their impedances are approximately equal. When a differential pressure is applied to the sensor, the diaphragm deflects away from one coil and towards the opposite. The diaphragm material is magentically permeable, and its presence nearer the one coil increases the magnetic flux density around the coil. The stronger magnetic field of the coil, in turn, causes its inductance to increase, which increases the impedance of one coil. At the same time, the opposite coil is decreasing its impedance. The change in coil impedances brings the half-bridge out of balance, and a small AC signal appears on the signal line.
The change in coil impedance is directly proportional to the position of the diaphragm, so the amplitude of the signal is directly proportional to the applied pressure. The phase of the signal with respect to the excitation is determined by the direction of movement of the diaphragm. The output of a variable reluctance circuit at its full scale pressure is 20 mV/V or more. This is about ten times more output than is typical for strain gage transducers. Sensitivity to Very Low Pressures
Because the diaphragm need move only one or two thousandths of an inch to produce a full scale output, the thickness and area of the diaphragm determines the full scale pressure range. A large diaphragm made of thin foil will respond to extremely low pressures; Validyne offers standard full scale ranges as low as 0.1 In H2O. Conversely, a thick diaphragm with a small area responds to very high pressures and Validyne offers one model having a full scale of up to 10,000 Psid.
Changeable Ranges
Becasue the diaphragm is the only moving part, Validyne transducers are designed so that the user may change the pressure range by substitution of diaphragms. The coil housings are clamped together with bolts and may be disassembled in the field. A series of diaphragms are available to allow a wide variety of pressures to be sensed with a single transducer. One Validyne model has 23 different diaphragms
available covering full scale pressures from 3.5 In H2O to 3200 psi. Frequency Response & Durability
The diaphragm is the only moving part of a variable reluctance pressure sensor; there are no linkages or other items that must move in response to pressure. In addition, the diaphragm must move only one or two thousanths of an inch over its full pressure range. This means that the diaphragm must be accelerated and moved but a small distance to reflect a change in pressure. This results in a high natural frequency. There are no internal fluids or hydraulic constraints inside the sensor, which enchances its high natural frequency response. Similarly, there are no delicate mechanisms inside the variable reluctance sensor to be damaged by shock or impact. The transducer may be dropped or otherwise mishandled without damage. In the event of an overpressure, the only parts likely to be damaged, the diaphragms, o-rings, etc, may be easily replaced.
