Digital Pressure Gauges Information
Image Credit: Century Control Systems, Inc. | Cooper Instruments & Systems | Testo Inc.
Digital pressure gages are devices that convert applied pressure into signals. Readouts are then displayed numerically.
Many pressure-gauging technologies are available. Devices that use mechanical deflection include an elastic or flexible element such as a diaphragm that responds to changes in pressure. Digital pressure gages that include a bridge circuit also use a diaphragm, but to detect changes in capacitance. Typically, strain gages or strain-sensitive variable resistors are used as elements in Wheatstone bridge circuits that perform measurements. Other digital pressure gages use pistons, vibrating elements, microelectromechanical systems (MEMS), or thin film to sense changes in pressure.
Some devices use piezoelectric sensors to measure dynamic and quasi-state pressure. Generally, these sensors have two modes: charge and voltage. Charge mode generates a high-impedance charge and voltage mode uses an amplifier to convert the high-impedance charge into a low-impedance output voltage.
Measurement and Display
Digital pressure gages are capable of performing various pressure measurements and displaying amounts in different units.
- Absolute pressure is a pressure measurement that is relative to a perfect vacuum. Typically, vacuum pressures are lower than the atmospheric pressure.
- Gage pressure, the most common type of pressure measurement, is relative to the local atmospheric pressure. By contrast, sealed gauge pressure is relative to one atmosphere of pressure at sea level.
- Differential pressure reflects the difference between two input pressures. In terms of units, some digital pressure gages display measurements in pounds per square inch (PSI), kilo pascals, bars or millibars, inches or centimeters of mercury, or inches or feet of water.
- Other devices display measurements in ounces per square inch or kilograms per square centimeter.
Selecting digital pressure gages requires an analysis of performance specifications and optional features. For example, devices differ in terms of maximum allowable pressure, accuracy, vacuum range, and operating temperature.
Accuracy, the difference between the true value and the indication expressed as percent of the span, is often denoted by a lettered grade. Some digital pressure gages feature ASME B40.1 and DIN accuracy grades, or list the largest reported percentage error.
In terms of optional features, digital pressure gages can include temperature outputs, temperature compensation, alarm switches, and output switches that are compatible with transistor-transistor logic (TTL). Negative pressure outputs are available for devices that measure differential pressure.
Digital pressure gages can produce several types of electrical signals, including analog voltage and analog current. Output signals can also be encoded via amplitude modulation (AM), frequency modulation (FM), or some other modulation scheme such as sine wave or pulse train.
Serial and parallel interfaces are available, and common communication protocols include Ethernet, Fieldbus, and DeviceNet.
Digital pressure gages are used in a variety of industries and have pharmaceutical, food processing, and automotive applications. Digital pressure gages are also used in the containment and monitoring of hazardous materials.
JIS B 7547 - Procedures for characterization and calibration for digital pressure gauges
A-A-59480 - Tester, pressure gage, hydraulic, portable (digital or deadweight)