Pressure Transmitters

Pressure transmitters translate low-level electrical outputs from pressure sensing devices to higher-level signals that are suitable for transmission and processing. They use many different sensing technologies and can measure the pressure of liquids and/or gases. Mechanical deflection devices such as diaphragms, Bourdon tubes or bellows consist of an elastic or flexible element that is deflected mechanically by a change in pressure. Devices that use sealed pistons or cylinders are also available. Strain gauges are often bonded to a larger structure that deforms as pressure changes. Piezoresistive devices sense shifts of electrical charges within a resistor. Piezoelectric pressure transmitters measure dynamic and quasi-static pressures. Their common modes of operation are charge mode, which generates a high-impedance charge output; and voltage mode, which uses an amplifier to convert the high-impedance charge into a low-impedance output voltage. Thin film devices consist of an extremely thin layer of material, usually titanium nitride or polysilicon, deposited on a substrate. Pressure transmitters that use microelectromechanical systems (MEMS), variable capacitance, and vibrating elements are also available. 

Pressure transmitters 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 (oz) at sea level. Differential pressure reflects the difference between two input pressures. Compound pressure instruments can display both positive and negative pressures. Some pressure transmitters display values in pounds per square inch, 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.

Performance specifications for pressure transmitters include working pressure range, vacuum range, accuracy, and operating temperature. Working pressure is the maximum, allowable pressure at which pressure transmitters are designed to operate. Typically, devices should not exceed 75% of their maximum, rated range. Vacuum range, another important measurement, covers the lowest vacuum pressure and the highest vacuum pressure. Accuracy, the difference between the true value and the indication, is expressed as a percentage of span. In cases where the accuracy differs between the middle span and the first and last quarters of the scale, the largest percentage error is reported.  

Pressure transmitters can produce several types of electrical signals, including analog voltage and analog current. These output signals can be encoded via amplitude modulation (AM), frequency modulation (FM), or some other modulation scheme such as sine wave or pulse train. Common communication protocols include Ethernet, DeviceNet, FOUNDATION Fieldbus, the process fieldbus (PROFIBUS^®), and highway addressable remote transmission (HART^®). HART is a registered trademark of the HART Communication Foundation. PROFIBUS is a registered trademark of PROFIBUS International. Several serial and parallel interfaces for pressure instruments are available. RS232 and RS485 are serial, digital protocols. Parallel protocols include the general-purpose interface bus (GPIB), a standard which is also known as IEEE 488. 

Pressure transmitters differ in terms of display types and features. Analog meters use a simple visual indicator such as a needle. Digital displays present numeric or application-specific values. Cathode ray tubes (CRT), liquid crystal displays (LCD), and other multi-line displays are also available. In terms of features, some pressure transmitters include temperature compensation or provide temperature measurement and negative pressure outputs. Others include built-in audible or visual alarms and an output switch compatible with transistor-transistor logic (TTL). Pressure transmitters that are rated for hazardous materials are also available.

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Part Numbers for Pressure Transmitters