Pressure Instruments Information
Pressure instruments are used to measure, monitor, record, transmit or control pressure. There are several types of devices. Recorders are used to measure values or accumulate totals. Controllers are used to regulate positive or negative (vacuum) pressure. They receive sensor inputs, provide control functions, and output control signals. Complete pressure instruments are self-contained devices that provide an output such as a display. 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. Some pressure instruments are suitable for measuring the pressure of liquids or gases. Others are rated for hazardous duty. Import specifications to consider include working pressure range, accuracy, vacuum range, and operating temperature. Temperature outputs, temperature compensation, negative pressure outputs, alarm indicators, and transistor-transistor logic (TTL) switches are commonly available.
Pressure instruments use many different sensing technologies. 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. Sealed pistons or cylinders are also available. Strain gauges are bonded to a larger structure that deforms as pressure changes. Piezoresistive devices sense shifts of electrical charges within a resistor. Piezoelectric pressure instruments 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 instruments that use microelectromechanical systems (MEMS), variable capacitance, and vibrating elements are commonly available.
Pressure instruments 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 instruments display values 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.
Pressure instruments 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, and highway addressable remote transmission (HARTÒ). HART is a registered trademark of the HART Communication Foundation. 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. Some pressure instruments output TTL signals. Others provide outputs that change the state of a switch or alarm.