Pressure Transmitters Information
Pressure transmitters translate low-level electrical outputs from pressure sensing devices to higher-level signals that are suitable for transmission and processing. In general, a transmitter is a signal converter for translating the signal into something that can be sent over a distance. Many pressure transmitters have an integral sensor such that they can measure a pressure signal as well as transmit this signal some distance. Transmission may be via an analog signal such as 4-20ma or a digital signal such as Ethernet.
Pressure Transmitter Technology
Pressure transmitters use many different sensing technologies and can measure the pressure of liquids and/or gases. These include mechanical deflection devices such as
- Bourdon tubes
- Sealed pistons or cylinders
- Strain gauges
- Piezoresistive devices
- Piezoelectric pressure transmitters
- Thin film devices
- Microelectromechanical systems (MEMS)
- Variable capacitance
- Vibrating elements
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 at sea level.
- Differential pressure reflects the difference between two input pressures.
- Compound pressure instruments can display both positive and negative pressures.
Please refer to Engineering360's How to Select Pressure Sensors for full details on pressure sensing technologies.
Some pressure transmitters display values in pounds per square inch (lb/in2), kilo pascals (kPa), bars or millibars, inches or centimeters of mercury (in of Hg), 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- 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- Vacuum range, another important measurement, covers the lowest vacuum pressure and the highest vacuum pressure.
- Accuracy- 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.
- Operating temperature- Operating temperature is the full-required range of ambient operating temperatures. Temperature and pressure are directly related to each other. If the temperature of the operating environment increases the pressure in the system will increase. In order to prevent equipment damage, it is important to know the extreme temperature ranges of the area.
Types of Electrical Signals
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.
The output voltage is a simple (usually linear) function of the measurement.
Analog current levels or transmitters such as 4 – 20 mA are suitable for sending signals over long distances. Current is imposed on the output circuit proportional to the measurement. Feedback is used to provide the appropriate current regardless of line noise and impedance.
RS232 and RS485 are serial communication protocols that transmit data one bit at a time.
Parallel outputs such as printer ports and Centronics ports transmit data in groups of bits. The general-purpose interface bus (GPIB) or IEEE 488 is a popular parallel interface for connecting computers, peripherals, and laboratory instruments.
The highway addressable remote transducer (HART®) protocol uses 1200 baud frequency shift keying (FSK) based on the Bell 202 standard to superimpose digital signals on conventional 4-20 mA analog signals. This enables two-way communication and facilitates the transmission of information beyond normal process variables to and from smart field instruments. The HART protocol communicates without interrupting the analog signal and allows a host application (master) to receive two or more digital updates per second from a field device (slave). Because they are phase-continuous, the digital FSK signals do not interfere with the analog 4-20 mA signals. HART is a registered trademark of the HART Communication Foundation.
The process fieldbus (PROFIBUS®) is a popular, open communication standard used in factory automation, process automation, motion control, and safety applications.
DeviceNet uses controller area network (CAN) network protocol to connect industrial devices such as limit switches, photoelectric cells, valve manifolds, motor starters, drives, and operator displays to programmable logic controllers (PLCs) and personal computers (PCs).
The FOUNDATION fieldbus is a serial, all-digital, two-way communication system that serves as a local area network (LAN) for factory instrumentation and control devices. It uses a line or tree topology and distributed data transfer (DDT). In the hierarchy of digital networks, the FOUNDATION fieldbus is on the lower end.
Ethernet is a local area network (LAN) protocol that uses a bus or star typology and supports data transfer rates of 10 Mbps. The Ethernet specification is the basis for the IEEE 802.3 standard, which specifies the physical and lower software layers. To handle simultaneous demands, Ethernet uses carrier sense multiple access / collision detection (CSMA/CD) to monitor network traffic.
The output signal is encoded via amplitude modulation (AM), frequency modulation (FM), or some other modulation scheme such as sine wave or pulse train; however, the signal is still analog in nature.
Devices produce digital outputs other than standard serial or parallel signals. Examples include transistor-transistor logic (TTL) outputs.
Display Types and Features
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) are commonly found in computer monitors
- Liquid crystal displays (LCD) use electrons to release energy in the form of photons.
In terms of features, some pressure transmitters include
- TTL-compatible switches are compatible with transistor-transistor logic.
- Built-in audible or visual alarms that signal when the switch or sensor has been turned on or off. This is important when the vacuum pressure of a system needs to be closely monitored.
- Temperature measurement outputs allow the user to observe the temperature of the system and adjust temperature and/or vacuum level as needed.
- Temperature compensation includes built-in factors that prevent pressure measurement errors due to temperature changes.
- Negative pressure outputs are available only with vacuum sensors that provide differential pressure measurements.