A transmitter is a device that converts readings from a primary sensor or transducer into a standard signal and sends the signal to a monitor or controller. Standard signals include the 3-15psi pressures, 4-20mA current and Field bus digital signals.
There are different types of transmitters in process industry, which are used to transmit different process variable:
- Pressure transmitter
- Temperature transmitter
- Flow transmitter
- Level transmitter
The types of different pressure transmitters used in industries are,
Absolute pressure transmitter:
This transmitter measures the pressure relative to the perfect vacuum pressure (0 psi or no pressure).
Gauge pressure transmitter:
This transmitter measures the relative pressure at a given atmospheric pressure at a given location. When the pressure gauge indicates 0 PSI, it is really the atmospheric pressure.
Differential Pressure transmitter:
This transmitter measures the difference between two or more pressures introduced as inputs to the detection unit, for example, by measuring the pressure drop through an oil filter. The differential pressure is also used to measure the flow or level in pressure vessels.
Level transmitters are of two types of transmitters, these transmitters can measure the level continuously or at determined points.
Point level transmitters provide the output when a specific level measurement is reached. This output is generally in the form of an audible alarm or an electrical charge to turn on a switch.
Continuous level transmitters measure level within a specified range and provide the output as a continuous reading of the level.
Different types of level transmitters are,
Ultrasonic: Ultrasonic level transmitters are used for non-contact level sensing of highly viscous liquids, as well as bulk solids. They are also widely used in water treatment applications for pump control and open channel flow measurement.
Conductive: These are ideal for point-level detection of a wide range of conductive liquids, such as water, and are especially suitable for highly corrosive liquids, such as caustic soda, hydrochloric acid, nitric acid, ferric chloride and similar liquids.
Pneumatic: These transmitters are intended to be used in hazardous environments, where there is no electric power or its use is restricted, and in applications involving heavy sludge or slurry.
A temperature transmitter is a device that captures a signal from a sensor such as a thermocouple or RTD, calculates the temperature based on this signal, and then converts it to a 4-20 mA type of signal for output to a receiving device. There are some types of temperature transmitters. They are,
Thermocouple type: Thermocouple measures the electromotive force, and uses this electromotive force data to calculate the temperature.
RTD: RTD measured the electrical resistance change in the conductor due to temperature change. The temperature transmitter passes a very small electric current through the RTD to measure electrical resistance.