A temperature transmitter works by connecting to it some form of temperature sensor. For example a RTD (Resistance temperature device) or Thermocouple. In the case of a RTD connected to the transmitter the transmitter measures a change in resistance of the RTD proportional to the change in temperature measured. The transmitter then derives a current output (generally 4-20mA) which can be measured by an instrument, such as a PLC, loop indicator ect. In the case of a thermocouple a Milli voltage is produced at a junction of two dissimilar metals, this change in Milli voltage again proportional to the change in temperature and the transmitter again derives a current output measurable by a instrument.
The transmitter will be ranged by a programming device, say for example an application where process temperatures need to be measured between 0-100 degrees, the transmitter will be ranged 0-100 degrees and thus give an output proportional to the temperature measured by the sensing element. 0 degrees = 4mA, 100 degrees = 20mA.
A excitation supply will be connected to the transmitter and the 4-20mA will flow in this loop back to the measuring instrument.
It is necessary to use the correct type of sensor for the application to get the best accuracy when measuring. RTD's are generally used for measuring lower temperatures where as thermocouples are used for measuring higher temperatures. There are several types of RTD's such as 2 wire, 3 wire and 4 wire and an even greater range of thermocouples depending on the temperatures and measuring environment.
What is RTD? Resistance Temperature Detectors(RTDs) are temperature sensors that contain a resisor that changes resistance value as its temperature changes. they have been used for many years to measure temperature in laboratory and industrial processes,and have developed a reputation for accuracy, repeatability and stability. Why use an RTD in stead of a thermocouple or thermistor sensor? Each type of temperature sensor has a particular set of conditions for which it is best suited. RTDs offer several advantages: 1, A wide temperature range(-50~500°C for thin-film and -200 to 850°C, RTDs can be used in all but the highest-temperature in dustrial processes. When made using metals such as platinum, they are very stable and are not affected by corrosion or oxidation. Other materials such as nickel, copper, and nickel-iron alloy have also been used for RTDs. However, these materials are not commonly used since they have lower temperature capabilities and are not as stable for repeatable as platinum.......Read More