Temperature Transmitters General Overview
Analog (4 – 20 mA) transmitters provide a number of benefits for use in industrial sensing.The nature of current (versus voltage) loops is to maintain the proper current, even if there is significant electrical resistance in the line, up to its maximum voltage capability.For this reason, current-based control loops can be used over much greater distances (measured in miles versus feet).
The 4 mA “live zero” provides a method to determine equipment failure, should the signal drop to zero mA.
Transmitters are scalable and provide an opportunity to increase resolution by spreading the predicted measurement range over the entire 4 – 20 mA range.That is, the entire 4 – 20 mA can be applied to a small temperature range measured at the sensor.Some transmitters are field programmable, while others have a specified range, and are programmed at the factory.
Analog input cards are very commonly used in industry and provide an opportunity to standardize and simplify control systems within your facility.
Choosing the right transmitter for your application will depend on many considerations, a few of which are: installation conditions, required input, desired output, field accessibility and your plant communication protocols.
RTD Transmitters
The use of this transmitter is generally recommended when lead wire length approaches 100 feet, or if a 4 – 20 mA output is desired.This transmitter receives the resistance from the RTD and converts it into a scalable 4-20 mA analog output.
Thermocouple Transmitters
The use of this transmitter is generally recommended when the cost of thermocouple wire and plc input card exceeds the cost of the transmitter and “common” wire, or if a 4 – 20 mA output is desired.This transmitter receives the voltage from the thermocouple and converts it into a scalable 4-20 mA analog output.
Thermocouple & RTD Transmitters
These transmitters will accept signals from either an RTD or thermocouple and will transform it into a scalable analog output of 4 – 20 mA.They provide an opportunity to further standardize process controls in your facility, by keeping one type of transmitter for all applications.