Industrial Temperature Primer

Chapter 2.

Temperature Signal Conditioning

Now that we have a very broad overview of available industrial temperature sensors, let's discuss briefly the instrumentation typically used with temperature sensors. Since liquid in glass thermometers use the graduation on the glass tube as the indicator and bimetallic and filled system dial thermometers use an analog scale built into the sensor assembly as the indicator, we will not spend further time on these types of sensors. The other types of sensors, however, have a broad array of instrumentation that may be associated with them. We will attempt to give an overview of each type starting with Temperature Transmitters.

The first instrument in line after the sensor is often some type of signal conditioning instrument. Most often it is a device called a temperature transmitter. Temperature transmitters are used to convert the signal produced by the sensor to an electrical signal recognizable to the processing instrumentation. Temperature transmitters may be of two basic types, four wire and two wire.

Four wire transmitters use a power input that is separate from the signal transmitting wiring. Two wire transmitters use a DC power supply that supplies power to the transmitter over the same two wires that are used to transmit the signal. (Figure 10)

In fact, more than one two wire transmitter may be powered by the same DC power supply as long as the total possible current draw of the transmitters does not exceed the specifications of the power supply. (Figure 11)

Thermocouple and RTD transmitters offer a some unique advantages over transmitting the sensor signal directly to the receiving instrument by means of thermocouple extension wire, in the case of thermocouples, and regular copper wire, in the case of RTDs.

First of all, we must remember that with thermocouples, we are dealing with a very low level emf measured in millivolts. When these small millivolt signals are transmitted by way of thermocouple extension wire over long distances, they are very susceptible to outside interference from electrical noise generated by surrounding machinery. This electrical noise can render the thermocouple signal useless. Thermocouple circuits are also prone to ground loop problems which can result in erroneous readings.

Thermocouple transmitters convert the small millivolt output of a thermocouple to a current signal (typically 4-20 mADC) which is immune to noise and voltage drops over a long distance. Isolated thermocouple transmitters eliminate the ground loop problems by isolating the transmitter input from the transmitter output.

RTD transmitters convert the RTD resistance measurement to a current signal and thereby eliminate the problems inherent in RTD signal transmission via leadwire which is lead resistance. Errors in RTD circuits (especially two and three wire RTDs) are often caused by the added resistance of the leadwire between the sensor and the instrument. (Figure 12)

Another fact that often makes the use of transmitters in thermocouple and RTD circuits advantageous is cost. Thermocouple extension wire is very expensive because its conductors are constructed of the same alloys as the element itself (refer to the Law of Homogeneous Circuits in chapter 1). Also, if lead lengths are long, a heavy gage (typically 16 awg) must be used to resist voltage drop in the circuit. This not only increases the cost of the extension wire but it makes it more difficult to install as well. If the distance between the sensor and the receiving instrument is substantial, then the difference in cost between thermocouple extension wire and the copper wire used with a transmitter can more than pay for the addition of a transmitter to the circuit. Pretty much the same holds true in RTD circuits. The extension wire used is copper but most often three conductors must be run instead of two. Also larger gage size is required to lessen the lead resistance effect.

Another reason that transmitters must often be used is that many instruments will not accept the signals produced by thermocouples and RTDs directly. Much of today's temperature instrumentation consists of computer based systems and programmable logic controllers (PLC). These systems normally handle the current input from a transmitter with no problem. Thermocouple and RTD signals often cannot be inputted directly to these devices. Even when they can, it often requires the addition of expensive electronic circuitry to convert the thermocouple and RTD signals to one that is usable by the system.