Thermocouples and Temperature Sensing
In many applications it is necessary to sense and control temperature accurately. Processing of many products and materials can often involve the use of ovens, adhesive or plastics melting, curing, annealing or other heat treatments. Refrigeration or cryogenic applications also require the use of temperature sensing and control.
As part of the control process it is necessary to use various types of temperature sensors as inputs to either stand-alone temperature/process controllers or system controllers such as PLCs and DCSs. Following is some of the information I have so far included in my book.
From The Primer:
There are a variety of devices that can be used to measure temperature. One of the most widely used is the thermocouple. A thermocouple is a junction between two different metals that produces a voltage related to a temperature difference. Thermocouples are a widely used type of temperature sensor and can also be used to convert heat into electric power. They are cheap and interchangeable, have standard connectors, and can measure a wide range of temperatures. The main limitation is accuracy; System errors of less than one kelvin (K) can be difficult to achieve.
Any circuit made of dissimilar metals will produce a temperature-related difference of voltage. Thermocouples for practical measurement of temperature are made of specific alloys, which in combination have a predictable and repeatable relationship between temperature and voltage. Different alloys are used for different temperature ranges, and to resist corrosion. Where the measurement point is far from the measuring instrument, the intermediate connection can be made by extension wires, which are less costly than the materials used to make the sensor. Thermocouples are standardized against a reference temperature of 0 degrees Celsius; practical instruments use electronic methods of cold-junction compensation to adjust for varying temperature at the instrument terminals. Electronic instruments can also compensate for the varying characteristics of the thermocouple, and so improve the precision and accuracy of measurements.
Thermocouples are widely used in science and industry; a few applications would include temperature measurement for kilns, measurement of exhaust temperature of gas turbines or diesel engines, and many other industrial processes.
The most common type of thermocouple in use is the K thermocouple (Chromel-Alumel). This covers temperature ranges from -200 to 1350 degrees Celsius. It is inexpensive and available in a variety of styles. J thermocouples (Iron-Constantan) are less popular than K due to their lower temperature range of -40 to 750 degrees Celsius. Other types include E, N, B, R,S, T, C, M and Chromel-Gold/Iron.
Thermocouples are not linear devices and the voltage curve must be linearized in the input instrument. Temperature loop controllers contain linearization algorithms for the most common types of thermocouples. Selection of the thermocouple type can be made by setting dipswitches or software parameters.
One note on thermocouple polarity: there is a polarity labeled + and – for connection to input terminals. Counter to the common thought that the red wire is positive in many DC circuits, red is always the negative lead for thermocouples. Not every thermocouple pair has a red wire, but when using the ANSI (American National Standards Institute) color code the red lead will always be negative.
Thermistors are a type of resistor with resistance proportional to its temperature. Thermistors are usually made of a ceramic or polymer material. They have a high precision over a limited temperature range.
RTDs, or Resistance Temperature Detectors also change resistance proportionally with temperature, but are made of pure metals. They are useful over a wider temperature range than thermistors but are less accurate. RTDs and thermistors may both be used with standard analog inputs and an excitation voltage because of their linearity, unlike thermocouples which must use a special input to linearize the signal.
Infrared thermocouples or infrared temperature sensors are used as non-contact methods of sensing temperature. They use the thermal emission from the target to scale temperature to a readable value.
In addition to this article I have also included a handy thermocouple chart in the appendix. This shows color codes and materials along with the appropriate temperature ranges for these sensors. Another great resource is the Omega catalog and website.