Sensor Selection For Temperature Control
Have you ever set out to specify a temperature controller or recorder? Most likely, one of the first questions asked was, "What is the input?" or "What type of sensor will be used with this device?"
Temperature is fundamental to many processes; it affects the physical properties of most materials and is one of the most prevalent measurements made in industry today. It is not only important to measure temperature but also to have the ability to control, alarm and record temperature values throughout a process. Therefore, it is important to select the proper sensor and to ensure that it is compatible with the rest of the instrumentation.
Many techniques have evolved over the years for measuring temperature. Perhaps the one dating back the farthest is the glass thermometer. In 1714, Gabriel D. Fahrenheit invented the liquid-filled thermometer, in which a liquid such as mercury or alcohol expands with temperature and moves up a glass tube. His system had a fixed volume with the vapor pressure above the liquid reduced to near zero, allowing the liquid freedom to expand. Graduations along the glass tube gave an indication of the temperature. This was a good tool for viewing the current temperature, but the limitations of such a device were soon obvious, as there was no way to record or control the temperature.
The next advancement in temperature sensing attempted to not only measure but also control. Bimetallic elements, made by bonding two materials that have different coefficients of expansion, were used to form a temperature switch. As the temperature increased, one of the metal elements expanded more rapidly than the other, causing a bending motion that closed a contact. A system such as this was simple, rugged and low cost but was limited to simple on/off type control.
Today, many types of sensors are used to record and control temperature. Infrared thermometers, also referred to as radiation detectors or optical pyrometers, measure the temperature of an object without coming into physical contact with the product. These devices are based on the fact that every object emits radiant energy, and the intensity of this radiation is proportional to the temperature. This can be an advantage when a product is moving -- for example, along an assembly line or in a location where other types of sensors cannot be attached easily. The down side to this technology is usually the size and cost of the sensor.
The most common method used to measure temperature throughout industry today is the thermocouple, and the second most common is the resistance temperature detector (RTD). The advantages and disadvantages of thermocouples and RTDs as well as how they interface with today's temperature recorders and controllers will be reviewed.
Thermocouple CompositionBack in 1821, T.J. Seebeck discovered that by connecting two dissimilar metallic conductors such as iron and constantan, an EMF (actually a small millivoltage) would develop when the junction is heated. He further noted that any change in the temperature at that junction would affect the EMF value. Since then, the development of the thermocouple has made it the most widely used electrical temperature sensor in industry.
The basic thermocouple is composed of two dissimilar metals wires welded together (figure 1). The circuit develops a small DC voltage proportional to the temperature at the measuring junction whenever a temperature differential exists between the measuring junction (T1) and reference junction (T2). This small DC voltage then can be used by instrumentation devices or computer systems to determine process temperature.
Thermocouple Construction TypesA thermocouple usually will not be used as an exposed sensor in any application. Normal practice dictates it be inserted into some type of housing that will provide protection from the environment.
There are two types of basic constructions of the protective housing: pipe and magnesium oxide insulator. Selection of construction is based on the specific application. Generally, the pipe type will be larger in size, withstand higher temperatures and be stronger mechanically. The magnesium-oxide-insulated type is lighter, easier to handle and will bend easily to satisfy unique applications.
Pipe-Type Housing. This construction consists of a protection tube that could be made from many different materials. Typically, carbon steel, 316 or 304 stainless steel, and cast iron tubes are used for temperatures below 1,000oF (538oC). For higher temperature applications, ceramic tubes and metals such as Inconel are available.
Lengths of these assemblies typically begin at about 12" and increase in 6" increments to about 60". Intermediate lengths are available depending on the manufacturer. Longer lengths also can be provided in many materials but consideration must be given to special handling and shipping.
Magnesium-Oxide-Insulated Housing. This type of thermocouple is constructed by insulating the wires with magnesium oxide insulators and slipping them into a metal tube. The tube is then drawn to a smaller diameter, thereby crushing and compacting the insulation around the wire, creating one solid mass.
Typically, this type of thermocouple is available in sizes of 0.0625 to 0.375" OD but can be fabricated in smaller sizes down to 0.010". Because of its construction, it is available only with metal protection such as stainless steel.
Thermocouple SelectionIn addition to the physical size and construction of the thermocouple, consideration must be given to the calibration type, which is determined by the combination of the two dissimilar metals that make up the probe. The calibration will determine the suitable temperature range of the thermocouple as well as its ability to survive in different environments. The most common types of thermocouples used in applications below 1,000oF are listed in table 1. It should be noted that the gauge (diameter) of the wire influences the maximum temperature rating.
Type T Copper-Constantan. Type T provides high resistance to corrosion from atmosphere moisture or moisture condensation. It can be used in either oxidizing or reducing atmospheres.
Type J Iron Constantan. Type J is suitable where free oxygen is deficient. Because oxidation of the iron conductor increases rapidly above 1,000oF, heavier gauge wire should be used at high temperatures.
Type E Chromel-Constantan1. Primarily used for oxidizing atmospheres, Type E does not corrode at subzero temperatures.
Type K Chromel-Alumel2. Type K is recommended for use between 1,000 and 2,000oF (1,093oC) in oxidizing atmospheres.
Another characteristic that is influenced by the calibration type is the accuracy of the thermocouple. Typically, the higher the temperature, the greater the degree of error. In general, it is best to select a thermocouple type with the lowest maximum temperature rating that will still meet your application requirements.
Resistance Temperature SensingIn addition to thermocouples, another popular sensing device is the resistance temperature detector, or RTD. About half a century after Seebeck started his work on thermocouples, Sir William Siemens, based on research conducted by Sir Humphrey Davy, discovered that a relationship existed between the temperature of a metal and its electrical resistance. While this principle was true for many metals, Siemens went on to establish platinum as the element of choice for the resistance thermometer. As the temperature of the platinum element increased, its resistance also increased. Today, RTDs are made using several different metals, including platinum, copper and nickel.
Unlike the thermocouple, the RTD is a passive device. Typically connected as one leg of a bridge circuit, it requires a small current to produce a measurable resistance change proportional to the change in temperature. The power supply used to generate this small current typically resides in the measuring device such as the temperature controller or recorder.
Care must be taken to ensure that the wires connecting the RTD to the measuring device do not affect the reading. These lead wires can add resistance, which can change the temperature. For this reason, RTDs often are provided in a three-wire version, and the extra lead is used to offset any potential for error by balancing the bridge (figure 3).
Thermocouple vs. RTD. While thermocouples are more widely used, RTDs have become quite popular. Thermocouples are simple, rugged and inexpensive devices that respond quickly to temperature changes. They tend to provide a single-point temperature measurement but typically are less stable and less repeatable than RTDs. RTDs are more expensive than thermocouples but are more stable, tend to sense the area temperature surrounding the element and tend to be more repeatable. On the down side, RTDs typically are slower to respond to temperature changes, require an external power source and require a third (and sometimes a fourth) lead be run to offset additional errors.
Universal InputsWhile the selection of the proper sensor -- be it a thermocouple, RTD, optical device or other type of sensor -- can be critical to measuring process temperatures, today's instrumentation selection has become somewhat easier when it comes to the input.
Many controllers and recorders on the market today tout the ability to work with most of the sensors commonly used in temperature processes. Instrument companies offering such universal inputs provide the user with the ability to simply select the type of sensor being used. If your process requires a Type K thermocouple, simply select Type K from the instruments menu or set a configuration code. If you change to Type J, no problem -- simply change the code.
Some manufacturers make it easy to change the input type by providing easy-to-follow prompts in their instruments. Additionally, today's technology enables temperature instrumentation to deliver higher control accuracy, plant-wide communications and diagnostics, and ever-increasing levels of functionality.