July 2008

Is it hot in here?

Transducers measuring temperature include RTDs, thermocouples, thermistors, integrated circuit sensors, and infrared sensors. In contrast to other physical parameters, there is no common, universally accepted definition of temperature. Definitions for temperature can range from an object's relative sense of warmth or coolness to a measure reflecting the average kinetic energy associated with the motion of the molecules that make up a material. Temperature is an intrinsic property of matter that quantifies the ability of one body to transfer heat energy to another body. Heat flows from a high temperature area or body to a low temperature area or body. If a high-temperature body comes in contact with a low-temperature body, heat energy will decrease in the hot body and increase in the cooler body until reaching a thermal equilibrium. At the molecular level, the velocity of the molecules in the hot body will decrease as they collide with the molecules in the cooler body. The velocity of the molecules in the cooler body will increase as molecules in the hot body hit them. At equilibrium, the velocities are the same in both bodies, and we can say the temperatures of the two bodies are the same. If heat energy joins a body, then the velocity of the molecules increases, and the temperature of the body increases.

Thermocouples

Thermocouples are probably the most widely used type of temperature transducer. Their construction involves joining two wires made of different materials and placing the junction at the point at which we want to measure the temperature. A small voltage will appear at the ends of the wires opposite the junction if there is a difference in temperature between the junction location and the measuring location. The magnitude of the voltage is proportional to the temperature difference and will vary depending on the types of material that see use in the two wires. The relationship between voltage and temperature is nonlinear, and the curves are different for each pair of wire material you use.

The wires (as opposed to the junction) generate the electromotive force (emf), and the emf is present whenever a temperature gradient exists along the wire. Two different materials will produce different emfs for the same temperature gradient. It is this difference we measure. Tables of emfs versus temperature are available from manufacturers or standards organizations (see ISA-MC96.1-1982, Temperature Measurement Thermocouples).

Resistance temperature detectors

The resistance of a metal will increase when its temperature increases. Resistance temperature detectors or RTDs are temperature transducers that apply this phenomenon in order to measure temperature. Platinum is the most common metal used to construct RTDs. Platinum RTDs can be  in the form of a coil of fine wire or as a film of metal deposited on a substrate. The coil or substrate is usually encased in a thin-walled stainless steel tube filled with a powdered insulating material such as magnesium oxide. This tube is sealed at the coil end to protect the coil from the measurement environment. Wires from each end of the coil or film are brought out of the other end of the tube, and a moisture resistant seal is made around the wires.

RTDs are made with a two-wire, three-wire, or four-wire configuration. In the two-wire configuration, one wire connects to each end of the coil and comes out of the protective tube. In the three-wire configuration, one wire connects to one end of the coil, and two wires connect to the other end of the coil. In the four-wire configuration, two wires connect to each end of the coil. The different configurations are designed to provide various means of compensating for the resistance of the lead wires that join the RTD to the signal conditioner or readout.

Thermistors

The name thermistor is a contraction of thermally sensitive resistor. Like RTDs, thermistors are sensors that change resistance when their temperature changes. The resistance can increase with an increase in temperature (positive temperature coefficient or PTC) or decrease with an increase in temperature (negative temperature coefficient or NTC). The most commonly used NTC thermistors are semiconductor resistors made from the oxides of manganese, cobalt, copper, and nickel.

The NTC thermistor has a high resistance at room temperature (thousands of ohms) and produces a large resistance change per degree of temperature change. This high sensor resistance and large temperature coefficient mean the errors caused by lead-wire resistance are not an issue with NTC thermistors as they are when using RTDs.

The basic thermistor element can take the form of a tiny glass bead, a disk, or a chip. These elements are usually too fragile to be used in these forms, however, so the element is encased in a thin-walled steel sheath for protection. The elements' small size means the sensor has a very small response time compared to the RTD.

Thermistors' temperature range is typically from 50ºC to 200ºC-smaller than that of the RTD and thermocouple.

SOURCE: Fundamentals of Test Measurement Instrumentation, by Keith R. Cheatle, ISA 2006. To purchase a copy of this book, visitwww.isa.org/books.