Temperature Sensors

Temperature sensors are available in wide range of types in different housings and applications are range from die temperature measurement of micro chips to extreme temperature measurements of engines.

RTD

This is resistance type of temperature sensors. Sensor element is made of platinum or cheaper types have copper wires. These sensors are placed in metallic housings and typically available as probes. Connectors are wires or lids and output is analog.

Thermo Couple types

A beaded wire thermocouple is the simplest form of thermocouple. It consists of two pieces of thermocouple wire joined together with a welded bead. The bead of the thermocouple is exposed so the beaded wire thermocouple should not be used with liquids that could corrode or oxidize the thermocouple alloy. Metal surfaces can also be problematic. Often metal surfaces, especially pipes are used to ground electrical systems. In general, beaded wire thermocouples are a good choice for the measurement of gas temperature. Since they can be made very small, they also provide very fast response time.

A thermocouple probes consists of thermocouple wire housed inside a metallic tube. The wall of the tube is referred to as the sheath of the probe. Common sheath materials include stainless steel and Inconel alloys. Inconel alloys supports higher temperature ranges than stainless steel, however, stainless steel is often preferred because of its broad chemical compatibility. For very high temperatures, other exotic sheath materials are also available. The tip of the thermocouple probe is available in three different styles. Grounded, ungrounded and exposed. With a grounded tip the thermocouple is in contact with the sheath wall. A grounded junction provides a fast response time but it is most susceptible to electrical ground loops. In ungrounded junctions, the thermocouple is separated from the sheath wall by a layer of insulation. The tip of the thermocouple protrudes outside the sheath wall with an exposed junction. Exposed junction thermocouples are best suited for air measurement.

Thermistors and IC type

Like the RTD, the thermistor is also a temperature sensitive resistor. While the thermocouple is the most versatile temperature transducer and the PRTD is the most stable, the word that best describes the thermistor is sensitive. Of the three major categories of sensors, the thermistor exhibits by far the largest parameter change with temperature.

Thermistors are generally composed of semiconductor materials. Although positive temperature coefficient units are available, most thermistors have a negative temperature coefficient (TC); that is, their resistance decreases with increasing temperature. The negative T.C. can be as large as several percent per degree Celsius, allowing the thermistor circuit to detect minute changes in temperature which could not be observed with an RTD or thermocouple circuit.

The thermistor is an extremely non-linear device which is highly dependent upon process parameters. Consequently, manufacturers have not standardized thermistor curves to the extent that RTD and thermocouple curves have been standardized.

The NTC (negative temperature coefficient of resistance) thermistors which are discussed herein are composed of metal oxides. The most commonly used oxides are those of manganese, nickel, cobalt, iron, copper and titanium. The fabrication of commercial NTC thermistors uses basic ceramics technology and continues today much as it has for decades. In the basic process, a mixture of two or more metal oxide powders are combined with suitable binders, are formed to a desired geometry, dried, and sintered at an elevated temperature. By varying the types of oxides used, their relative proportions, the sintering atmosphere, and the sintering temperature, a wide range of resistivities and temperature coefficient characteristics can be obtained.

The PTC (positive temperature coefficient of resistance) thermistors have been used in a wide variety of applications over the years. PTC thermistor applications make use of the characteristics inherent in their composition. Generally, applications are broken up into two distinct categories that utilize different characteristics of the PTC. The first category is those applications that utilize the voltagecurrent or current-time characteristics of the PTC thermistor. These are sometimes known as selfheated applications. The other general category is zero power or sensing applications. Unlike the NTC thermistor, the resistance versus temperature characteristic of a PTC thermistor is not well defined and applications that utilize the resistance versus temperature characteristic tend to utilize only a small portion of the R vs T curve and utilize broader tolerances than that of NTC thermistors.

Thermistors are available commonly in housings used in electronic components. Most of them are made of ceramics. Connectors are lids and pins and generally mount directly on to PCB. Applications include military, medical, robotics and generally almost all applications involve relatively low temperatures can use this types of sensors. Out put signals are vary from analog outputs to processed digital information systems on die.

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