Thermocouple

http://en.wikipedia.org/wiki/Thermocouple
http://www.picotech.com/applications/thermocouple.html
http://www.efunda.com/designstandards/sensors/thermocouples/thmcple_intro.cfm

A thermocouple is a junction between two different metals that produces a voltage related to atemperature difference. Thermocouples are a widely used type of temperature sensor for measurement and control^[1] and can also be used to convert heat into electric power. They are inexpensive^[2] and interchangeable, are supplied fitted with standard connectors, and can measure a wide range of temperatures. The main limitation is accuracy: system errors of less than one degree Celsius (C) can be difficult to achieve.<sup>[3]


Thermocouples for practical measurement of temperature are junctions of specific alloys which have a predictable and repeatable relationship between temperature and voltage. Different alloys are used for different temperature ranges. Properties such as resistance to corrosion may also be important when choosing a type of thermocouple.</sup>


Thermocouples are suitable for measuring over a large temperature range, up to 2300 °C.Thermocouples are widely used in science and industry; applications include temperature measurement for kilns, gas turbine exhaust, diesel engines, and other industrial processes.

Pros and Cons

• Pros:   - Low cost.   - No moving parts, less likely to be broken.   - Wide temperature range.   - Reasonably short response time.   - Reasonable repeatability and accuracy.   • Cons:   - Sensitivity is low, usually 50 µV/°C (28 µV/°F) or less. Its low voltage output may be masked by noise. This problem can be improved, but not eliminated, by better signal filtering, shielding, and analog-to-digital (A/V) conversion.   - Accuracy, usually no better than 0.5 °C (0.9°F), may not be high enough for some applications.   - Requires a known temperature reference, usually 0°C (32°F) ice water. Modern thermocouples, on the other hand, rely on an electrically generated reference.   - Nonlinearity could be bothersome. Fortunately, detail calibration curves for each wire material can usually be obtained from vendors.