Electrical Resistance Thermometers
1.Resistance Temperature Detector (RTD)
The resistance of metal wires change with temperature. Change in resistance can be expressed by the following equations:
Rt= R0 (1 + AT + BT2 + CT3)
Rt = R0(1 +a(T - T0))
where A, B, C anda are constants based on material properties of the RTD, T is temperature and Rt is the resistance, R0 is the reference resistance, and T0 is a reference temperature.
Platinum, nickel and copper wires are used in RTD devices. Platinum has a linear curve in the range of -200 to 850¡C, and is accurate to .01 ¡C. Several sources of error are possible for RTD devices: they include poor wire contact, thermoelectric electromotive forces and self heating of the wire from current flow through the wire. RTDs can be designed using wires and thin or thick films. Very small probes down to the micrometer scale can be built.
Thermistors are temperature sensitive resistors made of semiconductor materials. Oxides of manganese, nickel and cobalt have been used in the fabrication of thermistors. Thermistors can have a negative temperature coefficient (NTC) or positive temperature coefficient (PTC). NTC thermistors are generally made from metallic oxides, while PTC thermistors are formed from barium and strontium titanate mixtures. Single crystal semiconductors thermistors have also been used.
The relationship of resistance to temperature is of the form:
Rt = R0 eb/t
where Rt is the resistance of the thermistor, R0 is a reference resistance of the thermistor andb is a constant based on material properties and t is the temperature.
The relationship between thermistor resistance and temperature is exponential and therefore nonlinear. Various techniques for linearization of a portion (over a specific temperature range) of the thermistor output are employed. However, the sensitivity of the device may be sacrificed as a result of linearization. Thermistors can be configured with operational amplifiers to give current or voltage outputs. The physical size of thermistors start from the sub-millimeter range. Temperature response times range in the milliseconds.
Figure 3 Thermistor output and linearizing circuit
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Support for the development of this module was provided by the National Science Foundation and The Cooper Union for the Advancement of Science and Art.
Please send questions or comments to Professor Ron Adrezin or Professor Daniel Raichel.