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Temperature sensors use a sensing element (such as a diode-connected transistor) to measure temperatures external to the sensor (e.g., on a circuit board or on equipment in a wellbore). This article describes the types of temperature sensors used in measuring reservoir temperature.
The first bottomhole thermometers were mechanical. They were identical to bottomhole mechanical pressure gauges, except that a thermometer sensor was substituted for the pressure sensor. This type of thermometer has been mostly replaced by sensors and recording elements that are easier to use and have higher precision and accuracy.
Thermistors are temperature-sensitive resistive elements made of semiconductor material with a negative coefficient of resistance. The physical effect governing a thermistor’s change of resistance is the increased number of conducting electrons for a corresponding increase in temperature. Thermistors can be built up to 100 times more sensitive to temperature change, for the same resistivity change, than resistance temperature detectors (RTDs), which are described next. The main drawback of thermistors is their operating temperature limitation of approximately 300°F.
Resistance temperature detectors
Resistance temperature detectors (RTD) rely on the increase in resistance of metals in response to increasing temperature. The resistor consists of a coil of fine metal wire or a film of pure metal deposited on a nonconductive surface. Different metals with different resistances are used, but platinum has become the most popular because of its excellent accuracy, large linear range of operation, and wide temperature range (higher than 1000°F is possible). The RTD is usually encased in a probe that is directly exposed to the well fluids. The RTD is commonly the active leg of a Wheatstone bridge.
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