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The Difference Between Thermistor & Resistance Temperature Detectors

Thermometry, otherwise known as the measurement of temperature, is achieved by using either a thermistor or a resistance temperature detector. These two devices are both temperature sensitive and used in different applications, such as food, construction, petrochemical and steel industries. They are designed to provide precise quantifiable parameters of electrical signals not commonly provided by standard thermometers alone. Despite the aforementioned similarities, they are distinct from one another in several aspects.

Temperature Measurement
- A thermistor provides good accuracy in quantifying heat energy at -100oC up to 500oC. A resistance temperature detector (RTD) is able to provide a wider parametric measurement at -240oC to 649oC. At this temperature range, RTD provides better linearity since the thermistor is nonlinear, meaning that when an excitation current passes through a thermistor, the voltage or amount of electrons that passes across the sensor may get dissipated before being quantified. This energy loss can produce either a positive or negative temperature coefficient.
  
  The low thermal mass of thermistors enables them to have fast reaction times compared to an RTD̵7;s slow response time; however, this can be a setback, as thermistors become vulnerable to self-heating errors, while industrial RTDs are less prone to this error.
Material Composition
- The base materials used in creating thermistors are metal oxide semiconductor elements enclosed within beads of glass or epoxy composition. The basic elements found in RTDs are copper, nickel and platinum. The electrical resistance of these three metals, when excitation current is applied, forms the basis at which the RTD operates. Platinum RTDs -- otherwise known as platinum resistance thermometers -- are the most common material types at specifications of either 100 or 1000 ohms (one mega-ohm).
Applications
- Thermistors are required for applications that have specified temperature levels and those that do not need recalibration in interchangeable measurements. The narrow range provided by this measuring device is especially useful for medical and industrial applications where narrow point sensing and best sensitivity is a requisite.
  
  For more general purpose sensitivity specifications, where the highest accuracy is taken alongside averaging of temperature measurements, using an RTD can be better.
Computation and Cost
- RTD parameter computations use the Callendar-Van Dusen mathematical model of resistance as taken against temperature. The Steinhart-Hart formula forms the basis for computing the values obtained from measuring thermistor resistance parameters against temperature.
  
  Thermistors in their most standard form generally cost less than RTDs but can prove to be even costlier than an RTD when specified models are of narrower interchangeability and more extended temperature measurement ranges. Because RTDs are manufactured using plain copper extension wires without requiring cold junction allowances, they are relatively cheaper.

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