Which Sensor? Thermistor, RTD or Thermocouple

Expert Denver Plumber

Due to the nature of the basic process the number of conducting electrons increases exponentially with temperature; the characteristic therefore exhibits a strongly rising form. This pronounced non-linearity is a disadvantage of NTC resistors and limits their useful temperature span to about 100°C. They can of course be linearised by an automation computer. However, accuracy and linearity generally do not meet the requirements over larger measurement spans. Their drift under alternating temperatures is also larger than for RTD’s. Their field of use is limited to monitoring and indicating applications where the temperatures do not exceed 200°C. In such simple applications they are actually preferable to more expensive thermocouples and RTD’s in view of their low cost and the comparatively simple electronic circuitry required. In addition they can be produced in very small designs with a fast response and low thermal mass. Thermocouples are based on the effect that the junction between two different metals Thermistors, Thermocouples, RTD'sproduces a voltage which increases with temperature. Compared with resistance thermometers they offer the clear advantage of a higher upper temperature limit, up to several thousand degrees Celsius. Their long-term stability is somewhat worse (a few degrees after one year), the measuring accuracy is slightly poorer (on the average +0.75% of the measurement range). They are frequently used in ovens, furnaces, flue gas measurements and other areas with temperatures above about 250°C. Web Form Generator 3.0 -->

RTD’s employ the property that the electrical resistance of metals varies with temperature. They are positive temperature coefficient (PTC) sensors whose resistance increases with temperature. The main metals in use are platinum and nickel. The most widely used sensor is the 100 ohm or 1000 ohm RTD or platinum resistance thermometer. RTD’s are the most accurate sensors for industrial applications and also offer the best long-term stability. A representative value for the accuracy of a platinum resistance is +0.5 percent of the measured temperature. After one year there may be a shift of +0.05°C through aging. Platinum resistance thermometers can cover temperature ranges from -200 to 800°C.

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