National Instruments Module SCXI-1503 User Manual

Also see for Module SCXI-1503: CALIBRATION PROCEDURE

Contents

National Instruments Module SCXI-1503 User Manual Manual pdf 40 page image

Chapter 4 Theory of OperationSCXI-1503 User Manual 4-10 ni.comThermistorsA thermistor is a piece of semiconductor made from metal oxides, pressedinto a small bead, disk, wafer, or other shape, sintered at high temperatures,and finally coated with epoxy or glass. The resulting device exhibits anelectrical resistance that varies with temperature.There are two types of thermistors: negative temperature coefficient (NTC)thermistors, whose resistance decreases with increasing temperature, andpositive temperature coefficient (PTC) thermistors, whose resistanceincreases with increasing temperature. NTC thermistors are morecommonly used than PTC thermistors, especially for temperaturemeasurement applications.A main advantage of thermistors for temperature measurement is theirextremely high sensitivity. For example, a 2,252 Ω thermistor has asensitivity of –100 Ω/°C at room temperature. Higher resistancethermistors can exhibit temperature coefficients of –10 kΩ/°C or more.In comparison, a 100 Ω platinum RTD has a sensitivity of only 0.4 Ω/°C.Also, the physically small size and low thermal mass of a thermistor beadallows a very fast response to temperature changes.Another advantage of the thermistor is its relatively high resistance.Thermistors are available with base resistances (at 25 °C) ranging fromhundreds to millions of ohms. This high resistance diminishes the effect ofinherent resistances in the lead wires, which can cause significant errorswith low resistance devices such as RTDs. For example, while RTDmeasurements typically require 3- or 4-wire connections to reduce errorscaused by lead-wire resistances, 2-wire connections to thermistors areusually adequate.The major trade-off for the high resistance and sensitivity of the thermistoris its highly nonlinear output and relatively limited operating range.Depending on the type of thermistor, the upper range is typically limited toaround 300 °C. Figure 4-4 shows the resistance-temperature curve for a2,252 Ω thermistor. The curve of a 100 Ω RTD is also shown forcomparison.