Thermocouple vs Thermistor

Looking at the thermocouple vs thermistor for temperature sensing in the world of battery design. Cell temperature is a critical parameter. Although you might not be able to measure every cell, you need to measure a few in order that you can estimate the maximum and minimum.

Thermocouple

A thermocouple functions based on the Seebeck effect, involving two dissimilar metal wires joined at one end, forming a ‘hot junction.’ When this junction is heated, it creates a temperature difference between it and the other end, the ‘cold junction.’ This temperature difference leads to the generation of an electromotive force (EMF). The behavior of electrons in each wire differs due to heat exposure:

In one wire, electrons move more freely at higher temperatures, resulting in a more negative charge build-up at the cold junction. This wire is identified as the negative wire lead.

The other wire, with slower-moving electrons under the same conditions, builds up less charge and is known as the positive wire lead.  This variance in electron mobility between the two wires leads to a difference in charge, which manifests as a voltage across the junction. By measuring this voltage with a voltmeter and knowing the temperature at the cold junction, the actual temperature at the hot junction can be calculated

thermocouple and equation

Where E(T) is the electromotive force (EMF) in millivolts and T is the temperature in degrees Celsius. The coefficients a0, a1, a2, a3, an are specific to the Type of thermocouple.

Solving the above equation and using.a0, a1, a2, a3, an from NIST database, we get the following output:

thermocouple output voltage versus temperature

Sample mV vs temperature for type K thermocouple

Thermistor

A thermistor is a type of resistor whose resistance varies significantly with temperature. It’s a temperature-sensitive device composed of semiconductor material, which exhibits a large change in resistance in response to a small change in temperature. Thermistors can be classified into two types:

  1. Negative Temperature Coefficient (NTC) Thermistors: Their resistance decreases as the temperature increases. They are commonly used for temperature measurement and control.
  2. Positive Temperature Coefficient (PTC) Thermistors: Their resistance increases with an increase in temperature. They are often used as self-resetting fuses and in overcurrent protection.

The operation of a thermistor is based on the change in electrical resistance of semiconductor materials with temperature. This property allows them to be used in a wide range of applications, from simple temperature measurements to more complex electronic circuits for temperature control and compensation. The resistance-temperature relationship of a thermistor is typically non-linear and must be calibrated for accurate temperature readings. This relationship can often be described using the Steinhart-Hart equation, a third-order polynomial, providing precise temperature measurements over a specified range.

thermistor equation

Where

  • R(T) is the resistance at temperature T (Kelvin),
  • R0 is the resistance at a reference temperature
  • T0 (Kelvin)
  • B is a constant (typically given in the thermistor’s datasheet).
thermistor typical resistance versus temperature

Comparison

comparison of thermistor and thermocouple

The near linear response, robustness and wide operating temperature range make the thermocouple ideal for test environments. However, the need for more electronics and overall higher cost mean that the thermistor is better suited to the production environment.

CharacteristicThermocoupleThermistor
Temperature RangeLargeNarrow -40° to 250°C
LinearityFairPoor
SensitivityLowVery High
Response TimeMedium to FastMedium to Fast
StabilityFairPoor
DurabilityExcellentPoor
CostLow to high (depends on type)Low

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