Thermocouple vs RTD vs Thermistor: A Detailed Comparison

This article compares Thermocouples, RTDs (Resistance Temperature Detectors), and Thermistors, highlighting their differences in function, advantages, and disadvantages. We’ll explore the strengths and weaknesses of each temperature sensor device.

Thermocouple

A thermocouple is a temperature sensor that is very cost-effective for a wide range of temperatures and offers reasonable accuracy. They are used in applications like boilers, ovens, water heaters, and aircraft engines. Figure 1 shows a simple thermocouple construction.

As seen in the figure, a thermocouple has two junctions: the “measurement” (or Hot) junction and the “reference” (or Cold) junction. Two wires made of dissimilar metals are joined to form the Hot junction. At the other end, the wires are not joined but are connected to signal conditioning circuits, typically made of copper. This junction connecting the thermocouple metals to copper is called the “cold junction.”

A thermocouple is a differential device. The voltage generated at the reference junction depends on the temperatures at both the measurement and reference junctions.

Figure 2 depicts temperature vs. voltage graphs for different thermocouple types, including types E, J, K, R, and S.

Depending on the metal wires used, thermocouples can measure temperatures from -200°C to +2500°C. The most popular thermocouple is the type-K, composed of chromel® and Alumel®, which measures temperatures from -200°C to +1250°C. For more details, refer to the difference between thermocouple types.

Advantages of Thermocouples

• Wide temperature range: Support from -200°C to +2500°C, depending on the metal wires used.
• Hazardous environments: Can be used in hazardous environments because they are rugged and immune to shock and vibration.
• Rapid response: Respond very quickly to temperature changes due to their small size and low thermal capacity (response time in the hundreds of milliseconds).
• Self-powered: Do not require external power, making them safe compared to RTDs and thermistors.
• Simplicity: Simple in design and operation.
• Variety of forms: Available in a wide variety of physical forms.

Disadvantages of Thermocouples

• Least stable and least expensive: Generally the least stable and accurate among the three sensor types.
• Signal conditioning: Substantial signal conditioning is needed to convert the voltage into a usable temperature reading.
• Reference junction accuracy: Measurement accuracy is limited by the accuracy of the reference junction temperature measurement (approximately within 1°C to 2°C).
• Corrosion: Corrosion can occur in certain environments due to the use of dissimilar metals, leading to deterioration in measurement accuracy. This necessitates care, maintenance, and protection.
• Noise: Microvolt-level measurements are susceptible to noise from stray electric and magnetic fields. Twisted wires and shielded cables are used to reduce magnetic and electric field interference, respectively.

RTD (Resistance Temperature Detector)

RTD stands for Resistance Temperature Detector or sensor. RTDs are passive devices, typically made of platinum wire, whose resistance varies with temperature. Platinum RTDs are the most common and accurate. They measure temperatures from -200°C to +850°C with an accuracy of +/-0.02°C.

Figure 3 shows the resistance versus temperature relationships for different RTD materials, including nickel, Balco, copper, and platinum. The X-axis represents temperature, and the Y-axis represents normalized resistance relative to the resistance at 0°C (or 32°F).

Advantages of RTDs

• Linearity: Reasonable linearity.
• Accuracy: Higher accuracy than thermocouples and thermistors.
• Drift: Lower drift over time.
• Stability: Most stable among the temperature sensor devices.
• Precision applications: Suitable for precision applications.
• Linearity vs Thermocouples: More linear compared to thermocouples.

Disadvantages of RTDs

• Signal conditioning: Requires signal conditioning.
• Cost: Higher cost compared to thermocouples and thermistors.
• Slow response: Slower response time than thermocouples and thermistors.
• Current source: Requires a current source for operation.
• Wiring: Often need four wires for accurate measurement.
• Self-heating and lead wire resistance: Self-heating and lead wire resistance can cause errors in measurements.

Thermistor

Thermistors are devices whose resistance changes significantly with temperature variations. They are temperature-dependent resistors.

There are two types of thermistors: NTC (Negative Temperature Coefficient) and PTC (Positive Temperature Coefficient). In NTC thermistors, resistance is inversely proportional to temperature, so resistance decreases as temperature increases. In PTC thermistors, resistance is directly proportional to temperature, so resistance increases as temperature increases.

Most thermistors are of the NTC type. They are available for temperatures ranging from -100°C to +150°C. They are used in batteries, coolants, engines, freezers, and incubators for over-temperature shutdown and temperature monitoring and control. For more information, refer to thermistor basics and manufacturers.

Advantages of Thermistors

• Inexpensive: Relatively inexpensive.
• Small size: Available in small packages.
• High output: Provide higher output signal compared to thermocouples.
• Fast response: Fast response time.
• Two-wire measurement: Need only two wires for resistance measurement.

Disadvantages of Thermistors

• Limited temperature range: Achieve higher precision but within a limited temperature range.
• Non-linear: Nonlinear, and their nonlinearity needs to be addressed using software or hardware circuits.
• Self-heating: Resistive devices requiring excitation current to read the voltage, leading to self-heating. Care must be taken to limit the sensing current to lower values to reduce this error.
• Fragile: More fragile than thermocouples and RTDs.
• Current source: Require a current source for operation.

Comparison Chart

The above figure depicts voltage/resistance versus temperature curves for thermistors, RTDs, and thermocouples (TC) in a single plot.