Views: 30 Author: Site Editor Publish Time: 2025-10-31 Origin: Site
1. Types of Temperature Sensors
Sensor Type | Principle | Range | Accuracy | Pros | Cons |
Thermocouple (TC) | Seebeck Effect: Voltage generated at the junction of two dissimilar metals. | -200°C to +2300°C | Low to Medium (0.5°C to 5°C) | Wide range, rugged, fast response, low cost | Non-linear, requires cold junction compensation, low sensitivity |
RTD (Pt100/Pt1000) | Change in electrical resistance of a pure metal (Platinum) with temperature. | -200°C to +850°C | High (0.1°C to 1°C) | Very accurate, stable, relatively linear | Slower response, more expensive than TCs, self-heating error |
Thermistor | Change in electrical resistance of a ceramic/semiconductor material. | -40°C to +125°C | -40°C to +125°C | Very High (0.1°C to 0.5°C) in limited range | High sensitivity, high accuracy in its range, fast | Very non-linear, limited range, fragile, self-heating |
IC/Semiconductor | Silicon band-gap properties: The base-emitter voltage of a transistor is proportional to temperature. | -55°C to +150°C | Medium to High (0.5°C to 2°C) | Linear output, easy to interface (digital or analog), low cost | Slowest response, limited range, self-heating |
Infrared (IR) Non-Contact | Detects infrared radiation emitted by an object. | -50°C to +3000°C | Low to Medium (1% of reading) | Measures without touch, very fast, measures moving objects | Measures surface temp, affected by emissivity, ambient temp, and dust |
2. Key Technical Concepts & Selection Criteria
1. Accuracy vs. Precision:
Accuracy: How close the measurement is to the *true* value.
Precision: How repeatable the measurements are (low scatter).
Tip: You can have high precision but low accuracy (consistent wrong readings). Good systems need both.
2. Response Time (Time Constant - τ tau):
The time it takes for the sensor to reach 63.2% of a step change in temperature.
Tip: A small, exposed sensor (like a bead thermistor) has a fast response. A sensor in a metal sheath (like a thermowell) is much slower. For rapidly changing temperatures, response time is critical.
3. Stability/Drift:
How well the sensor maintains its calibration over time.
Tip: RTDs are the most stable. Thermistors can drift significantly if they are overheated. Factor in re-calibration schedules for critical applications.
4. Linearity:
How much the sensor's output deviates from a straight line.
Tip: IC sensors are highly linear. Thermistors are extremely non-linear and require lookup tables or complex equations (Steinhart-Hart) for accurate conversion.
5. Noise Immunity:
Thermocouples produce a very small voltage (mV) and are highly susceptible to electrical noise. Always use shielded, twisted-pair cable.
Tip: For long runs in noisy industrial environments, use a temperature transmitter to convert the weak TC signal to a robust 4-20mA loop signal.
