Some temperature measurement devices, like many modern sensors, act as energy transducers—converting energy from one domain into another. Specifically, they transduce thermal energy (heat) into electrical signals that can be measured, amplified, and digitized. Alternatively, temperature measurement devices can change a physical property (e.g., resistance) in response to temperature changes. The nature of this transduction or property change defines the sensor's operating principle and often its range and precision.
In this section, we will introduce various devices used for measuring temperature, all of which you will use in the lab. Later, we will explore their operating principles in more detail.
Thermometers are direct-reading devices that measure temperature based on the physical expansion of a liquid (typically mercury or alcohol) in response to temperature changes. The liquid expands within a calibrated capillary tube to indicate temperature. These are widely used in laboratory and household settings, though their use is declining due to mercury toxicity concerns.
Alcohol thermometers are commonly used in cold environments due to their lower freezing point compared to mercury.
Infrared (IR) thermometers determine temperature by detecting the intensity of thermal radiation (infrared light) emitted by an object. They are non-contact instruments and are particularly useful for measuring moving objects, hazardous surfaces, or objects at a distance. IR thermometers require emissivity calibration for accurate readings.
Thermocouples exploit the Seebeck effect, wherein a thermoelectric voltage is generated across two dissimilar metal junctions subjected to a temperature difference. This voltage is a function of the materials and temperature gradient. Thermocouples are compact, durable, and capable of measuring extreme temperatures. They require reference junction compensation (cold-junction compensation) for accurate results, typically handled via onboard electronics or ice baths.
The voltage generated is typically in the millivolt range and must be interpreted using calibration tables or polynomial fits specific to the thermocouple type. The Seebeck coefficient \(\alpha\) varies with material and temperature.
Common thermocouple types are standardized according to material pairings and performance characteristics:
\[ V = \alpha (T_{\text{hot}} - T_{\text{cold}}) \]
Thermistors are thermally sensitive resistors, typically made from ceramic semiconductors. Their resistance changes with temperature in a highly non-linear manner, with most exhibiting negative temperature coefficients (NTC), meaning resistance decreases as temperature increases. They offer high sensitivity and precision over narrow temperature ranges and are commonly used in digital thermometers, automotive sensors, and battery protection circuits. Their nonlinear behavior requires careful calibration and often limits their use in high-temperature applications.
\[ R(T) = R_0 \exp\left[\beta \left(\frac{1}{T} - \frac{1}{T_0}\right)\right] \]
The parameter \(\beta\) is a material-specific constant, and temperature in this model is assumed to be in kelvin.
Thermistors are used in HVAC, medical devices, consumer electronics.
Resistance temperature detectors (RTDs) operate based on the principle that the electrical resistance of a metal increases approximately linearly with temperature. Platinum is the most commonly used metal due to its chemical stability, repeatability, and near-linear response. RTDs such as the PT100 (100 Ω at 0 °C) are standard in industrial and laboratory applications where precision and stability are required. RTDs are more accurate and stable than thermocouples but are generally more expensive and slower to respond.
While the linear relation is convenient, precise modeling often uses a higher-order polynomial such as the Callendar–Van Dusen equation, particularly for platinum RTDs like the PT100.
\[ R(T) = R_0 (1 + \alpha T) \]
RTDs are typically connected in 2-, 3-, or 4-wire configurations to reduce or eliminate the effects of lead wire resistance. The 2-wire configuration is simplest but introduces the most error; the 3-wire configuration compensates for this error under the assumption of matched lead resistances; and the 4-wire configuration provides the highest accuracy by fully separating the measurement and excitation circuits.
Thermal imaging cameras detect long-wave infrared radiation (8–14 μm) emitted by all objects above absolute zero and convert it into an electronic signal to create a thermal image. These systems use sensors such as microbolometers that change resistance with temperature. They are calibrated using blackbody radiation sources to ensure measurement accuracy. Applications range from preventive maintenance (identifying hot spots in electrical systems) to medical diagnostics (detecting inflammation or circulatory problems) and energy auditing (visualizing heat loss in buildings). While they do not measure contact temperature directly, they provide valuable spatial temperature distribution information.
Unlike infrared thermometers, which measure the temperature at a single spot, thermal cameras provide a two-dimensional spatial distribution of temperature. This allows for real-time visualization of temperature gradients across a surface.
Thermal imaging sensors often use microbolometers or thermopiles, and may be calibrated via infrared blackbody sources.
As with infrared thermometers, measurement accuracy can be affected by surface emissivity, especially on shiny or metallic surfaces.
The table below summarizes key characteristics of the most common temperature measurement devices:
| Device Type | Operating Principle | Typical Range | Accuracy | Response Time | Cost | Notes |
|---|---|---|---|---|---|---|
| Thermometer | Liquid expansion | -80 to 300 °C | ±1 °C | Slow | Low | Visual readout; limited precision |
| Thermocouple | Seebeck effect (voltage) | -200 to 2300 °C | ±1 to ±2 °C | Fast | Low | Wide range, nonlinear, needs reference |
| Thermistor | Resistance change (NTC semiconductor) | -50 to 150 °C | ±0.1 to ±0.2 °C | Fast | Low | Highly sensitive, nonlinear |
| RTD | Resistance change (metal, usually platinum) | -270 to 850 °C | ±0.01 to ±0.1 °C | Moderate | Medium | Stable, accurate, 2-/3-/4-wire options |
| Thermal Camera | Infrared radiation (imaging) | -20 to 2000+ °C | ±2 °C or 2% | Fast | High | Spatial resolution, emissivity-sensitive |
This table can aid in selecting the appropriate sensor type based on the application’s range, precision, speed, and budget.