| Thermometer | Thermometric property |
|---|---|
| Liquid in glass | Expansion of liquid |
| Bimetallic strip | Expansion of solid |
| Bourdon | Pressure change of gas (Fig. 2) |
| Resistance | Change in resistance of wire |
| Thermistor | Change in resistance of metal oxides |
| Thermocouple | Seeback effect |
| Infra-red | Thermal radiation |
Liquid volume increases in proportion with temperature. The liquid thermometer (alcohol / mercury) places a volume of liquid in a bulb. Measuring the volume change is achieved by allowing a very thin (small volume) column of the liquid to escape along a channel. Since the volume of the channel is very small, the change in volume it contains reflects the change in the volume of the liquid in the bulb and is linear with respect to temperature more information
.Solids can be described in terms of a linear coefficient of expansion the change in length per Kelvin). Different solids expand at different rates and this is exploited by firmly joining two strips of dissimilar material. As the expand and contract at different rates, the combined strip experiences a torque or bending force, which can be used as a pointer or operate as a switch (e.g. in an electric iron).
Bimetallic strip
The Bourdon gauge exploits the change in pressure in a gas with temperature. As a sealed, coiled metal tube is heated, the gas within it expands and tends to straighten it. This mechanical change can operate a pointer as in Figure 2
Bourdon temperature guage
Most materials show an increased resistance with temperature. This can be used to measure temperature as in the platinum wire thermometer (using a very thin platinum wire). Unfortunately, although simple and providing a linear temperature response, this device is relatively insensitive to small changes in temperature. By contrast, thermistors are cheap, robust and sensitive, but are non-linear and generally show a decrease in resistance with temperature (so-called negative temperature-coefficient or NTC devices). It is possible to manufacture positive temperature-coefficient devices (PTC), but they are uncommon. Thermistors are generally very stable over time, although some MCQ's may suggest otherwise !
Thermistors
Thermistors resistance characteristics
This depends on the Seebeck effect. This is the production of a voltage (and current) when a material experiences a temperature gradient. Thus, in Figure \ref{Fig:Thermocouple} a voltage exists across the hot and cold junctions of Metal A and Metal B. The junction itself is unimportant (it is required to create an electrical circuit to allow the measurement, but NOT for producing the voltage itself). By using two dissimilar metals, there will be a difference in the voltage along their lengths when they experience identical temperature gradients. If the cold junctions are at a known temperature, then the temperature of the hot junction can be measured Good explanation here.
Thermocouple
An infrared thermometer detects the infrared radiation emitted by an object. It uses this data to calculate the object’s surface temperature, utilizing the relationship between radiation intensity and temperature described by Planck’s law and Stefan-Boltzmann law.
Planck’s Law describes the distribution of radiation intensity over various wavelengths based on temperature.
tefan-Boltzmann Law gives the total power radiated per unit area of a black body, proportional to the fourth power of its absolute temperature.
The thermometer has a sensor (often a thermopile or pyroelectric detector) that is sensitive to infrared radiation. When pointed at an object, the sensor detects the IR radiation being emitted. This is then converted into an electrical signal. Based on the intensity of the IR radiation received, the device calculates the surface temperature of the object.
Since real-world objects are not perfect black bodies, they have an emissivity value (a measure of how efficiently they emit radiation) less than 1. Infrared thermometers often have a setting for emissivity to account for this. If the emissivity is unknown, it is usually set to a standard value (like 0.95 for many organic materials) for general use.
A thermistor
A thermistor
The thermistor in cardiac output measurement
A thermistor
Highly technical description of all aspects of temperature measurement