Any material or system that changes with temperature can be used to measure temperature, i.e. create a thermometer. E.g. Mercury changes its volume and metal wires undergo changes in resistance when temperature changes. ==Types of Thermometers== ===Mercury or Ethanol=== The mercury-in-glass or mercury thermometer was invented by physicist Daniel Gabriel Fahrenheit in Amsterdam (1714). It consists of a bulb containing mercury attached to a glass tube of narrow diameter; the volume of mercury in the tube is much less than the volume in the bulb. The volume of mercury changes slightly with temperature; the small change in volume drives the narrow mercury column a relatively long way up the tube. The space above the mercury may be filled with nitrogen or it may be at less than atmospheric pressure, a pencilvacuum. [image:http://upload.wikimedia.org/wikipedia/commons/thumb/5/5f/Quicksilvertermometer_Osaby.JPG/170px-Quicksilvertermometer_Osaby.JPG]In order to calibrate the thermometer, the bulb is made to reach thermal equilibrium with a temperature standard such as an ice/water mixture, and then with another standard such as water/vapour, and the tube is divided into regular intervals between the fixed points. These thermometers are '''sensitive''' but have a '''limitied range''' (-40C to 350C for Mercury, -120C to 80C for Ethanol). ===Metal Resistance=== Resistance thermometers, also called resistance temperature detectors (RTDs), are sensors used to measure temperature by correlating the resistance of the RTD element with temperature. A metal's resistance changes with temperature.. Most RTD elements consist of a length of fine coiled wire wrapped around a ceramic or glass core. The element is usually quite fragile, so it is often placed inside a sheathed probe to protect it. The RTD element is made from a pure material, typically platinum, nickel or copper. The material has a predictable change in resistance as the temperature changes and it is this predictable change that is used to determine temperature. [image:http://upload.wikimedia.org/wikipedia/commons/thumb/3/3c/Wire_Wound_PRT.png/220px-Wire_Wound_PRT.png] They typically have a '''wide range''': -260C to 1700C but have a '''poor sensitivity''' over small range. They can also be to '''hard to calibrate''' as the relationship is non-linear. ===Thermistors=== A thermistor is a type of resistor whose resistance varies significantly with temperature, more so than in standard resistors. '''As temperature increases, the resistance of the thermistor decreases.''' Thermistors differ from resistance temperature detectors (RTDs) in that the material used in a thermistor is generally a ceramic or polymer, while RTDs use pure metals. The temperature response is also different; RTDs are useful over larger temperature ranges, while thermistors typically achieve a higher precision within a limited temperature range, typically −90 °C to 130 °C. [image:http://upload.wikimedia.org/wikipedia/commons/thumb/3/3b/NTC_bead.jpg/200px-NTC_bead.jpg] Thermistors can be very small components and have a rapid response to changing temperatures. They are also very sensitive to small changes in temperatures. ===Thermocouple=== A thermocouple is a temperature-measuring device consisting of two dissimilar conductors that contact each other at one or more spots, where a temperature differential is experienced by the different conductors (or semiconductors). It produces a voltage when the temperature of one of the spots differs from the reference temperature at other parts of the circuit. Thermocouples are a widely used type of temperature sensor for measurement and control, and can also convert a temperature gradient into electricity. Commercial thermocouples are inexpensive, interchangeable, are supplied with standard connectors, and can measure a wide range of temperatures. In contrast to most other methods of temperature measurement, thermocouples are self powered and require no external form of excitation. The main limitation with thermocouples is accuracy; system errors of less than one degree Celsius (°C) can be difficult to achieve. Any junction of dissimilar metals will produce an electric potential related to temperature. Thermocouples for practical measurement of temperature are junctions of specific alloys which have a predictable and repeatable relationship between temperature and voltage. Different alloys are used for different temperature ranges. [image:http://i.imgur.com/9rstqS2.png]
Credit: Tristan O'Hanlon