Thermometry

What is Thermometry?
Thermometry is a method of measuring temperature. Temperature is basically a measure of the amount of kinetic energy, particles possess. Nowadays, there are many different types of thermometers, for example glass in tube, thermistor, thermocouple, radiation thermometer etc.

History of Thermometry
Temperature measurement can be of 3 types:

a) Thermometers

b) Probes

c) Non-contact

Thermometers are the oldest of the group. The need to measure and quantify the temperature of something started around 150 A.D. when Galen determined the ‘complexion’ of someone based on four observable quantities. The first actual thermometer was an air-thermoscope. Up to 1841, there were 18 different temperature scales being used. Sprites of wines were eventually replaced by mercury, for better linear rate of thermal expansion. His calibration techniques was known to be a certain mixture of the melting point of a mixture of sea salt, ice, and water and the armpit temperature of a healthy man as calibration points. The two main calibration points used were boiling point of water and melting point of ice. About 1740 Anders Celsius proposed the centigrade scale. Its not clear who invented the scale, but it divided the range of the melting point of ice (100) to the steam point of water (0) into 100 parts, hence ‘centigrade’. Linnaeus inverted the scale so that 0 was the ice point and 100 was the steam point.Then soon enough the centigrade scale was renamed to Celsius scale.

Liquid-in-glass
Liquid-in-glass thermometer is the oldest and widest type of temperature measurement device used nowadays. A fair reading is given within -200 to 600°C. It is generally possible to read the temperature from eye level. In this thermometer, the thermally sensitive element is a liquid contained in a graduated glass envelope. The main principle used is that of the apparent thermal expansion of the liquid used. It is the difference between the volumetric reversible thermal expansion of the liquid and its glass container that makes it possible to measure temperature. To make sure that mercury wont evaporate at room temperature easily, an inert gas like argon or nitrogen are filled above mercury.

Introduction
A thermocouple is a temperature-measuring device that produces a voltage when the temperature on one contact is different from the reference temperature on another contact. The contacts are made of two dissimilar conductors. Commercial thermocouples are inexpensive, interchangeable and can measure a wide range of temperatures. Thermocouples don't have to rely on any outside sources to make work, unlike other measurement devices. However, thermocouple has its downside, it isn't very accurate.

Working
The working of a thermocouple is based on a Seebeck's principle, which says that whenever conductor material experiences a temperature difference i.e. temperature gradient, it produces voltage which can be measured by making use of another conductor. The second conductor material exposed to the same temperature difference would also produce a voltage which would be different from the first one. The difference between the two voltages is calculated and correlated to the corresponding changes in temperature i.e. temperature gradient. Hence, it is evident that a thermocouple is designed in such a way that it can only measure temperature differences and require a known reference temperature for accurate measurement.

=== Platinum resistance thermometer(resistance temperature detectors (RTDs) ===

Introduction
 A platinum resistance thermometer (PRT) is a device which determines the temperature by measuring the electrical resistance of a piece of pure platinum wire. The piece of platinum wire is referred to as a temperature sensor. When manufactured carefully these devices offer an excellent combination of sensitivity, range and reproducibility.

Working
The electrical resistance of many metals (e.g. copper, silver, aluminium, platinum) increases approximately linearly with absolute temperature and this feature makes them useful as temperature sensors. The resistance of a wire of the material is measured by passing a current (AC or DC) through it and measuring the voltage with a suitable bridge or voltmeter, and the reading is converted to temperature using a calibration equation. The most reproducible type of sensor is made from platinum because it is a stable unreactive metal which can be drawn down to fine wires but is not too soft. Using very pure wires, thermometers can be made with closely similar resistance characteristics and achieve good reproducibility in use. The length and diameter of the platinum wire used in a thermometer are often chosen so that the resistance of the device at around 0 ºC is 100 ohms. Such a sensor is a called a PT100 sensor, and its resistance changes by approximately 0.4 ohms per degree Celsius.

Introduction
A thermistor is simply an electrical resistor whose resistance changes rapidly with temperature. Usually thermistors are made from small beads of complex materials and although they are not as reproducible as platinum resistance thermometers, they are much more sensitive. Commonly the resistance falls exponentially with increasing temperature and such devices are said to have a negative temperature coefficient (NTC). The strong change in temperature is useful for establishing sensitive temperature control of a system.

Working
The thermistor acts as the temperature sensor and it is placed on the body whose temperature is to be measured. It is also connected in the electric circuit. When the temperature of the body changes, the resistance of the thermistor also changes, which is indicated by the circuit directly as the temperature since resistance is calibrated against the temperature. The thermistor can also be used for some control which is dependent on the temperature.

Introduction
 Radiation Thermometers (Pyrometers, if you will) are non-contact temperature sensors that measure temperature from the amount of thermal electromagnetic radiation received from a spot on the object of measurement. This group of sensors includes both spot or "point" measuring devices in addition to line measuring radiation thermometers, which produce 1-D and, with known relative motion, can produce 2-D temperature distributions, and thermal imaging, or area measuring, thermometers which measure over an area from which the resulting image can be displayed as a 2-D temperature map of the region viewed.

Working
A radiation thermometer consisted of an optical system to collect the energy emitted by the target; a detector to convert this energy to an electrical signal; an emittivity adjustment to match the thermometer calibration to the specific emitting characteristics of the target, and an ambient temperature compensation circuit, to ensure that temperature variations inside the thermometer due to ambient conditions did not affect accuracy.

Introduction
 A two terminal integrated circuit temperature transducer that produces an output current proportional to absolute temperature. The sensor package is small with a low thermal mass and a fast response time. The most common temperature range is 55 to 150°C (-58 to 302°F). The solid state sensor output can be analog or digital.

Working
The analog IC solid state sensors provide an output as a voltage or current that is proportional with temperature without additional circuitry. The digital IC sensors provide an output that has been processed thru an integral A-D converter and is ready for input into digital control and monitoring systems. The IC sensors do not require linearization or other circuitry.