Thermometry

Thermometry

''' What is Thermometry? ''' Thermometry is the science and practice of temperature measurement. Any measurable change in a thermometric probe (e.g. the dilatation of a liquid in a capillary tube, variation of electrical resistance of a conductor, of refractive index of a transparent material, and so on) can be used to mark temperature levels, that should later be calibrated against an internationally agreed unit if the measure is to be related to other thermodynamic variables.

 History of Thermometry  There are a wide variety of temperature measurement probes in use today depending on what you are trying to measure, how accurately you need to measure it, if you need to use it for control or just man monitoring, or if you can even touch what you are trying to monitor. Temperature measurement can be classified into a few general categories: 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 actual science of ‘thermometry’ did not evolve until the growth of the sciences in the 1500’s The first actual thermometer was an air-thermoscope described in Natural Magic (1558, 1589). This device was the fore runner of the current class of glass thermometers. Up to 1841 there were 18 different temperature scales in use. An instrument maker, Daniel Gabriel Fahrenheit learned to calibrate thermometers from Ole Romer, a Danish astronomer. Between 1708 and 1724 Fahrenheit began producing thermometers using Romer’s scale and then modified that to what we know to day as the Fahrenheit scale. Fahrenheit greatly improved the thermometer by changing the reservoir to a cylinder and replaced the spirits used in the early devices with mercury. This was done because it had a nearly linear rate of thermal expansion. His calibration techniques were a trade secret, but it was known that he used 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. When the scale was adopted by Great Britain the temperature of 212 was defined as the boiling point of water. This point as well as the melting point of plain ice was used as two known calibration points. About 1740 Anders Celsius proposed the centigrade scale. It is 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. In 1948 the name of the centigrade scale was changed to Celsius.

Types of thermometers ''' Liquid-in-glass Introduction ''' Liquid-in-glass thermometer is the simplest and most commonly employed type of temperature measurement device. It is one of the oldest thermometers available in the industry. It gives fairly accurate results within the temperature range of -200 to 600°C. No special means are needed to measure temperature via these thermometers. One can read temperature readings easily with human eyes. They find their use in variety of applications such as medicine, metrology and industry. The foremost liquid-in-glass thermometer was introduced in the year 1650 in which the liquid filled in was spirit from wine. Later on, more linear thermometers were developed with the use of mercury as a liquid inside the thermometer. In the LIG thermometer the thermally sensitive element is a liquid contained in a graduated glass envelope. The principle used to measure temperature is that of the apparent thermal expansion of the liquid. It is the difference between the volumetric reversible thermal expansion of the liquid and its glass container that makes it possible to measure temperature. Construction A typical liquid-in-glass thermometer is shown in the figure below. It mainly comprises: •	A bulb which acts as a container for the functioning liquid where it can easily expand or contract in capacity. •	A stem, “a glass tube containing a tiny capillary connected to the bulb and enlarged at the bottom into a bulb that is partially filled with a working liquid”. •	A temperature scale which is basically preset or imprinted on the stem for displaying temperature readings. •	Point of reference i.e. a calibration point which is most commonly the ice point. •	A working liquid which is generally either mercury or alcohol. •	An inert gas, mainly argon or nitrogen which is filled inside the thermometer above mercury to trim down its volatilization.

Thermocouple

Introduction 

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. Working The working of a thermocouple is based on a Seebeck's principle which was recognized by Thomas Johann Seebeck in the year 1821. The Seebeck’s principle 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.

 Thermistor 

 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.

'''Radiation Thermometers (RTs) 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.

'''Integrated Circuit Temperature Sensors (IC Sensors) 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.