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The thermocouple is a type of temperature measuring sensor or thermoelectric sensing element consisting of two dissimilar materials (metals) with two junctions. One junction is referred to as the Cold Junction or reference junction and the temperature at this junction is constant. The other is referred to as the Hot Junction or measuring junction. When these two junctions are at different temperatures a voltage is generated and this voltage is used to interpret temperature.
The effect of temperature difference in creating a voltage is referred as the thermoelectric effect and is the basic principle which is utilised in the use of the thermocouple for the measurement of temperature. Thermocouples can be designed to measure temperatures ranging from -200? to 2000 ?. They have the widest range of temperature measurement of all temperature sensors.
The principle of operation is based on discovery made by a German physicist, Johann Seebeck. He discovered in 1821 that when different metals are joined at the ends and having a temperature difference between the joint, a magnetic field is observed. This he referred to as thermo-magnetism. It was later discovered that this magnetic field was a result of thermo-electric current thereby reinforcing the theory that two dissimilar metals when joined, create a voltage at their junction. It is this voltage that is of practical use in determining temperature. Temperatures can be very low or very high.
Voltages generated are generally very small, and in the microvolt range. The magnitude of the voltage depends upon the type of materials used.
Arising out of Seebeck’s discovery is what is known as the Seebeck Effect which is actually an example of an electromotive force (emf). An electromotive force leads to measurable currents or voltages.
Under open-circuit conditions where there is no internal current flow, the gradient of voltage (?V) is directly proportional to the gradient in temperature (?T): ?V = -S(T)?T
Where S(T) is a temperature dependent material property known as the Seebeck coefficient.
The standard measurement configuration shows four temperature regions and therefore four voltage contributions.
The first and fourth contributions cancel out directly because these regions involve the same temperature change and the same metal. As a result Tmeter does not influence the measured voltage. The second and third contributions do not cancel as they involve different materials.
The measured voltage turns out to be : V = Where S+ and S- are the Seebeck coefficients of the conductors attached to the positive and negative terminals of the voltmeter respectively (chromel and alumel in the figure)
To obtain the measurement for Tsense it is necessary that temperature at the reference junctions be known. Voltage measurement by itself does not make possible the determination of Tsense.
There are two (2) methods of establishing the temperature at the reference junctions:
The first is the use of an ice bath to create a fixed temperature. The reference junction block is immersed in a semi frozen bath of distilled water at atmospheric pressure. The precise temperature of the melting point phase transition acts as a thermostat fixing Tref. to 0 ?.
The second method is to use a sensor (known as “cold junction compensation”). The reference junction block is allowed to vary in temperature, but its temperature is measured using a separate temperature sensor. Knowledge of this temperature is used to compensate for temperature variation at the block.
Issues affecting the use of thermocouples are related to uncertainties in the manufacture of alloys used, thus, impurities affect each batch of alloy manufactured, the aging on the accuracy of measurements, and circuit design mistakes such as error on the estimation of Tref.
The use of a particular type of thermocouple is influenced by a few considerations namely cost, availability, melting point, chemical properties, stability and output. Different types are best suited for specific applications. Selection is normally based on temperature range and sensitivity required.
Standard thermocouple types (with positive electrode first followed by negative electrode) are:
Typical range is 0 – 2315?. They are used in vacuum furnaces and hydrogen and inert atmospheres.
Others include thermocouples using chromel-gold/iron-alloy, platinum/molybdenum-alloy, iridium/rhodium alloy, pure noble metal Au-Pt, Pt-Pd.
In general, thermocouples are suitable for measuring over a large temperature range from -270 to up to 3000? (for a short time in an inert atmosphere). Examples of areas of use are in kilns, gas turbine exhaust, diesel engines and fog machines. They are less suitable for applications where smaller temperature differences are to be measured with high level of accuracy, for example in the range 0-100? with an accuracy of ±1?. Such applications require the use of thermistors, silicon band-gap temperature sensors or resistance thermometers.
The iron and steel industry makes extensive use of type B, S, R and K thermocouples to monitor temperatures throughout the steel making process. For the electric arc furnace use is made of disposable, immersible, type S thermocouples.
In gas fed heating appliances, thermocouples are used as a safety mechanism. The thermocouple is set up in a fail -safe circuit to detect when the pilot flame goes out. The tip of the thermocouple is placed in the flame generating a voltage which operates the supply valve to the pilot. As long as the flame is lit the valve remains open. If the flame goes out the thermocouple temperature drops, causing the voltage across the thermocouple to drop and the valve to close.
In manufacturing, thermocouples, for example, can be used to monitor and confirm that temperature limits are not exceeded.
In power production a thermocouple can produce a current to drive some process directly, for example, power from the thermocouple can activate a valve when a difference in temperature occurs.
In process plants such as chemical production and petroleum refineries thermocouples are used to monitor the hundreds of temperatures associated with the process.
The value of the thermocouple comes in its fast reaction time, its wide range of temperature measurement and its ability to be used in many applications. One disadvantage for temperature measurement is that its signal is non -linear and careful calibration is required for desired accuracy.
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