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The thermistor can be placed anywhere in the bridge with three constant resistors, but different placements can produce different behaviour in the bridge. For example, different placements might cause the output voltage to go in different directions as the temperature changes.
1. 4.Resistance Temperature Detector
RTD stands for Resistance Temperature Detector. RTDs are sometimes referred to generally as resistance thermometers. An RTD is a temperature sensor that measures temperature using the principle of the resistance of a metal changes with temperature.In practice a small electric current transmitted through a piece of a metal which are platinium ,copper or nickel(the rtd elemt or resisitor ) located in the area where temperure is to be measured.The resistance value of the RTD elementis then measured by an instunment.This resistnce value is then correlated to temperaute.They are used in many industrial applications below 600 °C, due to higher accuracy and repeatability.platinium is the most stable resistance-temperature relationship over the largest temperature range and best metal for RTDs because it follows a very linear resistance-temperature relationship and it follows the R vs T relationship in a highly repeatable manner over a wide temperature range and the unique properties of platinum make it the material of choice for temperature standards over the range of -272.5 °C to 961.78 °C, and is used in the sensors that define the International Temperature Standard. Platinum is chosen also because of its chemical inertness.
2. Working principle
RTDs work on a basic correlation between metals and temperature. As the temperature of a metal increases, the metal's resistance to the flow of electricity increases. Similarly, as the temperature of the RTD resistance element increases, the electrical resistance, measured in ohms (Ω), increases. RTD elements are commonly specified according to their resistance in ohms at zero degrees Celsius (0° C). The most common RTD specification is 100 Ω, which means that at 0° C the RTD element should demonstrate 100 Ω of resistance.Most traditional RTD operation is based upon a linear relationship between resistance and temperature, where the resistance increases with temperature.
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Ohm's Law: V=IR. This can be re-arranged to I=V/R. This states that the current (I) through a circuit is equal to the voltage across the circuit (V) divided by the resistance in the circuit (R). Since I is inversely proportional to R, then for a fixed V, the current will decrease as the resistance increases. One of the things which increases electrical resistance is the increased motion of the molecules and free electrons of a conductor caused by heating. This is due to their random collisions with the electrons making up the current which tends to interfere with their forward progress. So, it is true that heating of the metal(resistance element) causes the free electrons to move faster, but in random directions. As a result, the net electron current will be diminished due to the increased frequency of random collisions.From the figure below shows that when a resistance element come incontact to a higher temperture where there is a constant current flow through the circuit, the collision or movemnet of the electrons increases and the same way the electrical resistance also increase but the current will decrases at the end side of the resistance element.The change in electrical resistance of the metal when there is a temperature difference at different situation sensed and measured.
Figure 2.Effect of temperature on Resistance and electric change in monocular
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3. Sensitivity
The relative change in resistance (temperature coefficient of resistance) varies only slightly over the useful range of the sensor. The R vs T relationship is defined as the amount of resistance change of the sensor per degree of temperature change. Then estimate of RTD sensitivity can be noted from typical values of ao, the linear fractional change in resistance with temperature. For platinum, this number is typically on the order of 0.004/°C, and for nickel a typical value is 0.005/°C. Thus, with platinum, for example, a change of only 0.4 W would be expected for a 100-W RTD if the temperature is changed by 1°C. Usually, a specification will provide calibration information either as a graph of resistance versus temperature or as a table of values from which the sensitivity can be determined. For the same materials, however, this number is relatively constant because it is a function of resistivity.
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