Resistance Temperature Detector (RTD)
RTD stands for Resistance Temperature Detector. RTDs are sometimes referred to generally as resistance thermometers. An RTD is a temperature sensor which measures temperature using the principle that the resistance of a metal changes with temperature. In practice, an electrical current is transmitted through a piece of metal (the RTD element or resistor) located in proximity to the area where temperature is to be measured.
The resistance value of the RTD element is then measured by an instrument. This resistance value is then correlated to temperature based upon the known resistance characteristics of the RTD element.
The construction is typically such that the wire is wound on a form (in a coil) on notched mica cross frame to achieve small size, improving the thermal conductivity to decrease the response time and a high rate of heat transfer is obtained. In the industrial RTD’s, the coil is protected by a stainless steel sheath or a protective tube.
So that, the physical strain is negligible as the wire expands and increase the length of wire with the temperature change. If the strain on the wire is increasing, then the tension increases. Due to that, the resistance of the wire will change which is undesirable.So, we don’t want to change the resistance of wire by any other unwanted changes except the temperature changes.
This is also useful to RTD maintenance while the plant is in operation. Mica is placed in between the steel sheath and resistance wire for better electrical insulation. Due less strain in resistance wire, it should be carefully wound over mica sheet.
An RTD takes a measurement when a small DC current is supplied to the sensor. The current experiences the impedance of the resistor, and a voltage drop is experienced over the resistor. Depending on the nominal resistance of the RTD, different supply currents can be used. To reduce self-heating on the sensor the supply current should be kept low. In general, around 1mA or less of current is used.
An RTD can be connected in a two, three, or four-wire configuration. The two-wire configuration is the simplest and also the most error prone. In this setup, the RTD is connected by two wires to a Wheatstone bridge circuit and the output voltage is measured. The disadvantage of this circuit is that the two connecting lead wire resistances add directly two the RTD’s resistance and an error is incurred.
The variation of resistance of the metal with the variation of the temperature is given as,
Where, Rt and R0 are the resistance values at t°C and t0°C temperatures. α and β are the constants depends on the metals.
- Two wire RTDs are least commonly specified and are generally used where only an approximate value for temperature is needed.
- Three wire RTDs are the most common specification for industrial applications. Three wire RTDs normally use a Wheatstone bridge measurement circuit to compensate for the lead wire resistance as shown below. 3 wire RTD configuration, Wires “A” & “B” should be close to the same length. These lengths are significant because the intention of the Wheatstone bridge is to make the impedances of wires A and B, each acting as an opposite leg of the bridge, cancel the other out, leaving Wire “C” to act as a sense lead carrying a very small (microamperage range) current.
- Four Wire RTDs are even more accurate than their 3 wire RTD counterparts because they are able to completely compensate for the resistance of the wires without having to pay particular attention to the length of each of the wires. This can provide significantly increased accuracy at the relatively low cost of increased copper extension wire.
- Very high accuracy
- Suitability for remote measurement
- Excellent stability and reproducibility
- Ability to be matched to close tolerances for temperature difference measurements.
- Ability to measure narrow spans
- Need for lead wire resistance compensation
- Susceptibility to signal noise
- Susceptibility to mechanical damage
- Unsuitability for bare use in electrically conducting substance
- Need for power supply
- Generally not repairable
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