Resistance Temperature Detector (RTD)
As the name suggests RTD (Resistance temperature detector), gives output as resistance after detecting the temperature.
Resistance Temperature Detectors (RTDs) are characterized by a linear change in resistance with respect to temperature. RTDs are positive temperature coefficient, that means Resistance increases with increase in temperature.
Platinum is Commonly used in RTDs because its temperature Resistance characteristic is more linear in comparison to copper and nickel.
RTD types are broadly classified according to the different sensing elements used. Platinum, Nickel, and Copper are the most commonly used sensing elements. Platinum is considered the best as it has the widest temperature range. This is shown in the resistance versus temperature graph above.
Platinum type RTD is also known for its best interchangeability than copper and nickel. It also has the highest time stability. PRT’s can also be used in unsuitable environments where it can reduce atmospheric metallic vapors and also capitalizable vapors if the element is bare. It can also be used in radioactive environments. In industrial applications, a PRT is known to measure temperatures as high as 1500 degree Fahrenheit while copper and Nickel can measure only to a maximum of 400 degrees Fahrenheit.
How to check RTD
There are three wires in RTD. Out of the two wires are short-circuited as shown in the figure. Same color wires are short-circuited.
white color wires are short-circuited
We will check whether two same color wires are short-circuited or not. If no then connect a wire between these two same color wires. This wire can be connected either on RTD Side or Indicator Side.
We will check whether the different color wire is showing the same resistance with the same color wires. If no then change the RTD.
If different color wires are showing the same resistance then we will calculate the temperature according to the table or formula provided.
Since platinum is used in RTDs so RTDs are PT 100, PT 500, PT1000.
PT 100 means it will show 100 ohms at 0 Degree Celsius. PT 500 will show 500 ohms.
We will check the resistance to calculate the temperature. Third wire shows the same resistance with the other two wires. And for calculating temperature the formula is
Rt = R0(1+@t)
Where t is temperature, R0 is resistance at 0 Degree Celsius. Rt is resistance at t Degree Celsius. R0 is 100 Degree Celsius for PT 100.
@ is temperature Coefficient.
The other method of calculating temperature is
For PT 100
t = (Rt-100)*2.6
similarly for PT 500
that means we will check the resistance between a white and red wire, it will show some value more than 100 ohms, this resistance is called as Rt.
suppose Rt is 138.5,
then t= (138.5-100)*2.6
approximately 100 degrees Celsius.
Types of RTDs
RTDs are of three types.
2 wire RTD.
3 wire RTD.
4 wire RTD.
Commonly used RTD Materials:
Platinum (most popular and accurate)
2 wire RTD is used for short distances.
3 wire RTD is used for long distances. The third wire is used for Resistance compensation. Resistance compensation means when the signal is coming from large distance wire resistance is not added.
4 wire RTD is used for large distances.
The various wiring arrangements are designed to reduce and/or eliminate any errors introduced due to resistance changes of the lead wires when they also undergo temperature changes. RTDs used for electrical equipment generally use either a three-wire system or a four-wire system having paired lead wires.
The four wire system is little affected by temperature induced resistance changes in lead-wires, and, of all the arrangements, it is affected least by stray currents. It, therefore, is used to measure temperature differences and is used generally for making very accurate measurements. The three-wire system is generally satisfactory for industrial measurement using a secondary instrument that is remote, say, more than 3 meters distant from the RTD. Although the error caused by temperature change in the leads is virtually eliminated in a 3-wire arrangement, a slight non-linearity in the resistance change is introduced with this scheme.
RTDs are available in different sizes according to their length. Length of an RTD is in millimeters (mm). This Sensing element is nothing but a PT100 capsule. All RTDs are simplex and duplex.
Simplex and Duplex RTD
In simplex type RTDs, there will be only one element with 3 wires.
In duplex type RTD, there will be 2 temperature sensing elements of 3 wires each.
- Very high accuracy
- Ability to measure narrow spans
- Suitability for remote measurement
- Ability to be matched to close tolerances for temperature difference measurements.
- Excellent stability and reproducibility
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