Resistance thermometer commonly dubbed as the resistance temperature detector and abbreviated as RTD are made of materials that have very high sensitivity to temperature. Change in the temperature is one of the cause of resistance change in conductors and some like Pt are more sensitive to these changes. RTDs are one of the temperature sensors (Read here) working on thermo - resistive principle other than Thermistors.
Working principle of RTD
RTDs work on the principle that metals change their resistance on change in temperature. A conductor of length l and cross sectional area A has a basic resistance R
In these passive resistance based sensing elements an output of the change in resistance is measured through bridge network. This change is proportional to the temperature. Metals with positive temperature coefficient are preferred for RTDs
The resistance of an RTD is given as: Rt = Ro (1 + αt + βt^2 )
For pure platinum: α = 3.94 Χ 10-3 /⁰C, β = – 5.8 Χ 10-7 /(⁰C)^2
The above equation can be rewritten as:
Rt = Ro (1 + C tpt)
where C = mean resistance temperature coefficient between 0 ⁰C and 100 ⁰C.
tpt = platinum temperature coefficient and is given by
where Rt, Ro, R100 are the resistance at t ⁰C, 0 ⁰C, 100 ⁰C.
Temperature coefficient of RTD
They differ from thermistor from the material used as the sensing element. RTDs use metals where as thermistors use ceramic and polymer. Positive temperature coefficient is when the resistance increases with increase in temperature . Platinum (Pt), Copper (Cu), Nickel (Ni), are some materials commonly used. Tungsten (W) is also used but only at high temperature as it is very brittle.
| Material | Temperature Coefficient of Resistance |
|---|---|
| Platinum | 0.00392 for reference grade |
| Tungsten | 0.0048 |
| Copper | 0.0043 |
| Nickel | 0.0067 |
| Lead | 0.0042 |
| Silver | 0.0041 |
| Gold | 0.004 |
| Iron | 0.002 to 0.006 |
| Aluminum | 0.0045 |
| Manganin | ± 0.00002 |
Selection criteria for materials used in RTDs:
1. Sensing Element :
- The material must be malleable to be formed into small wires.
- The materiel must have its resistance vs temperature characteristics to be repeatable and stable slope or curve
- The material should also be resistant to corrosion, inert at high temperatures also.
Platinum is highly preferable due to its sensitivity but using beyond 600 °C makes it open to contamination from lamination material (Ceramic or polymer) . Tungsten is used at high temperature applications only because it is very brittle. Silver and Gold are not used because of their small resistivity and Copper can only be used until 120 °C. Nickel and Nickel alloy is also used in low temperature applications.
2. Connection leads / Lead wire:
- Compatible with the sensing element
- Temperature resistant
3. Insulating layer:
- Abrasive resistant
- Temperature compatibility PVC, silicone rubber or PTFE are used for temperatures below 250°C and above this glass fibre or ceramic are used.
Normally RTDs are used only till 600 °C this is because, above this temperature Pt is susceptible to contamination from the surrounding materials.
Construction of a RTD Probe:
A RTD probe is composed of an element, a sheath, a lead wire, and a termination or connection. The wiring and packaging requirements need to be determined based on the element selected. There are different types of element used.
Element types used in RTD:
1. Wire wound Element : The wire wound sensor is the simplest sensor design with the sensing wire wrapped around an electrical insulating core that can be either round or flat. To minimize the mechanical strain the coefficient of thermal expansion of the sensing wire and winding core material is matched. The core is selected according to the temperature range.
The sensing wire is connected to the element lead or wire that is compatible with sensing wire. If not, the combination generate an emf that would distort the measurement.
The wire wound sensor is the only configuration that can be made with all of the sensing materials.
Reasons for Error:
Strain on the element wire
2. Coiled Element : To produce a “strain free” design, a coiled sensor is introduced . A strain free design allows the sensing wire to expand and contract freely without contributing to the error. The sensing element is a small coil of platinum sensing wire that resembles a bulb filament . The housing is aluminum oxide tube with four equally spaced holes that run
through the axes. The coil is inserted in the holes of the mandrel (housing) and are packed with a very fine ceramic powder. This assembly permits the sensing wire to move while still
remaining in good thermal contact with measurant.
3.Thin Film Element : The thin film sensing element is made by depositing a very thin layer of platinum of 10 to 100 angstroms thick on a ceramic substrate. The platinum film is then coated with epoxy or glass to protect the film and as a strain relief for the external leadwires.
Disadvantage is that they are not stable and only available for one type of Pt. Advantages are that they can be easily manufactured into small packages and have fast response as the thermal mass is less.
What is Pt100 :
Common type of industrial Platinum based sensing element for which the nominal resistance is 100 Ω at 0 °C with a sensitivity of 0.385 Ω/°C. Similarly Pt 1000 which has a resistance of 1000Ω at 0 °C is also available.
Lead wire configurations of RTD:
The selection of leadwire configuration depends on accuracy needed and instrumentation used for the measurement. The more the lead wire the accuracy increases. 1. Two wire RTD: In this, one wire is attached to each side of the element. A change in resistance can be taken by any device that measure resistance, including basic Volt Ohm Meters (VOM). This is the least accurate way of as the leadwire resistance is in series with the sensing element and at a different temperature than the sensing element as well as having different resistance Vs temperature characteristics. The longer the leadwire the greater the effect on the measurement. Compatibility of the sensing element and the lead wire is to be considered here.
2. Three Wire RTD: The three wire RTD is the most popular configuration for use in industrial applications. This can eliminate the series resistance. This permits an accurate measurement of the sensing element. Two of the leads are connected to one side of the sensing element and the third lead to the other side. The resistance in L1 and L3 should be matched as close as possible, this will cause the lead resistance to cancel themselves.
The color code for a three wire RTD is two red wires and one white.
3. Four Wire RTD: A four wire RTD is the most accurate method to measurement.It is used in laboratories mostly for its accuracy. A four wire RTD circuit removes the effect of mismatched resistances on the lead wires. A constant current is passed through L1 and L4. L2 and L3 measure the voltage drop across the RTD element.
The color code for a four wire RTD is usually two red wires and two white wires
Calibration of RTD
RTDs are calibrated to generate an R vs. T table or to determine if they are within a predefined tolerance. There is no adjustment to an RTD after it is built so any calibration is a check of the resistance at a given temperature. Types of RTD calibration
- Characterization • Calibrate at several temperatures and use equations for R vs. T
- Tolerance check • Compare resistance to defined R vs. T such as IEC 60751 or ASTM 1137
- Rule of thumb • If your minimum uncertainty of measurement is less than .1C you will want to use ITS-90. Otherwise you can use IPTS-68 For Calibration of platinum RTD with Step by step instruction click on this pdf
See Also
For comparison between thermistor and thermocouple and RTD: Click here For Passive Vs Active Transducers comparison : Click here For other resistance based transducer: Click here For other temperature transducers : Click here