Resistance Temperature Detector (RTD) is replacing thermocouples in many industrial applications due to their higher accuracy and stability. The RTD allows accurate and consistent temperature measurement. This post will discuss Resistance Temperature Detector (RTD), its components, working , wire configurations, applications, advantages and disadvantages.
What is Resistance Temperature Detector (RTD)
Resistance Temperature Detector (RTD) is a temperature sensor that is used to measure temperature by associating the resistance of the Resistance Temperature Detector (RTD) element with temperature.
Fig. 1 – Resistance Temperature Detector (RTD)
The elements used in Resistance Temperature Detectors are specified in to their resistance in ohms at zero degrees Celsius (C). The RTD specification that is most commonly used is 100 ohms. This means that a Resistance Temperature Detector element should have a resistance of 100 Ohms at 0° Celsius.
The most common metal which is used as Resistance Temperature Detector element is Platinum. Platinum is a noble metal and has stable resistance-temperature relationship over a large temperature range. In addition to Platinum, Nickel, Copper and Balco is also used as RTD element.
Platinum is the most common choice because of following properties:
- Its inertness to foreign chemicals.
- Its Linear resistance change with temperature.
- Its high Temperature coefficient of resistance. This avoids any sudden spike in resistance with respect to applied temperature.
- Its stability.
Fig. 2 – Resistance Temperature Detector Module and Devices
Thin film elements are commonly used today, although ceramic wire wound elements still have their place. Ceramic wire wound RTDs are used when the temperature measured is higher or if there is vibration in the application.
Components of Resistance Temperature Detector (RTD)
The components of Resistance Temperature Detector (RTD) are:
- Resistance Element
- Outside Diameter
- Tubing Material
- Process Connection
- Wire Configuration
- Cold End Termination
Resistance Element is the main temperature sensing portion of the Resistance Temperature Detector. The Element’s length ranges from 1/8″ to 3″. The standard resistance is 100 Ω at 0° C and the standard temperature coefficient is an alpha of .00385.
The outside diameters range from .063″ to .500″. In US, the most common RTD outside diameter in use is 0.25″ while in other countries it is normally 6mm.
316 Stainless steel is used for assemblies up to 500° F. Above 500° F Inconel 600 is used.
Process connection fittings consists all regular fittings that are utilized with thermocouples.
Fig. 3 – Components of Resistance Temperature Detector (RTD)
Resistance Temperature Detectos (RTDs) are available in three different wire configurations. These configurations are 2, 3 and 4 wire configurations.
The most commonly used materials for wire insulation are Fiberglass and Teflon. Teflon is used up to 400° F as it is moisture resistant. Fiberglass is used up to 1000°F.
Cold End Termination
RTDs can terminate on the cold end with plugs, terminal heads, bare wires, and any of the reference junctions common to thermocouples.
Working Principle of Resistance Temperature Detector (RTD)
A Resistance Temperature Detector (RTD) functions on the resistance and temperature relationship in metals. It works on the principle of measurement which states that “The resistance of a material changes with temperature”.
When the metal’s temperature increases, the resistance to the electron’s flow also increases. Similarly, as and when the temperature of element used in the Resistance Temperature Detector increases, the resistance increases too.
The resistance of Resistance Temperature Detector (RTD) changes constantly with respect to the applied temperature and so the temperature is quite predictable by measurement of its resistance. It is this property which allows Resistance Temperature Detector (RTD) to measure temperature more accurately and consistently.
RTD’s resistance and the temperature can be determined by applying a constant current and evaluating the occurrence of voltage drop through the resistor.
Wire Configurations in Resistance Temperature Detector (RTD)
Resistance Temperature Detectors (RTDs) are available in following configurations:
- Two Wire Configuration
- Three Wire Configuration
- Four Wire Configuration
Two Wire Configuration
It is the most simple one and has the chances of errors. In this configuration, RTD is connected to a Wheatstone bridge circuit using two wires. The voltage output is monitored to get the the desired output.
Fig. 4 – Two Wire Configuration of Resistance Temperature Detector
The biggest disadvantage of two wire configuration is that the RTD’s resistance is the direct summation of the two connected wire’s resistance and so an error is most likely to occur.
Three Wire Configuration
In Three Wire Configuration, the wires “RL1” & “RL3″(as shown in figure below) should be nearly of same length. The lengths of the wire are to be carefully kept equal (approximately) as they are the most important part of the configuration.
It is done so that the impedances of wires RL1 and RL3 in the formed Wheatstone bridge should cancel each other, each acting as an opposite leg of the bridge,. This will leave Wire “RL2” to act as the sensing lead carrying a very small current.
Fig. 5 – Three Wire Configuration of Resistance Temperature Detector
Four Wire Configuration
Four wire Resistance Temperature Detectors are even more accurate than Three wire RTDs as they are able to completely compensate for the resistance of the wires without having to pay attention to the length of each of the wires. This provides significantly increased accuracy at low cost of increased copper extension wire.
Fig. 6 – Four Wire Configuration of Resistance Temperature Detector
Applications of Resistance Temperature Detector (RTD)
The applications of Resistance Temperature Detectors in various industries include:
- In Automotive Industry – As audio amplifiers and engine oil temperature sensors.
- In Communication and Instrumentation – As temperature sensors and amplifiers.
- In Consumer Electronics – For small appliance controls and Fire Detectors.
- In Industrial Electronics – For gas flow indicators and Plastic laminating equipment.
- In Medical Electronics – For blood dialysis equipment and Infant incubators.
Fig. 7 – Applications of Resistance Temperature Detector (RTD)
RTDs should be used when:
- Stability and accuracy are a requirement of the customer’s specification.
- Accuracy extends over a wide temperature range.
- High degree of standardization is desirable.
Advantages of Resistance Temperature Detector (RTD)
The advantages of Resistance Temperature Detectors include:
- Linearity over wide operating range
- Wide temperature operating range
- High temperature operating range
- Interchangeability over wide range
- Good stability at high temperature
Disadvantages of Resistance Temperature Detector (RTD)
The disadvantages of Resistance Temperature Detectors include:
- Low sensitivity
- Higher cost than thermocouples
- No point sensing
- Affected by shock and vibration
- Requires three or four-wire operation