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RTDs - Resistance Temperature Detectors
RTDs (Resistance Temperature Detectors), also known as resistance thermometers, are a type of temperature sensor that uses the principle that the resistance of a metal changes in a predictable fashion with temperature and by measuring the resistance of the element, the temperature of the sensor can be determined from tables, calculations or instrumentation.
How does an RTD work?
In simple terms, an RTD works by equating a change in resistance to a change in temperature. However, measuring the change in resistance can be more difficult than first expected. Consider the following examples:
2 Wire RTD Circuit
As can be seen in the 2-wire RTD schematic above right, the resistance of the wires adds to the sensing element’s resistance giving a larger total circuit resistance. This will be interpreted by the receiving instrument (ohmmeter) as a falsely high temperature reading. The classical method of avoiding this problem has been the use of a bridge where the bridge output voltage is an indirect indication of the RTD resistance, as shown on the left above.
To avoid subjecting the three bridge-completion resistors to the same temperature as the RTD sensor, the RTD is separated from the bridge by a pair of extension wires. These extension wires recreate the problem that we had initially: The impedance of the extension wires affects the temperature reading. Because of this 2 wire RTDs are rarely used in the process industry.
The effect of extension wire impedance can be minimised by using a three-wire bridge configuration.
3 Wire RTD Circuit
In a 3 wire RTD circuit, voltmeter "A" measures the voltage dropped across the RTD plus the voltage dropped across the bottom current-carrying wire. Voltmeter "B" measures just the voltage dropped across the top current-carrying wire. If both current-carrying wires are of the same material and of identical length they will have the same resistance, subtracting the indication of voltmeter "B" from the indication given by voltmeter "A" yields the voltage dropped across the RTD.
RTD resistance can then be calculated from the RTD voltage and the known current source value using Ohm’s Law.
3 wire RTDs are the most commonly used configuration in the process industries.
4 Wire RTD Circuit
In a 4 wire RTD cicuit wire resistances are completely inconsequential. 4 wire RTDs tend to be more expensive than 3 wire RTDs due to the extra wire employed.
The 4 wire RTD uses four wires to connect the RTD to the measuring instrument, which consists of a voltmeter and a precision current source. Two wires carry excitation current to the RTD from the current source while the other two wires carry the voltage signal to the voltmeter. RTD resistance is calculated using Ohm's Law: taking the measured voltage displayed by the voltmeter and dividing that figure by the regulated current value of the current source.
This configuration leads to greater accuracy than the other two, and is most often encountered in laboratory conditions.
What is a Pt100 RTD Temperature Sensor?
A Pt100 is an RTD made from platinum with a resistance of 100 ohms at 0°C.
Although an RTD can be constructed from any metal, platinum (chemical symbol Pt) is the most commonly used because it is stable, provides repeatable and measurable results and has a broad temperature range. Pt100s are also relatively immune to electrical noise and therefore well suited for temperature measurement around motors, generators and other high voltage equipment. Other metals occasionally used in RTDs are copper (Cu) and nickel (Ni).
The reason 100 ohms at 0°C is used is because this is a requirement from what is considered the world wide standard for platinum RTDs - DIN/IEC 60751. Other resistances are also availabe e.g. Pt1000 which has a resistance of 1000Ω at 0°C.
What are the Standards for RTDs
The two most widely accepted standards for platinum RTDs are: the European standard (IEC 60751) and the American standard (ASTM E1137). The European standard is considered the world wide standard for platinum RTDs.
IEC 60751 - Industrial Platinum Resistance Thermometers and Platinum Temperature Sensors
IEC 60751 defines the temperature accuracy and the resistance/temperature characteristic curve for several tolerance classes. The combination of resistance tolerance and temperature coefficient define the resistance versus temperature characteristics for the RTD sensor. Note that there are a number of standards that either copy or are predecessors of IEC 60751. Among them are IEC 751, DIN 43760, EN 60751, and BS EN 60751.
It requires an RTD to have an electrical resistance of 100.00 Ω at 0°C and a temperature coefficient of resistance (TCR) of 0.00385 Ω/°C between 0 and 100°C.
IEC60751 accuracy classes are AA, A, B and C. Class A and Class B are the most commonly encountered in the process industry. The tolerances for theses classes are:
Class A = ±(0.15 + 0.002*t)
Class B = ±(0.3 + 0.005*t)
For closer tolerance 1/3 DIN and 1/10 DIN standards are available from many manufacturers. The higher the element tolerance the broader it’s range to deviate away from the temperature-resistance curve increasing the level of uncertainty. Note this doesn’t make 1/10th DIN detectors any more accurate then class B detectors, however it does significantly reduce uncertainty of reading. Misinterpretation of tolerance can lead to unnecessary expenditure as some instrument engineers assume closer tolerance assures better accuracy.
ASTM E1137 - Standard Specification for Industrial Platinum Resistance Thermometers
ASTM E1137, uses the same nominal characteristic curve, but defines tolerances differently, and designates them as Grade A and Grade B.
Technical Library
The following pages on Control and Instrumentation.com give more detail on the techniques used in temperature measurement:
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Technical Bookshop
For those who want to read further about the theory and practice of measuring temperature, and broaden their understanding of the differing types of temperature instrumentation, then the following books from Amazon will be of interest:
