Resistance Temperature Detectors
Resistance temperature detectors, or RTDs, are a common form of temperature sensor used in all kinds of industrial process applications.
A RTD is basically a variable resistor whose resistance changes as its temperature changes.
In this lesson we will look at how resistance temperature detectors work, how they are commonly used, and potential problems to be aware of.
How It Works
Resistance temperature detectors increase their resistance as the temperature around them increases.
They are similar to positive temperature coefficient thermistors in this way.
For a more in-depth look at how an increase in temperature can increase the electrical resistance of a conductive material check out the thermistors lesson.
Resistance temperature detectors are typically made of a fine wire element wrapped around a ceramic or glass core and then surrounded by a porcelain insulator.
The key component, the wire, is made of nickel, copper or platinum.
Platinum is preferred as its resistance changes fairly linearly across a wide range of temperature, and it has low chemical reactivity, meaning the wire will not corrode over time.
The most common resistance temperature detector found in industry is the Pt100 RTD.
"Pt" refers to platinum.
"100" refers to the value of resistance, in ohms, of the RTD when at 0°C.
Though other platinum RTDs, such as the Pt500 or the Pt1000, are not rare.
RTDs have traditionally been a part of a Wheatstone bridge.
The RTD functions as one of the resistors used to build the bridge.
When the temperature being sensed causes the resistance of the RTD to change the bridge becomes unbalanced.
An unbalanced Wheatstone bridge develops a voltage difference between the two branches which we can detect by placing a voltmeter between them.
This detected voltage value is then used by the measuring device to calculate the temperature of the RTD.
More recently the use of a Wheatstone bridge has fallen out of favor, replaced by new transmitters which use the constant current source method.
With a constant current flowing across the RTD, resistance changes can be detected by measuring the voltage drop across the RTD.
A "voltmeter" or voltage detector has extremely high input impedance (resistance), ideally it would be infinite.
This ensures that the voltmeter doesn't create a parallel flow path for current, which would affect the circuit resistance.
Once again the detected voltage value is used by a measuring device to calculate the temperature of the RTD.
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