Tutorial: A closer look at RTDs
In recent months we have examined thermocouples in detail, so it’s time to dig a bit more deeply into another important sensor technology. RTDs (resistance temperature detectors or devices) use the fact that most metals increase their electrical resistance as they get hotter. The amount varies between materials, but is usually in the 0.3 to 0.5 Ohms / °C range. The most frequent sensing material used in industrial applications is platinum, due to its relatively high melting point and resistance to corrosion.
Making a sensor involves a small segment of coiled wire or a bit of sensing material deposited on an insulator substrate that has leads attached so it can be inserted into the process. A voltage is fed into the sensor so its resistance can be measured. Unlike a thermocouple, an RTD provides a direct temperature measurement rather than a temperature differential against a reference point. The signal is typically fed into a transmitter that uses a Wheatstone bridge or other method to form a precise resistance measurement. With an appropriate transmitter, a high-quality RTD can provide measurements of hundredths of a degree with high repeatability.
That provides a useful group of advantages:
Wide operating range (but not as high as a thermocouple);
High linearity; and
Can be configured in a small package for high sensitivity.
But, no technology is without its downside:
Like thermocouples, any physical changes or deterioration of the sensor wire will cause the reading to drift.
Since the reading is a resistance, it’s best to keep the transmitter near the sensor so you aren’t also reading the resistance of the lead wires. Some users try to correct for this by using the largest possible lead wire, but this can serve as a heat sink, drawing heat away from the sensor and creating an artificially low value.
Since the sensor is a resistive element, it is critical to use as little current as possible for the reading or the sensor will actually warm itself and create an artificially high value. This is particularly important when working with small sensors designed for fast response.
In some extreme situations, the sensor can create a voltage due to the Seebeck effect in the same way as a thermocouple. This can counter the resistance and throw off the reading. Using appropriate lead wires and careful sensor positioning can avoid this.
If you’re looking for a technology that uses the lowest possible current consumption because you need to operate on batteries with a wireless transmitter, do your math carefully. While an RTD can operate with very low current, it may not be low enough to ensure long battery life. A thermocouple may be better in this kind of situation.
RTDs are thoroughly standardized under IEC 60751, which creates specific performance and accuracy classes. Following these standards can allow you to use sensors from multiple suppliers and receive reliable performance.
All this does not come without a price. While RTDs generally have the best group of performance characteristics, they are also generally the most expensive, often by a substantial margin. The sensors and transmitters normally sell at a premium compared to thermocouples and thermistors.
Search for RTD suppliers at Control Engineering Supplier Search .
Read Challenges of temperature sensing for advice on how to calibrate an RTD and transmitter for extreme precision.
Read How does an RTD work?
—Peter Welander, process industries editor, PWelander@cfemedia.com ,
Control Engineering Process Instrumentation & Sensors Monthly
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