Resistance Temperature Detectors

Of the many ways to measure temperature, one of the most accurate is a resistance temperature detector, usually referred to as an RTD. In an RTD, the resistance of the device is proportional to temperature. The most common resistive material for RTDs is platinum, though some RTDs are made from nickel or copper.

11/01/2005

 AT A GLANCE Current source issues Low resistance Self-heating Error elimination

Of the many ways to measure temperature, one of the most accurate is a resistance temperature detector, usually referred to as an RTD. In an RTD, the resistance of the device is proportional to temperature. The most common resistive material for RTDs is platinum, though some RTDs are made from nickel or copper. RTDs have a wide range of temperatures. Depending on their construction, they can measure temperatures in the range of -270 to 850 °C.

RTDs require an external stimulus, usually a current source, to function properly. However, current generates heat in the resistive element, which causes an error in the temperature measurement. The measurement error is calculated by the formula:

ÄT = P x S

where T is temperature, P is the I2R power generated, and S is °C/milliwatt.

Measurement methods

There are several techniques for measuring temperature with an RTD. The first is a two-wire method that works by forcing current through the RTD and measuring the resulting voltage. The benefit is that it's a simple method using only two wires, making it easy to connect and implement. The main drawback is that the lead resistance is part of the measurement, which can cause some error.

 Schematic shows the setup for a typical two-wire resistance measurement.

An improvement on the two-wire method is the three-wire method. Here again, a current is forced through the device and a resulting voltage is measured. However, using a third wire provides compensation for the lead resistance. This requires either a three-wire compensating measurement unit or actually measuring the contribution from the third wire and subtracting it from the overall measurement.

A third technique is the four-wire method. Like in the other two methods, a current is forced through and a voltage measured. However, the current is sourced on one set of leads while the voltage is sensed on another set of leads. The voltage is sensed at the resistive element (RTD), not at the same point as the source current. This means the test lead resistance is completely out of the measurement path. In other words, the test lead resistance is not a part of the measurement.

For example, if the test lead resistance is about 100 milliohms and the RTD is a 100-Ohm RTD, then the test lead resistance will be about 0.1% of error. In the four-wire method, the test lead resistance is not part of the measurement. Therefore, it is a much more accurate technique for measuring the resistance of the RTD, because it completely eliminates lead resistance.

Pros and cons

RTDs have some distinct advantages over other temperature-measuring devices. For one, they are the most stable and most accurate of all the different temperature measurement devices. They also are more linear than thermocouples.

 A typical four-wire resistance measurement setup helps eliminate much of the noise and uncertainty in temperature measurements.

There are a few drawbacks, however. RTDs are more expensive than thermistors and thermocouples. Also, they require a current source. They have a small delta R, which means there is a low resistance to temperature change. For example, to change one degree C, the RTD might change by 0.1 Ohm. However, low absolute resistance could lead to measurement errors if using the two-wire method.

When using RTDs, several common occurrences are often not accounted for, the biggest of which is self-heating. If the RTD self heats with the test current, measurement inaccuracy could result. If measuring low temperature (for example, below 0 °C), the heat from the RTD could raise the expected temperature. Also, if there is no compensation for the test leads, even more error can be introduced into the measurement. Using a four-wire method helps eliminate this type of error. Another mistake is not selecting the proper RTD temperature range. Trying to measure outside of the RTD temperature range can result in more errors or even sensor damage. Always select the appropriate RTD for the expected measurement.

 Author Information Dale Cigoy is senior applications engineer, Keithley Instruments Inc.;

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