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CAS DataLoggers recommends three-wire RTDs

Aug. 9, 2012
To prevent inaccurate temperature measurements, CAS DataLoggers recommends using three-wire resistance temperature detectors (RTDs).

To prevent inaccurate temperature measurements, CAS DataLoggers recommends using three-wire resistance temperature detectors (RTDs) to those taking temperature measurements using RTDs.

These sensors are available for use with the firm’s inventory of temperature data loggers, but some users are unaware of this more accurate option. With this in mind, the company has put together a guide to show the need for three-wire RTDs.

RTDs make simple resistance measurements, usually at about 100 ohms, which is a relatively low level of resistance. Therefore, an RTD measurement error of 1 ohm or more is quite significant: for a regular RTD at room temperature, the resistance is 109.1 ohms, and even a 1-ohm error in that measurement will cause a temperature error of about 2.5° C. When using a two-wire RTD, users may find the resistance of the connections between sensor and datalogger directly affects the temperature measurement. This is easily avoided by using a three-wire RTD, which enables the connected data logger to compensate for the resistance of the circuit.

An example of this kind of measurement error occurs when connecting an RTD to a piece of 20-gauge hookup wire, with the resistance at about 1 ohm per 100 feet of cable. In this case, using a two-wire RTD with a 50-ft cable on it can result in a 2° to 2.5° C temperature error due to the wire’s resistance. So especially with longer cable runs, users can avoid this inaccuracy by using a three-wire RTD along with a data logger that provides automatic compensation for these applications.

Sophisticated data acquisition and control systems are also available. These include high-performance, real-time systems for situations where traditional test systems or programmable controllers are not suitable. Such systems are used in data acquisition, test, and control applications where microsecond precision is needed. Models are provided with eight to more than 400 analog input channels, analog output channels, digital inputs and outputs, counters, RS-232, RS-485 CANbus, and Profibus interfaces.

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