General
An RTD is a sensor that changes its resistance when the temperature changes. The resistance increases as the temperature increases in the sensor. Its resistance vs. temperature relationship is well known and it is repetitive over time. An RTD is a passive device. It does not produce an outlet in itself. External electronic devices are used to measure the resistance of the sensor where they generate a small electrical current that passes through the sensor generates a voltage which is proportional to the resistance.
Standard tolerances
RTDs are built for various standardized curves and tolerances. The most common standard curve is the ‘DIN’ curve. The curve describes the resistance versus temperature characteristics of a platinum sensor, 100 ohms, its tolerances and the measurable temperature range. The DIN standard specifies a base resistance of 100 ohms at 0 º C, and a temperature coefficient of .0038500 ohms / ohms / º C. The nominal values of an RTD DIN sensor are shown below:
There are three standard tolerance classes for DIN RTDs. These tolerances are defined as follows:
DIN Class A Temperature tolerances: ± (0.15 + 0.002 x | T | ° C)
DIN Class B Temperature tolerances: ± (0.3 + 0.005 x | T | ° C)
DIN Class C Temperature tolerances: ± (1.2 + 0.005 x | T | ° C)
Sensor Connections
The connections for RTD sensors are available in a number of different configurations. The most common configuration is the single element, connection to three wires. Schemes of the wire connection configurations are shown below:
Two wire connection is normally used in applications where accuracy is not critical. The two-wire configuration allows for a simpler measurement technique, but suffers from inherent inaccuracy due to the resistance of the sensor conductors. In the two-wire configuration, there is no way to compensate for cable resistance which will cause a wrong increase in resistance measurement.
Connection to three wires is made with a compensation circuit to allow measurement and compensate the resistance of the conductors. With this configuration, the controller / meter device makes two measurements. The first measurement measures the total resistance, the resistance of the sensor together with the connecting cables. The second measurement measures the resistance of the compensation circuit. Subtracting the resistance of the compensation circuit from the total resistance, the final resistance is that of the sensor. Three wire connection is the most common and is a good combination of precision and convenience.
The four-wire sensor configuration is a measurement technique that allows the measurement of the sensor resistance without the influence of the connecting cables. Although this technique is of the best precision, many industrial controllers and measuring devices do not measure four wires.
The transition between the sensor connection cables to the field wiring is typically done through a connection head connected to the sensor. Terminal blocks are used to facilitate connection. A typical terminal block / sensor connection is shown in the following figure.
Ensure accurate performance
To guarantee a good performance of the sensor, the sensor must have a good thermal contact with the process. When using a sensor in conjunction with a sheath, use an appropriate heat transfer compound to fill the air gap between the sensor and the well bore. This will improve the heat transfer between the process and the sensor, reducing the heat conduction error and improving the response time of the sensor. For moderate temperature applications (below 250 º C) a heat transfer silicone compound is the most used. Higher temperature applications will require a graphite-based material. An easy way to apply the heat transfer compound is to apply a generous amount to the tip of the sheath and insert the sensor into the sheath, pressing the sensor firmly into the sheath until the tip reaches the bottom of the sheath hole. The RTD sensor must be isolated from the sheath and other metallic components. Checking the insulation resistance of the sensor is a simple test that ensures the integrity of the sensor. Resistance measurement is made between the connection cables of the sensors and the metal sheath of the sensor. The resistance must be at least 100 megohms. Lower values of insulation resistance may be an indication that the sensor has been damaged.