• Technical Info
  • Sensor Design Overview
  • RTD Sensor
    • Selecting an RTD
    • RTD Installation and Maintenance
  • Thermocouples
  • RTD Tables
  • Thermocouple Tables
  • Temperature Tables

RTD Installation & Maintenance

General

An RTD is a sensor whose resistance changes as its temperature changes. The resistance increases as the temperature of the sensor increases. The resistance vs temperature relationship is well known and is repeatable over time. An RTD is a passive device. It does not produce an output on its own. External electronic devices are used to measure the resistance of the sensor by passing a small electrical current through the sensor to generate a voltage.

Standard Tolerances

RTDs are built to several standardized curves and tolerances. The most common standardized curve is the ‘DIN’ curve. The curve describes the resistance vs. temperature characteristics of a Platinum, 100 ohm sensor, the standardized tolerances, and the measurable temperature range. The DIN standard specifies a base resistance of 100 ohms at 0ºC, and a temperature coefficient of .0038500 ohms/ohms/ºC. The nominal output of a DIN RTD sensor is shown below:


There are two standard tolerance classes for DIN RTDs. These tolerances are defined as follows:

DIN Class A	Temperature tolerances:	+/-(0.15 + .002|T|°C)
DIN Class B	Temperature tolerances:	+/-(0.3 + .005|T|°C)
DIN Class C	Temperature tolerances:	+/-(1.2 + .005|T|°C)

Sensor Connections

RTD Sensors are available in a number of different leadwire configurations. The most common configuration is the single element, three lead configuration. Schematics of the available leadwire configurations are shown below:





Two wire sensors are typically used in applications where accuracy is not critical. The two wire configuration allows for the simplest measurement technique, but suffers from an inherent inaccuracy due to the resistance of the sensor leads. In the two wire configuration, there is no way to directly compensate for the resistance of the leadwires which will cause an offset increase in the resistance measurement.

Three wire sensors are built with a compensation loop to allow the measurement to factor out the resistance of the leads. With this configuration, the controller/measurement device makes two measurements. The first measurement measures the total resistance of the sensor and the connecting leadwires. The second measurement is the resistance of the compensation loop resistance. By subtracting the compensation loop resistance from the total resistance, an account net resistance is determined. Three wire sensors are the most common and give a good combination of accuracy and convenience.

The four wire sensor configuration and measurement techniques allow measurement of the sensor resistance without the influence of the leadwires. While this technique gives the best accuracy, many industrial controllers/measurement devices cannot make a true four wire measurement.

The transition from the sensor leadwires to the field wiring is typically done in a connection head attached to the sensor. Terminal blocks are used to facilitate the connection. A typical terminal block/sensor connection is shown in the following figure.

Mechanical connection

RTD sensor assemblies typically use pipe threads (NPT) for the mechanical connections between the connection heads, extensions and thermowells. For proper operation you should tighten the threads at least 1-2 turns past hand tight. This will ensure that the threads mate tightly, and that the proper sensor assembly length is obtained. To prevent thread galling RTD Company recommends that anti-sieze compound or Teflon plumbing tape is used on all NPT threads.

Ensuring accurate performance

To ensure good performance of the sensor, the sensor must be in good thermal contact with the process. When using a sensor in conjunction with a thermowell you should use a suitable heat transfer compound to fill the air space between the sensor and the bore of the thermowell. This will improve the heat transfer between the process and the sensor, reducing the stem conduction error and improving the response time of the sensor. For moderate temperature applications (less than 250ºC) a silicone based heat transfer compound should be 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 sensor and insert the sensor into the thermowell, pressing the sensor firmly into the well until the tip reaches the bottom of the thermowell bore. The RTD sensor should be isolated from the sensor sheath and other metal components. Checking the insulation resistance of the sensor is an easy test to ensure the integrity of the sensor. Measure the resistance between the sensor leads, and the metal sheath of the sensor. The resistance should measure at least 100 megohms (100,000,000 ohms). Lower insulation resistances could be an indication that the sensor has suffered damage.