Richards Thermocouple and Temperature Sensor Catalog 16

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RTDs

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Resistance Temperature Detectors (RTDs) - Platinum

Typical RTD Wiring Configurations and Richards Color Codes

The Two Wire RTD (Red & Black) The Richards two wire RTD is constructed with only one red and one black lead wire. This type of sensor is the least accurate form of RTD since it does not compensate for lead wire resistance. The total resistance of the lead wire will be added to the calibrated element’s resistance. This will result in a higher temperature reading than is ac- tually being measured. The error created by the lead wire can be quite substantial if it is not compensated. The two wire RTD is the least common type of lead configuration. The Three Wire RTD (Red, Black & Black) The Richards three wire RTD is constructed with two black lead wires and one red lead wire. This is the most common configuration for RTD sensors used today. The three wire RTD is more accurate than the two wire version since the instrument or transmitter can use this third lead to compensate for added resistance and ambient temper- ature changes along the lead length. The Four Wire RTD (Red, Red, Black & Black) The Richards four wire RTD is constructed with two black lead wires and two red lead wires. It is the most accu- rate RTD configuration since the instrument or transmitter can completely compensate for lead wire resistance and ambient temperature changes along the leads. The four wire RTD can also compensate for errors caused by mis- matched lead wires (this does not occur often). This is accomplished by passing a current through the outer leads (First Red + First Black) and measuring the voltage drop across the inner leads (Second Red + Second Black). The instrument or transmitter can then calculate the resistance of the element alone. These sensors are not as common as the three wire version. They are only used where very high accuracy is critical. Four wire RTDs can be used as three wire or two wire by removing one of the red leads, or one of the red and one of the black leads. Note: The lead color codes are Richards standards. There is not an industry standard for lead color codes.

Red

Red

Red

Red

2 wire

3 wire

4 wire

Black

Black

Black

Black

Black

Resistance Temperature Detectors (RTDs) - Thin Film Thin Film RTDs elements are less expensive than wire round, better on vibration, but their temperature range is narrower. Thin Film: -70°C to 500°C -94°F to 932°F Wire wound: -200°C to 650°C -328°F to 1202°F Wire wound RTDs elements in this catalog are the Richards standard. Thin Film elements add “ F ” in the catalog number after the curve letter “ E ” or “ A ”. Example: 4EF164-11-12-24

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