Resistance Temperature Detectors (RTDs) are used to measure temperature in a wide variety of applications. They tend to offer greater stability, accuracy, and repeatability than thermocouple sensors and can be used in many applications below 600°C.
A central premise of RTD design is variable resistance -- a change in temperature (∆T) results in an approximately linear change in the resistance of the sensor (∆R). This is called the temperature coefficient denoted in units of Ω/Ω/°C and is typically calculated by measuring the relationship between resistance and temperature at 0°C and 100°C.
When using RTDs, it is important to understand that the variability of the resistor is nearly linear but not exactly so. As such, use of a sensitivity value for a given temperature (20°C for instance), will result in small error in measurements at other temperatures. This error can be estimated using a series of calculations.
One way to improve the accuracy of RTD measurements is by using current-limiting resistors in series with the RTD. This has two benefits. First, by limiting the current flowing through the sensor, self-heating is minimized. Secondly, the change in resistance across the RTD versus the total resistance becomes less significant, and this results in a reduced error percentage over the range of temperature measurements.
Recommended Connection Diagram:
The following diagram illustrates the recommended connections for RTDs. RL1 and RL2 are the current limiting resistors placed in series with the RTD for accuracy improvement. When selecting current limiting resistors, we recommended using 0.1% resistors with a low thermal coefficient to help ensure that voltage-divider calculations for the system are as close to ideal as possible.
While it is possible to use the sensitivity value listed on the RTD datasheet, many times this value is calculated at 0°C. As most applications begin at ambient temperature (~20°C), using a sensitivity value centered at 0°C will result in a percentage error value in temperature measurement at ambient. Therefore, it is recommended to use a sensitivity value centered at either ambient temperature or at a temperature centered at the mid-range of expected measurements.
When using the recommended current-limiting resistors, the sensitivity value is also different than that listed on the RTD datasheet, as the series resistors affect current flow and, therefore, the relationship between ∆R and ∆T for the RTD.
For ease of calculations, DTS has created a sensor settings calculations table that can be used to provide Sensitivity and Initial EU values that should be entered into the Sensor Settings for your RTD.
Some important notes with respect to this calculation table:
- RTD Resistance Values:
- Temperature Coefficient Used: 0.00385 Ω /Ω/°C
- Values for the PT100 were taken from standard resistance tables available online. PT500 and PT1000 values are calculated based on the PT100 values used.
- The CUSTOM table is editable by the user, should they wish to use actual values associated with their specific RTD model. See the "RTD Resistance @ Temperature" worksheet.
- If a custom value is entered on the "RTD Data" worksheet and the table on the "RTD Resistance @ Temperature" worksheet is unchanged, the custom RTD resistance values will be calculated based on the standard PT100 values available.
- Standard PT ranges are 100, 500, and 1000.
- Custom range is available as an option, with a field for value entry.
- Temperature Range:
- -200°C to 320°C
- Current-Limiting Resistance Values:
- Standard resistor values of 0.1% accuracy between 4700Ω and 20,000Ω are available for selection.
Resistor values were chosen to limit the system current to no more than 0.5mA. Actual current limits will depend on the resistance value chosen.
Notes on 3- and 4-Wire RTD Sensors:
The purpose behind the additional leads on 3- and 4-wire RTD sensors is to increase reliability of voltage readings -- and, thus, temperature calculations -- read by the DAS by reducing or eliminating the error introduced by the inherent resistance in the RTD leads. This adjustment is achieved using an RTD amplifier circuit/IC that provides the compensation. DTS DAS do not implement such an amplifier circuit on input channels, thus the benefit to using RTDs with more than 2 leads is not possible.
When using 3- and 4-wire RTDs with DTS DAS, it is recommended to use the voltage divider schematic provided above and not connect the 3rd and/or 4th wires that are used for sensing.