Resistance thermometers (RTDs) are sensors used to measure temperature. They have high accuracy and repeatability. They consist of a wire coil encased in a protective material such as ceramic. The wire is made from a pure material with a known temperature-resistance relationship.
Due to it having the widest accurate resistance-temperature relationship platinum is the most commonly used material for RTDs. Since PT1000 is one of the most widespread RTDs available the MonoDAQ-U-X has internal electronics that allow a single PT1000 to be connected directly without the need for any external components.
Using external resistors up to eight RTDs of any type can be used at the same time with a single MonoDAQ-U-X device.
Single PT1000 – no external components
To use the PT1000 sensor simply select the “RTD” option in the function column drop-down menu for any of the odd-numbered channels (1,3,5,7). We used channel 1 but you can use whatever is most convenient for your application.
Note that connecting a PT1000 this way will require the use of the current and excitation pins. Any existing setups on those channels will be overwritten when selecting the RTD option. It is best practice to set CUR+ and EXC to “off” before selecting the RTD function.
Connect the PT1000 sensor as shown in the image below:
Notice that using the RTD function connects a 1 kΩ internal resistor between CUR+ and GND. The PT1000 (which is a 1000 Ω resistor at 0°C) and the internal resistor form a voltage divider. As the resistance of the PT1000 changes with the temperature so does the voltage drop over the PT1000 which is being measured by pins 1 and 2.
Keep in mind that wires have a non-neglible resistance! Make sure to keep them as short as possible or use thicker wires if need be. RTDs with lower resistances are more prone to error due to cable resistance.
The temperature-resistance relationship for the PT1000 is stored in the software so the temperature is calculated automatically.
Single PT1000 – external resistor
In the case where we need to use the current pins for something else or want to use a different type of RTD we can use alternate wiring:
The circuit functions the same as before but now we are using an external resistor. This way we don’t need to use the integrated resistor freeing up the current pins.
The RTD’s resistance can be calculated from the measured voltage. The temperature can be calculated from the RTD’s resistance
This also gives us more freedom with the choice of RTD. Any resistive temperature sensor (even NTC) can be used. Generally, the external resistor should be the same value as the temperature sensor at room temperature. Other values could be used to yield higher precisions in certain temperature ranges.
If the sensors will be calibrated manually there is no need for precision resistors. But if no calibration will be done then high precision resistors should be used.
External resistor software setup
To calculate the temperature with this method set up a math channel that calculates the resistance of the RTD.
If the temperature-resistance relationship of the RTD is known a handy feature can be used to calculate the temperature: scaling. Scaling can be found in Add Math > Additional > Scaling.
Like the name implies scaling allows you to scale values linearly or as a polynomial. Select the number of points (the degree of a polynomial) and input the coefficients. Finally, select the unit and that’s it.
This is a faster and easier way to calculate the temperature than manually doing the scaling with a formula.
Multiple RTDs – external resistors
The external resistor circuit can be expanded with multiple RTDs. Each RTD needs its own external resistor and voltage input. The EXC and GND pins are shared between all RTDs.
Above is an example using two sensors but up to eight total could be used. The total current draw must not exceed the 200mA which the U-X can provide. For most sensors, this should not be a problem. When using multiple low resistance sensors an external PSU might be necessary.