Before connecting any signals, locate your device pinout.
The following terminal types correspond with RTD measurements:
The next step is to physically connect the RTD to your DAQ device.
Connect the red RTD lead to the excitation positive. Place a jumper from the excitation positive pin to the channel positive on the DAQ device. Connect the black (or white) RTD lead to the excitation negative. Place a jumper from the excitation negative to the channel negative on the DAQ device.
Figure 2. 2-Wire RTD Measurement
In the 2-wire method, the two wires that provide the RTD with its excitation current and the two wires across which the RTD voltage is measured are the same.The easiest way to take a temperature reading with an RTD is using the 2-wire method; however, the disadvantage of this method is that if the lead resistance in the wires is high, the voltage measured, VO, is significantly higher than the voltage that is present across the RTD itself. The NI 9217 analog input module does not support 2-wire measurement configurations.
Connect the red RTD lead to the excitation positive. Place a jumper from the excitation positive pin to the channel positive on the DAQ device (Note: This step is not necessary with the NI 9217; it internally connects these two channels, see Figure 3). Connect one of the black (or white) RTD leads to excitation negative and the other to channel negative.Figure 3 shows the external connections for the measurement as well as the pinouts for the NI 9217 RTD module. The excitation positive is connected to RTD0+ because the NI 9217 internally connects this to the excitation terminal.
Figure 3. 3-Wire RTD Measurement
To connect this RTD, simply connect each of the red leads on the positive side of the resistive element to the excitation positive and channel positive on the DAQ device. Connect the black (or white) leads on the negative side of the resistive element to the excitation and channel negative on the DAQ device. The two additional leads from a 2-wire RTD increase the attainable accuracy. Figure 4 shows the external connections for the measurement as well as the pinouts for the NI 9217 RTD module.
Figure 4. 4-Wire RTD Measurement
The 4-wire method has the advantage of not being affected by the lead resistances because they are on a high-impedance path going through the device that is performing the voltage measurement; therefore, you get a much more accurate measurement of the voltage across the RTD.
You can use NI MAX to quickly verify the accuracy of your measurement system setup. Using an NI-DAQmx Global Virtual Channel, you can configure an RTD measurement without any programming. A virtual channel is a concept of the NI-DAQmx driver architecture used to represent a collection of device property settings that can include a name, a physical channel, input terminal connections, the type of measurement or generation, and scaling information.Follow these steps to begin:
Once the sensor is connected to the measurement device, continue with configuration as follows:
With NI-DAQmx global virtual channels, you can preview your measurements.
The following example code is provided in the LabVIEW NI Example Finder under Hardware Input and Output»DAQmx»Analog Input»RTD or Thermistor - Continuous Input.vi. The front panel of this example allows the user to:
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