1. RTD Overview
A platinum resistance temperature detector (RTD) is a device with a typical resistance of 100 Ω at 0°C. It consists of a thin film of platinum on a plastic film. Its resistance varies with temperature and it can typically measure temperatures up to 850 °C. The relationship between resistance and temperature is relatively linear as shown below for a sample 100 Ω RTD.
Figure 1. RTD Resistance Versus Temperature Relationship
This relationship appears relatively linear, but curve fitting is often the most accurate way to make an accurate RTD measurement.
2. Measuring Temperature with RTDs
An RTD is a passive measurement device; therefore, you must supply it with an excitation current and then read the voltage across its terminals. You can then easily transform this reading to temperature with a simple algorithm. To avoid self-heating, which is caused by current flowing through the RTD, minimize this excitation current as much as possible. The easiest way to take a temperature reading with an RTD is using the 2-wire method.
Figure 2. Making a 2-Wire RTD Measurement
Using 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 inaccuracy using 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. To get a more accurate measurement, use the 4-wire method.
Figure 3. Making a 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.
3. Which NI Products Can I Use with RTDs?
The following products support RTD measurements:
- SCXI-1503 - a 16-channel analog input module specific for RTD measurements
- NI 9217 - a 4-channel C Series analog input module for RTD measurements (use with NI CompactDAQ or NI CompactRIO)
- SCXI-1102 & SCXI-1581 - a multiplexer module with 32 inputs and a current excitation source with 32 outputs (Use both modules with SCXI-1300 terminal blocks)
- SCXI-1121 - an isolated amplifier, multiplexer module with four isolated inputs and four current/voltage excitations
- SCXI-1122 - an isolated amplifier, multiplexer module with sixteen inputs and one 1 mA current excitation source
- SCC-RTD01 - dual channel module that accepts 2, 3, or 4 wire platinum RTDs. Includes a 1mA current source.
- NI 4350/4351 - high precision DAQ device, 16 differential analog inputs, 24-bit analog-to-digital converter, built-in 25uA precision current source
- NI PXIe-4357 - high precision SC Express device, 20 RTD inputs, 24-bit analog-to-digital converter, 0.9mA current excitation per channel
- NI 4070 DMM & SCXI-1129 - a high precision 6.5 digit FlexDMM and a 256 crosspoint, matrix relay. (Use one current source and one DMM to take high precision measurement via the SCXI-1129 matrix relay)
- cFP-RTD-122 - an eight channel, 3-wire RTD and resistance input module that is a part of the distributed I/O product line
- cFP-RTD-124 - an eight channel, 4-wire RTD and resistance input module that is a part of the distributed I/O product line
- FP-RTD-122 - an eight channel, 3-wire RTD and resistance input module that is a part of the distributed I/O product line
- FP-RTD-124 - an eight channel, 4-wire RTD and resistance input module that is a part of the distributed I/O product line
These products support taking 2-wire, 3-wire and 4-wire measurements, with the exception of the FP-RTD-122, which only supports 3-wire measurements. The diagram below shows how to make a sample RTD measurement using a 4-wire, 3-wire or 4-wire configuration with the SCXI-1121 module.
Figure 4. SCXI-1121 Module 2-,3-, and 4-Wire RTD Configuration
4. Programming Considerations for RTDs
- Set up the gain settings in the data acquisition software (NI-DAQ) for the SCXI module or data acquisition (DAQ) device.
- Read the voltage (you can use programmable gain settings if you use lower level VIs such as AI_Config) with any low or high level VIs for analog input.
- Scale the voltage to temperature using the LabVIEW data acquisition utility VI, RTD_Convert. This VI uses the Calendar Van Dusen equation for accurately converting voltage to temperature.
- Set up any gains in NI-DAQ for the SCXI module or DAQ device.
- Read the voltage using the single point voltage commands or multiple point (buffered) commands. For single point read use SCXI_Load_Config, SCXI_Single_Chan_Setup, AI_Read, SCXI_Scale (example SCXI_AIOnepoint.prj which ships with CVI) and for multiple point (buffered) read use SCXI_Load_Config, SCXI_SCAN_Setup, SCXI_MuxCtr_Setup, SCAN_Op, SCAN_Demux, SCXI_Scale (example SCXI_SCANsingleBuf.prj).
- Scale the voltage to temperature using the RTD_Convert functions in the convert.fp instrument driver that is part of the LabWindows\CVI\toolslib folder. There is a version of this function for both single point and buffered operations. These routines also use the Calendar Van Dusen equation for accurately converting voltage to temperature.
See the LabWindows/CVI example. The only missing piece is the RTD conversion from voltage to temperature so you can implement your own algorithm for that.
5. Thermistor Overview
Thermistors are similar to RTDs in that they are resistors whose resistivity changes with temperature. The main difference between thermistors and RTDs is that they are made of metal oxide semiconductor material that is coated with glass or epoxy. They also come in two different types, negative temperature coefficient (NTC) and positive temperature coefficient (PTC). NTC thermistors have a resistivity that decreases with increasing temperature and the PTC thermistors have increased resistivity with increasing temperature. Thermistors have a much higher sensitivity to temperature than RTDs and a much higher nominal resistance. Thermistors are less sensitive to lead resistance noise effects. With sensitivities on the order of 10 W/°C to 10 kW/°C, thermistors are well suited to high accuracy temperature measurements. The major disadvantages of thermistors are they have a small temperature range and a highly nonlinear output. Below is a typical thermistor temperature curve compared to a typical 100 Ω RTD temperature curve.
Figure 5. Resistance Versus Temperature for a Typical Thermistor and RTD
6. Measuring Temperature with Thermistors
A thermistor, like an RTD, is a passive measurement device. You must supply it with a constant voltage or current excitation, and then read the voltage across its terminals to get a temperature reading.
7. Temperature Measurement Systems
National Instruments offers several hardware platforms to measure RTDs and thermistors. The Temperature Measurement Resource Page (link below) shows NI temperature measurement hardware for a variety of needs, as well as additional temperature measurement information.
Temperature Measurement Resource Page