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. Passing current through an RTD generates a voltage across the RTD. By measuring this voltage, you can determine its resistance and, thus, its temperature. The relationship between resistance and temperature is relatively linear.|
Figure 1. Physical Architecture of an RTD
RTDs operate on the principle of changes in electrical resistance of pure metals and are characterized by a linear positive change in resistance with temperature. Typical elements used for RTDs include nickel (Ni) and copper (Cu), but platinum (Pt) is by far the most common because of its wide temperature range, accuracy, and stability.
RTDs are constructed using one of two different manufacturing configurations. Wire-wound RTDs are created by winding a thin wire into a coil. A more common configuration is the thin-film element, which consists of a very thin layer of metal laid out on a plastic or ceramic substrate. Thin-film elements are cheaper and more widely available because they can achieve higher nominal resistances with less platinum. To protect the RTD, a metal sheath encloses the RTD element and the lead wires connected to it.
Popular because of their stability, RTDs exhibit the most linear signal with respect to temperature of any electronic temperature sensor. However, they are generally more expensive than alternatives because of the careful construction and use of platinum. RTDs are also characterized by a slow response time and low sensitivity, and, because they require current excitation, they can be prone to self-heating.
RTDs are commonly categorized by their nominal resistance at 0 °C. Typical nominal resistance values for platinum thin-film RTDs include 100 and 1000 Ω. The relationship between resistance and temperature is nearly linear and follows this equation:
For <0 °C RT = R0 [ 1 + aT + bT2 + cT3 (T - 100) ] (Equation 1)
For >0 °C RT = R0 [ 1 + aT + bT2 ]
Where RT = resistance at temperature T
R0 = nominal resistance
a, b, and c = constants used to scale the RTD
The resistance/temperature curve for a 100 Ω platinum RTD, commonly referred to as Pt100, is shown in Figure 2.
Figure 2. Resistance-Temperature Curve for a 100 Ω Platinum RTD, a = 0.00385
This relationship appears relatively linear, but curve fitting is often the most accurate way to make an accurate RTD measurement.
The most common RTD is the platinum thin-film with an a of 0.385%/°C and is specified per DIN EN 60751. The a value depends on the grade of platinum used, and also commonly include 0.3911%/°C and 0.3926%/°C. The a value defines the sensitivity of the metallic element, but is normally used to distinguish between resistance/temperature curves of various RTDs.
Temperature Coefficient (a)
2. How to Make an RTD Measurement
Measuring Temperature with RTDs
All RTDs usually come in a red and black or red and white wire-color combination. The red wire is the excitation wire and the black or white wires are ground wires. If you are not sure which wires are connected to which side of the resistive element, you can use a digital multimeter (DMM) to measure the resistance between the leads. If there is close to 0 Ω resistance, then the leads are attached to the same node. If the resistance is close to the nominal gage resistance (100 Ω is a common RTD nominal gage resistance), then the wires you are measuring are on the opposite side of the resistive element. In addition, reference the RTD specification to find the excitation level for that particular device.
Most instruments offer similar pin configurations for RTD measurements. The following example demonstrates an RTD measurement using an NI CompactDAQ chassis and the NI 9217 RTD module (see Figure 3). National Instruments provides many options to measure temperature from 1 to 1,000+ channels.
Figure 3. NI CompactDAQ Chassis and the NI 9217 RTD Module
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. There are essentially three different methods to measure temperature using RTDs.
Two-Wire – RTD Signal Connection
Connect the red RTD lead to the excitation positive. Place a jumper from the excitation positive pin to the channel positive on the data acquisition 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 data acquisition device.
Figure 4. Two-Wire RTD Measurement
In the two-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 two-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 does not support two-wire measurement configurations.
Three-Wire – RTD Signal Connection
Connect the red RTD lead to the excitation positive. Place a jumper from the excitation positive pin to the channel positive on the data acquisition device (Note: This step is not necessary with the NI 9217; it internally connects these two channels, see below). Connect one of the black (or white) RTD leads to excitation negative and the other to channel negative.
Figure 5 shows the external connections for the measurement as well as the pin-outs 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 5. Three-Wire RTD Measurement
Four-Wire – RTD Signal Connection
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 data acquisition device. Connect the black (or white) leads on the negative side of the resistive element to the excitation and channel negative on the data acquisition device. The two additional leads from a two-wire RTD increase the attainable accuracy. Figure 6 shows the external connections for the measurement as well as the pin-outs for the NI 9217 RTD module.
Figure 6. Four-Wire RTD Measurement
The four-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.
RTD Noise Considerations
RTD output signals typically run in the millivolt range, making them susceptible to noise. Lowpass filters are commonly available in RTD data acquisition systems and can effectively eliminate high-frequency noise in RTD measurements. For instance, lowpass filters are useful for removing the 60 Hz power line noise that is prevalent in most laboratory and plant settings.
You can also significantly improve the noise performance of your system by amplifying the low-level RTD voltages near the signal source. Because RTD output voltage levels are very low, you should choose a gain that optimizes the input limits of the analog-to-digital converter (ADC).
Getting to See Your Measurement: NI LabVIEW
Once you have connected the sensor to the measurement instrument, you can use LabVIEW graphical programming software to visualize and analyze the data as needed.
Figure 7. LabVIEW RTD Measurement