The NI 9217 does not support two-wire measurement configurations.Ĭonnect the red RTD lead to the excitation positive. 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. 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. Place a jumper from the excitation negative to the channel negative on the data acquisition device. Connect the black (or white) RTD lead to the excitation negative. Place a jumper from the excitation positive pin to the channel positive on the data acquisition device. There are essentially three different methods to measure temperature using RTDs.Ĭonnect the red RTD lead to the excitation positive. To avoid self-heating, which is caused by current flowing through the RTD, minimize this excitation current as much as possible. You can then easily transform this reading to temperature with a simple algorithm. NI CompactDAQ Chassis and the NI 9217 RTD ModuleĪn RTD is a passive measurement device therefore, you must supply it with an excitation current and then read the voltage across its terminals. For higher-channel-count measurement systems, NI provides the PXIe-4357 RTD input module.įigure 3. The following example demonstrates an RTD measurement using an NI CompactDAQ chassis and the NI 9217 RTD module (Figure 3). Most instruments offer similar pin configurations for RTD measurements. In addition, reference the RTD specification to find the excitation level for that particular device. 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. If there is close to 0 Ω resistance, then the leads are attached to the same node. 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.
The red wire is the excitation wire and the black or white wires are ground wires. The resistance/temperature curve for a 100 Ω platinum RTD, commonly referred to as Pt100, is shown in Figure 2.Īll RTDs usually come in a red and black or red and white wire-color combination. The relationship between resistance and temperature is nearly linear and follows this equation:Ī, b, and c = constants used to scale the RTD Typical nominal resistance values for platinum thin-film RTDs include 1 Ω. RTDs are commonly categorized by their nominal resistance at 0 ☌. 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. However, they are generally more expensive than alternatives because of the careful construction and use of platinum. Popular because of their stability, RTDs exhibit the most linear signal with respect to temperature of any electronic temperature sensor. To protect the RTD, a metal sheath encloses the RTD element and the lead wires connected to it. Thin-film elements are cheaper and more widely available because they can achieve higher nominal resistances with less platinum.
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. Wire-wound RTDs are created by winding a thin wire into a coil. RTDs are constructed using one of two different manufacturing configurations. 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 operate on the principle of changes in electrical resistance of pure metals and are characterized by a linear positive change in resistance with temperature.
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