RTD Temperature Measurement Using the SCXI-1520 Strain/Bridge Module

This concept document describes how to use a SCXI-1520 to measure an RTD in a Wheatstone bridge configuration for slight variations in temperature with high accuracy.

Background

A resistance-temperature detector (RTD) is a temperature sensing device whose resistance increases with temperature. Compared to other temperature devices, the output of an RTD is relatively linear with respect to temperature. Most RTD measurement configurations involve passing a known current through an RTD and then measuring the voltage across the device. By knowing both the current through the device and the voltage across it, the resistance is computed. The known resistance is used to compute a temperature.

Another method for calculating the resistance of an RTD is to place the device in a Wheatstone bridge that is excited by a known voltage. A Wheatstone bridge consists of four individual resistors. If the RTD is the only unknown resistance in the bridge, the voltage measured from the bridge, the excitation voltage, and the values of the three other resistors is used to determine the resistance of the RTD. This Wheatstone bridge configuration is known as a quarter-bridge configuration because the RTD is the only variable resistance and the resistances of the other three resistors is constant. This configuration is shown in Figure 1.

Voltage-to-Resistance Equations

In order to determine the resistance of the RTD in the bridge circuit, its value must be the only unknown variable. Using the known values of the excitation voltage, measured voltage, and the remaining three resistors, you can calculate the resistance of the RTD. The equation for the measured voltage is shown below. The measured voltage is expressed in terms of the excitation voltage and values of the four resistors.



Using this equation, the resistance of the RTD is expressed in terms of the excitation voltage, measured voltage, and the remaining three resistances.

Resistance-to-Temperature Equations

Once the resistance of the RTD is determined, the temperature of the RTD is calculated. Platinum RTDs use a linearization curve known as the Callendar-Van Dusen equation to measure the temperature of RTDs. The equation is as follows:

Temperatures below 0 °C:
R_T = R₀[1 + A × T + B × T² + C × T³ × (T – 100 °C)]

Temperatures above 0 °C:
R_T = R₀[1 + A × T + B × T²]

T = temperature in °C
R_T = RTD resistance at temperature T
R₀ = RTD nominal resistance at °C
A, B, and C = Callendar-Van Dusen coefficients

The Callendar-Van Dusen coefficients are dependent on the type of Platinum RTD you use. The type RTD chosen for this application has Callendar-Van Dusen coefficients: A = 3.9083×10^–3 , B = –5.775×10^–7 , C = –4.183×10^–12

The Resistance-Temperature Curve for a Platinum RTD is shown in Figure 2. The Platinum RTD has a resistance of 100 W at 0 °C and has fairly linear Resistance to Temperature relationship.

External Connections/Hardware Configuration

The hardware used for this application consists of the SCXI-1520 universal strain/bridge module and the SCXI-1314 terminal block. This hardware conditions the signal so that the data acquisition device can accurately measure the voltage from the bridge. The Wheatstone bridge circuitry resides within the SCXI-1314 terminal block. The quarter-bridge configuration is used in this application. In this particular configuration, you must supply a dummy resistor to balance the leg of the bridge below the RTD. A 120 W quarter-bridge completion resistor is used in this case. NI recommends that the nominal resistance of this dummy resistor be close in value to the resistance of the RTD at 0 °C. The external connections to the RTD in the quarter-bridge mode is shown in Figure 3.

Software (LabVIEW Sub-VIs)

In order to interpret the voltage measured from the Wheatstone bridge, LabVIEW analyzes, processes, and computes a temperature. The software architecture has three main sections:

The first step of software is to acquire a raw voltage from the Wheatstone bridge. This code can also have some additional functionality in order to make the measurement more accurate. Offset nulling and shunt calibration can be added in order to balance the Wheatstone bridge, reduce amplifier error, and improve overall accuracy. It is important that the voltage measured from the bridge is accurate for the voltage is used to calculate the temperature of the RTD.

The second step of the software is to take the measured voltage from the bridge and compute the resistance of the RTD. The equations used to compute the resistance of the RTD were examined in the Voltage-to-Resistance Equations section above.

The third step of the software is to take the calculated resistance of the RTD and calculate the corresponding temperature of the RTD using the Calledar-Van Dusen coefficients. The equations and constants used to compute the temperature of the RTD were examined in the Resistance-to-Temperature Equations sections above.

You can use the VIs attached to achieve these three main steps. The top level application integrates all three areas.

 

Results (Interpreting Data)

You must have accurately specified all of the voltages and resistances in the system in order to get an accurate measurement. Since there are two calculations based on multiple system variables such as the resistors in the Wheatstone bridge, the bridge excitation voltage, and the Callendar-Van Dusen constants. It is critical that you use the correct values for these constants. As long as these constants remain accurate, the results produced remain accurate. The precision of the system relies most heavily on the resistance of the RTD, which has very high precision when compared to other temperature measurement devices. You can use a temperature bath of a known value to null the RTD to a specific temperature. Using a temperature bath can help improve accuracy and alleviate offset error.

Reader Comments | Submit a comment »

Effect other measurements?
If we perform this task to measure temperature with RTDs, will it have any impact on the other channels that I am recording? For instances I am looking to make a channel for a torque, pressure, load cell, fluid flow, and then have two more for RTDs?
- mmbuga@roushfenway.com - Sep 22, 2008