Wednesday, August 18, 2010

Measure Temperature using a RTD, myDAQ, and LabVIEW

This document explains using a Platinum RTD to measure temperature with your National Instruments myDAQ in LabVIEW. The data will be acquired using the DAQ Assistant that is installed into LabVIEW with the NI DAQmx driver and converted to a temperature using basic programming in LabVIEW.

A RTD is a thermal sensing device constructed from a metal, in this case platinum, that varies linearly with temperature in a certain range.  There are a multitude of these RTDs that can accept a wide range of temperatures.  In this application, we will focus on a Pt-RTD 3750.  There is a basic equation used to calculate temperature from resistance, similar to the document that involves using myDAQ with a thermistor.  Due to the high value of platinum, the sensors are not necessarily low-cost, but they are commonly used in a variety of applications.

Figure 1: Omega Precision 3-Wire Platinum RTD
The user interface we created both shows the current temperature measurement taken as well as plotting the values over time on a waveform chart.  Temperature Chart stores previous values so that you can see the change in values over time. 
Figure 3: LabVIEW Front Panel

Coding Strategy:

In LabVIEW we need to measure the resistance signal coming from the Pt-RTD from 100Ω to 10kΩ.   This value is the converted to a temperature using the polynomial equation from the RTD specifications for the respective sensor, and the Callendar-Van Dusen equation.  Finally, we will output the result to a numeric indicator and a temperature chart on the front panel.

Figure 4: The Callendar-Van Dusen Equation

Figure 5: Coding Block Diagram

The LabVIEW block diagram looks very similar to the coding block diagram
Figure 6: LabVIEW 2009 Block Diagram
(The attached LabVIEW code snippet can be dragged-and-dropped to a LabVIEW block diagram, use attached PNG file.  After locating the PNG file, just drag the file icon onto a blank block diagram, as if you were dragging the file onto your desktop.)

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