Chapter 4 Example Measurements

The above equation can be implemented on the block diagram as shown in

Figure 4-17.

Figure 4-17. Measuring Fluid Level Without DAQ Named Channels

Alternatively, a DAQ Named Channel configured in the DAQ Channel Wizard can handle this scaling. In this case, the LabVIEW diagram is simplified to that of Figure 4-18.

Figure 4-18. Measuring Fluid Level Using DAQ Named Channels

How to Measure Resistance

It is simple to use either the NI 4050 or NI 4060 DMM to measure resistance. Figure 4-19 shows an instrument control system to measure resistance.

Figure 4-19. Instrument Control System for Resistance

Chapter 4 Example Measurements

Figure 4-20 shows the LabVIEW diagram to measure resistance using the IVI class driver VIs. Notice that this diagram is similar to Figure 4-13. The difference is the measurement function has been changed to 2-wire resistance.

Figure 4-20. Measuring Resistance Using an Instrument

How to Measure Temperature

A thermocouple is formed when two dissimilar metals come in contact with each other, and a temperature related voltage is produced. Because they are inexpensive, easy to use, and easy to obtain, thermocouples are commonly used in science and industry. This section examines a simple approach to measuring temperature using a thermocouple. Refer to Application

Note 043, Measuring Temperature with Thermocouples – a Tutorial,

for more information about measuring temperature using a thermocouple. This application note can be found on our web site at

zone.ni.com/appnotes.nsf/

In this example, you will learn how to measure a single temperature value following the diagram of Figure 4-21.

Figure 4-21. Simple Temperature System

Chapter 4 Example Measurements

Figure 4-22 shows a typical wiring diagram for a thermocouple. Notice that the resistor, R, is only used if the thermocouple is not grounded at any other point. If, for example, the thermocouple tip were already grounded, using R would cause a ground loop and result in erroneous readings.

Figure 4-22. Thermocouple Wiring

Figure 4-23 shows the block diagram needed to measure the temperature if you are using DAQ Named Channels. In this case, the DAQ Named Channel handles all gain, linearization, and cold-junction compensation.

Figure 4-23. Measuring Temperature Using DAQ Named Channels

If you do not want to use DAQ Named Channels to measure temperature, you must write a VI that determines the gain needed for your temperature range, read the thermocouple voltage, read the cold-junction voltage,

and convert all this information into a temperature. Refer to the Single Point Thermocouple Measurement VI located in

examples\daq\solution\transduc.llb for an example of how to do this. Refer to the Single Point RTD Measurement VI located in examples\daq\solution\transduc.llb for an example of how to measure temperature using an RTD.