Essential Concepts in Data Acquisition for Chemical Engineering Research

1. Data Acquisition Seth Price Department of Chemical Engineering New Mexico Tech Rev. 10/9/14

2. Resolution • Resolution: the smallest “y” step measurable by measurement system • Resolution is related to the number of states (and thus number of bits)

3. Sampling Rate • How fast a system collects data • Generally expressed in Hertz, Hz • Means how many times a second • Trade off when choosing sampling rate • Sampling too fast -> generating larger files, some “useless” data, expensive hardware • Sampling too slow -> miss short-lived events, aliasing

4. Aliasing • Consider a 10 Hz waveform sampled at a few different sampling rates:

5. Nyquist (or Sampling) Theorem • MUST sample at a rate that is greater than twice the highest frequency you will see in the system to avoid aliasing S > 2*fm

6. DAQ In LabVIEW • Using DAQ Assistant Express VI: • Handles much of the low-level configuration for you • Much easier than traditional DAQ • Does not work on all equipment • Can be the source of problems

7. DAQ in LabVIEW • Place a DAQAssistant Express VI on the block diagram • Select whether you will be generating a signal or acquiring a signal • Select the type of signal (voltage, current, etc.) • Select the physical channel

8. DAQ in LabVIEW • Choose a Sampling Method • For this class, choose “On Demand” • Choose a Grounding Method • For this class, choose “Let NI Choose” • Test in DAQAssistant Window • Press “OK”

9. Data Conversion • LabVIEW uses a data type called “Dynamic Data” when interfacing hardware • We want a number (in this case), so we must convert it

10. Data Conversion • For “Acquiring Signals”, use “From DDT” • On the popup menu, select “Single Scalar” • For “Generating Signals”, use “To DDT” • On the popup menu, select “Single Scalar”