During slow scan rates, are events captured between scans?

[Willard]

We are using 32 thermocouples to measure a heated cabinet performance and are using the Continuous Acquisition to Spreadsheet VI. I am saving the data to a text file while plotting the data to a chart in Labview 6.1. I only wanted to read temperature every 60 seconds (0.01667 scans/sec) but set the scan rate rounds off to 2 decimal places, so at 0.02 scan rate we read data every 50 seconds. I set the buffer size to 1 and the number of scans to write at a time to 1. Does this mean I have a lag of one scan in the buffer or am I seeing the latest scan written to the chart? Also, temperature events between writes to the screen sometimes show up on the chart, sometimes not. It seems to depend on how long the e
vent takes place or how close to the next write. I would appreciate any insight on how the software handles the events between writes. Thanks.

[RIO-Todd]

Willard,

You can actually change the digits of precision in LabVIEW to display more than 2 decimal places. If you right-click on the scan rate control and select 'Format and Precision' you can alter the digits of precision to include 5 decimal places.

By setting the buffer size to 1 you are setting a memory size to hold only 1 scan at a time. By setting the number of scans to write at a time to 1 that means that every time the AI Read VI is called within the while loop, the VI will only read 1 scan from the buffer. Therefore, the hardware is set-up to read 1 scan every 50-60 seconds and so the hardware places that scan in the buffer. Then every time the AI Read VI is called in software, it reads 1 scan from the buffer. So, if the AI Read VI already read
the 1 scan from the buffer, it will wait until the buffer gets the next scan from hardware (50-60 seconds) and then plot it on the chart. So, with the set-up you have, you will not see any lag of temperature data. Once the hardware scans, the software will almost immediately read that scan. I hope this helps.

Regards,

Todd D.
Applications Engineer
National Instruments