Use time offsets to compare data in absolute time

In absolute time mode, you can use a time offset to compare the same data from different time periods. For example, you may want to compare data from a shift at 10:00 a.m. to data from a shift at 11:00 a.m. The time offset feature enables you to adjust the time period for one of the shifts so that the data appears as if it occurred during the same time period as the other shift. Using a time offset enables you to easily see the differences between the data on the trend chart.

To use a time offset

1. Create a trend for batch of data that you want to use as the basis for comparison.

   In this example, the chart is configured to show data for the ReactLevel tag between 6/22/2005 10:00:00 AM and 6/22/2005 11:00:00 AM.

   

2. Add the same tag again to the trend chart for the same time period.

   In this example, the ReactLevel tag was added again to the chart.

3. Because the data is identical, you only see single trend curve in the chart.

   

4. Determine the time span for the data you want to compare with the base batch of data.

5. To specify the time offset for the data to compare, double-click on the tag in the Tag List. The Tag Properties dialog box appears.

6. In the Time offset box, configure the amount of time that the data shown in the chart is to be offset from the actual query time. For more information on the format, see Time offset formats.

   In this example, this data is to be compared with the base batch that occurred an hour before, so the time offset is set to one hour.

7. Click OK.

   In this example, the data for the ReactLevel tag between 6/22/2005 11:00:00 AM and 6/22/2005 12:00:00 AM includes an annotation made around 11:30:00 AM.

   

8. Stack the traces so that you can see both sets of data separately and then select the first tag that you added to the chart.

   In this example, the trend curve for the later set of data (shown in green) appears on the chart, even though the time axis reflects the time of the base batch of data (shown in orange).

   

9. To view the chart in relative mode, on the View menu, click Relative Time. The time axis now shows the time span for the base batch starting at 0:00:00 instead of the actual time.

   

You can also use the offset to compare a trend curve against another curve either forward or backward in time. To do this, set the time offset of the primary batch of data so that the start time is the same as the start time for the batch of data you want to compare.

In the following example, the time offset for the complete batch is set to a value of -01.00.44. The complete batch appears as the top curve in the chart.

The incomplete curve at the bottom of the chart is plotted in live mode next to the complete curve at the top.