For delta logging, data is logged (as normal) using the appropriate NumericCOV (Numeric Change Of Value) or NumericInterval extension. Then, in the Chart view, you simply select the Delta command to plot delta values. Similarly, in an exported CSV file, you can review the delta values instead of the running total value, as shown below. Delta values are computed by taking the difference between one numeric record and the next. The timestamp of the last record (of the two) is used as the timestamp for the delta value.

Figure 9.   Delta logging history values shown in History Table view

Two other properties that apply to delta logging are related to the concept of rollover.

Rollover occurs when a running total reaches a defined maximum number and then resets to zero or another defined number. The defined maximum number is represented in the history extensions by the Max Rollover Value property. The reset value (which is often zero) is represented in the history extensions by the Min Rollover Value property. These properties allow you to specify the behavior of the delta logging when the rollover occurs. If you do not know these values or if they are not specified, then select the null option for these properties.

Figure 10.   Using rollover value properties with delta logging

Consider the following example. If you are logging energy consumption with the Max Rollover Value property set to 999,999 and the Min Rollover Value set to 100, then when a rollover is detected, the delta logging bases its delta calculations on a maximum value of 999,999 and a subsequent initial value of 100.