When you are logging data, such as electric consumption (kWh) or other information that uses a running total, you may want to know the difference between consecutive timestamped values instead of the actual running total. The delta logging feature is provided for this type of calculation. For delta logging, data is logged (as normal) using the appropriate NumericChangeOfValue or NumericInterval extension. Then, in the history chart or history table view, you simply select the Delta option box (or use the toolbar icon) to display the delta values instead of the running total value, as shown in Figure 166. Figure 165Delta 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.

Two other parameters 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 parameter. The reset value (which is often zero) is represented in the history extensions by the Min Rollover Value parameter. These parameters 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 parameters.

Consider the following example. If you are logging energy consumption with the Max Rollover Value parameter 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.

Refer to Configure history extensions for a list and short description of all the history extension parameters.