ClearSCADA has built-in historic algorithms that you can use with historic Trends and other historic features such as Historic Views. Each historic algorithm performs a calculation on the relevant raw historic values and returns the result, which can be used as a data source on a Trend or displayed in Queries. (For more information on raw historic values, see Raw Historic Data on Trends).
ClearSCADA uses the proprietary Quality filter feature to define which raw values are used by the algorithms and will only limit itself to good quality raw values if that is how the filter is configured. This method complies with the OPC HDA standard, as a result, if you are using a standard OPC HDA client or a part of ClearSCADA that does not support the quality filter then all raw values (regardless of quality) will be used.
Each historic algorithm is applied to the values in a defined 'resample interval' (a time period). The resample interval is defined in the configuration of the Historic item, for example the Historic Trend, Historic Calculation, Historic View, and so on.
If the resample interval is 30M (minutes) and you use the 1hAverage algorithm, ClearSCADA calculates the average hourly value at 30 minute intervals.
Take care with End and End Last Historic Algorithms used on clients, or with ClearSCADA features, that run in Local Time with Daylight Saving Time. Ensure that the start and end times specified for the sample interval do not coincide exactly with the time at which the clock jumps backward. Include a small offset in the sample time, to offset the calculation to avoid the sample end or start time exactly matching the time that the clock jumps backward at the transition from Daylight Saving Time to Standard Time. A small offset of 1s or even 1ms should suffice. For more information about Daylight Saving Time, see The Effects of Daylight Saving Time Adjustments.
The topics in this section explain the historic algorithms that are often used when working with historic data. For more information, see the topics that are listed in the gray footer section at the bottom of this topic. Select the relevant entry to display the topic that you require.
ClearSCADA also supports any custom historic algorithms that have been created on your system (see Historic Algorithms (Custom)). When you are configuring a Historic item, any custom historic algorithms are included in the list of available algorithms.