As you have seen from the previous labs, a Measure can summarize data from a data source and store the resultant information in your Flow System. In the previous labs, you created a Measure that summarized data every hour. This means that for every hour (described by your calendar definition), a summary value was created and stored in the Flow System for that Measure. However, Flow handles more than just hourly time periods. Let’s see …
The following Measure intervals can be configured in Flow (notice the associated icons):
Each Measure created in your Flow System will be associated with one of these intervals. Once created, a Measure’s interval cannot be modified. If you create an hourly measure for the average Boiler Temperature, but you also require the daily Boiler Temperature average, you will need to create a new daily measure.
It is important to note that “Minutely” in Flow does not imply a single minute time period, but rather a sub-hourly period that is defined by your calendars. The available “Minutely” durations are:
- 5 minutes
- 10 minutes
- 15 minutes
- 20 minutes
- 30 minutes
|The reason Flow has these “Minutely” time periods is to accommodate the aggregation (roll up) of data to an “Hourly” time period.|
In the previous labs, you created a Measure that retrieved detailed data from a data source (i.e. Historian) and summarized it into a single value stored in the Flow System. This Measure Type is known as “Retrieved”. However, Flow allows the configuration of other Measure Types as well, let’s discuss …
The following Measure types can be configured in Flow:
- Retrieved (from a data source)
- Manually Entered (by a person)
- Aggregated (or rolled up from other Flow Measures)
- Calculated (from other Flow Measures)
Retrieved Measures are used to collect data from the data sources you have connected to your Flow System. These measures are responsible for collecting, summarizing and storing data from the “outside world” in your Flow System.
Most Flow Data Source types represent tag-based time series data. For this reason, Flow has defined a number of aggregation methods to standardize how detailed data is summarized into Flow.
|Note that Flow does not replicate the detailed data from your data sources in the Flow System. It will only store the summarized information in the Flow System.|
Manually Entered Measures
It is not always possible to retrieve information automatically from a data source. Some information is just not available in a data source. For this reason, Manually Entered Measures are used to allow insertion of information into your Flow System by people.
Examples of where you would use a Manually Entered Measure include:
- Instrument data that isn’t available in your control systems and historians
- Calculation factors that you will want to change over time
Aggregated Measures are used to perform roll up calculations of other measures in your Flow System. For example, you have already created a measure that summarizes the Boiler Temperature from your Historian every hour. You can configure a new Aggregated Measure to calculate the daily average of these hourly values. This means that the new daily measure does not have to re-query the detailed data from the Historian to provide the daily summary values, it rather uses the information Flow has already collected from the Historian for the hourly values. Furthermore, if any of the hourly values need to be edited via the Flow Client Tool, Flow will automatically re-calculate the daily Aggregated Measure, thus maintaining the integrity of the information stored in your Flow System.
|By using Aggregated Measures, you not only reduce the retrieval load on your data sources, but also increase the performance of your Flow System.|
Calculated Measures are used to configure your own calculations based on one or more measures in your Flow System. This is an extremely powerful feature which allows you to generate additional information, just like you would be able to do in Excel. For example, if you had an hourly measure for production counts, and an hourly measure for electricity used, you could create an hourly calculated measure for the ratio of electricity used per unit of production.
In addition to allowing the configuration of calculations, Flow has a built-in library of common functions, similar to what you would find in Excel. You can even create your own User Defined Functions to standardize and simplify your calculation expressions.
|Flow allows you to use Calculated Measures inside other Calculated Measures, making nested calculations possible.|