32. Virtual Meter Datum Filter¶

The Virtual Meter Datum Filter provides a way to derive an accumulating "meter reading" value out of an instantaneous property value over time. For example, if you have an irradiance sensor that allows you to capture instantaneous W/m² power values, you could configure a virtual meter to generate Wh/m² energy values.

Each virtual meter works with a single input datum property, typically an instantaneous property. The derived accumulating datum property will be named after that property with the time unit suffix appended. For example, an instantaneous irradiance property using the Hours time unit would result in an accumulating irradianceHours property. The value is calculated as an average between the current and the previous instantaneous property values, multiplied by the amount of time that has elapsed between the two samples.

This filter is provided by the Standard Datum Filters plugin.

32.1 Settings¶

Each filter configuration contains the following overall settings:

32.2 Virtual Meter Settings¶

The Virtual Meter settings define a single virutal meter.

Setting	Description
Property	The name of the input datum property to derive the virtual meter values from.
Property Type	The type of the input datum property. Typically this will be Instantaneous but when combined with an expression an Accumulating property can be used.
Reading Property	The name of the output meter accumulating datum property to generate. Leave empty for a default name derived from Property and Time Unit. For example, an instantaneous irradiance property using the Hours time unit would result in an accumulating irradianceHours property.
Time Unit	The time unit to record meter readings as. This value affects the name of the virtual meter reading property if Reading Property is left blank: it will be appended to the end of Property Name. It also affects the virtual meter output reading values, as they will be calculated in this time unit.
Max Age	The maximum time allowed between samples where the meter reading can advance. In case the node is not collecting samples for a period of time, this setting prevents the plugin from calculating an unexpectedly large reading value jump. For example if a node was turned off for a day, the first sample it captures when turned back on would otherwise advance the reading as if the associated instantaneous property had been active over that entire time. With this restriction, the node will record the new sample date and value, but not advance the meter reading until another sample is captured within this time period.
Decimal Scale	A maximum number of digits after the decimal point to round to. Set to 0 to round to whole numbers.
Track Only On Change	When enabled, then only update the previous reading date if the new reading value differs from the previous one.
Rolling Average Count	A count of samples to average the property value from. When set to something greater than 1, then apply a rolling average of this many property samples and output that value as the instantaneous source property value. This has the effect of smoothing the instantaneous values to an average over the time period leading into each output sample. Defaults to 0 so no rolling average is applied.
Add Instantaneous Difference	When enabled, then include an output instantaneous property of the difference between the current and previous reading values.
Instantaneous Difference Property	The derived output instantaneous datum property name to use when Add Instantaneous Difference is enabled. By default this property will be derived from the Reading Property value with Diff appended.
Reading Value	You can reset the virtual meter reading value with this setting. Note this is an advanced operation. If you submit a value for this setting, the virtual meter reading will be reset to this value such that the next datum the reading is calculated for will use this as the current meter reading. This will impact the datum stream's reported aggregate values, so you should be very sure this is something you want to do. For example if the virtual meter was at 1000 and you reset it 0 then that will appear as a -1000 drop in whatever the reading is measuring. If this occurs you can create a Reset Datum auxiliary record to accomodate the reset value.
Expressions	Configure as many expressions as you like, using the + and - buttons to add/remove expression configurations.

32.3 Virtual Meter Expression Settings¶

A virtual meter can use expressions to customise how the output meter value reading value is calculated. See the Expressions section for more information.

32.4 Filter parameters¶

When the virtual meter filter is applied to a given datum, it will generate the following filter parameters, which will be available to other filters that are applied to the same datum after this filter.

32.5 Expressions¶

Expressions can be configured to calculate the output meter datum property, instead of using the default averaging algorithm. If an expression configuration exists with a Property that matches a configured (or implied) meter configuration Reading Property, then the expression will be invoked to generate the new meter reading value. See the Expressions guide for general expression language reference.

32.5.1 Expression root object¶

The root object is a virtual meter expression object that lets you treat all datum properties, and filter parameters, as expression variables directly, along with the following properties:

The following methods are available:

32.5.2 Expression example: time of use tariff reading¶

Iagine you'd like to track a time-of-use cost associated with the energy readings captured by an energy meter. The Time-based Tariff Datum Filter filter could be used to add a tou property to each datum, and then a virtual meter expression can be used to calculate a cost reading property. The cost property will be an accumulating property like any meter reading, so when SolarNetwork aggregates its value over time you will see the effective cost over each aggregate time period.

Here is a screen shot of the settings used for this scenario (note how the Reading Property value matches the Expression Property value):

The important settings to note are:

Assuming a datum sample with properties like the following:

Then here are some example expressions and the results they would produce: