Calculated Fields

Calculated fields let you apply statistical functions and custom formulas to your device telemetry directly in Trendz, without modifying any data in ThingsBoard or adding extra load to the database. Results can optionally be written back to ThingsBoard as telemetry for use in dashboards, rule chains, and further analysis.

To access the calculated fields tool, click the √x icon labeled Calculation Fields in the left sidebar. The table lists all existing calculated fields. To create a new one, click Create Calculation Field.

The Function tab defines how the calculation is computed.

The Input tab lets you define and inspect the data fed into the calculation.

Data field mode:

• Real Data — shows read-only input values based on the configured parameters.
• Manual — lets you enter test values manually. Manual entries can be edited, copied, or deleted; editable fields have a gray background.

Built-in variables always available in the calculation function:

After clicking Run test, the Output tab shows three views:

The Tasks tab lists all background jobs created for this calculated field, along with their current status: completed, in progress, or pending.

Jobs are triggered in two situations:

• A calculated field is enabled for the first time — Trendz creates an initial job to compute and store historical results in ThingsBoard.
• You modify and re-save an enabled calculation and then manually run a Reprocess Task — a new job applies the updated formula to all historical data.

Without running the Reprocess Task after a change, the updated formula applies only to data recorded after the re-save. Historical values retain the results of the previous formula.

Convert boiler temperature from Celsius to Fahrenheit:

var celsius = avg(Machine.boilerTemp);
var fahrenheit = celsius * 1.8 + 32;
return fahrenheit;

Calculate total energy consumed by two meters per square meter of apartment area:

  ┌──────────────────────────────────────────────────────┐
  │  Apartment (area = 120 m²)                           │
  │  CF: (energyConsumption + heatConsumption) / area    │
  └──────────────────────┬───────────────────────────────┘
                         │ contains
             ┌───────────┴───────────┐
             ▼                       ▼
  ┌────────────────────┐   ┌────────────────────┐
  │     HeatMeter      │   │    EnergyMeter      │
  │ heatConsumption    │   │ energyConsumption   │
  └────────────────────┘   └────────────────────┘

  Result: (200 + 300) / 120 = 4.17 kWh/m²

var energy = sum(energyMeter.energyConsumption);
var heat = sum(heatMeter.heatConsumption);
var size = uniq(apartment.area);
return (energy + heat) / size;

To access a field inside a calculation function, use the three-part reference:

  avg(Machine.temperature)
  │    │        │
  │    │        └─ Field name (telemetry key or attribute)
  │    └────────── Entity name (Asset Type or Device Type)
  └─────────────── Aggregation function (avg, sum, uniq, …)

All three parts are required. Use uniq() when referencing an attribute, an entity name, or an owner name.

After configuring the calculation, click Save Field in the upper-right corner. The field appears in the business entity list under the name you provided. If a field with the same name or key already exists, the system warns you of the duplicate.

While saving, you can Enable the field. When enabled, Trendz writes computed results to ThingsBoard as telemetry under the key _ECD_{KEY}.

For full details, see Save Calculation to ThingsBoard.

1. Open the saved calculation and make your changes.

2. Click Save Field again. Trendz prompts you to propagate the changes to all view fields that use this calculation — confirm to update them automatically.

3. If the field was enabled, go to the Tasks tab and click Run Reprocess Task to apply the updated formula to all historical data.

Once saved, a calculated field appears in the business entities panel under the name you assigned it and can be added to any view.

Real-time (RT) vs ThingsBoard (TB) data:

Calculated fields appear in the business entities list under the name assigned during creation. Toggle RT to compute the result from current telemetry instead of reading stored ThingsBoard data.

To adjust a calculation directly from a view field, click Go to calculation field in the view field configuration panel.

A calculated field that is not enabled still appears in the business entities list and can be dragged into any chart. However, a red bell icon appears next to it in the chart-building area as a reminder that the field is not persisted to ThingsBoard.

Click the red bell icon to navigate directly to the calculated field and enable it.