Calculate Differences

Node-RED Flow: Calculate Differences

This flow is designed to calculate the difference between consecutive numerical values along with the time elapsed between them.

Flow Overview

1. Inject Nodes

   • Several inject nodes send predefined numerical values (0, 10, 20, 50, 100) into the flow to simulate data input.
   • Each inject node triggers the flow manually when clicked.

2. Set Value Node (Function Node: "set val")

   • Converts the incoming numerical value into an object:

     {
       "value": <input_value>
     }
     

   • Adds a timestamp (msg.timestamp) representing the current time.

3. RBE (Report-by-Exception) Node

   • Ensures that only changes in the payload value are forwarded to the next stage, filtering out duplicate values.

4. Calculate Differences (Function Node)

   • Computes the difference (diffValue) between the current and previous values.
   • Computes the time difference (diffTimestamp) between the current and previous timestamps.
   • Outputs an object with:

     {
       "diffValue": <difference>,
       "diffTimestamp": <time_difference>,
       "newValue": <current_value>,
       "newTimestamp": <current_timestamp>,
       "prevValue": <previous_value>,
       "prevTimestamp": <previous_timestamp>
     }
     

   • Stores the current value and timestamp in the node’s context for the next calculation.

5. Debug Node ("Show Output")

   • Displays the computed differences in the Node-RED debug pane.

Expected Output

• On each new value input, the flow calculates and outputs the difference from the previous value along with the time difference between updates.
• If it’s the first received value, diffValue and diffTimestamp will be null.

This flow can be useful for tracking sensor data changes, logging rate-of-change metrics, or monitoring process values over time.

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