Derivative?

ESPHome
1

To extract a derivative of a sensor (or even simply delta), is it best to create a global variable to store previous value and compare with current value? Is there a better method?

2
3

For a delta sensor, if you need to do it in ESPHome you can put this on a copy sensor:

filters:
  - lambda: |-
      static float last_value = 0;
      float change = 0;
      change = x - last_value;
      last_value = x;
      return change;

And on ESPHome Discord if you search for “Delta Sensor (Change from previous value). Survive Deep Sleep” there’s also a way using globals that will survive a deep sleep.

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4

I didn’t realize that’s how the static keyword works in lambdas. Nice. That solves the issue.

Just out of curiosity; how can I use a copy sensor to get delta? Do I understand correctly that it simply mirrors the state of another?

EDIT: right, I think I get it: instead of creating a separate sensor that fetches the value of the other sensor, it’s easier to create a copy sensor for the delta purpose.

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5

Copy sensors are a newer thing. Before that you’d probably use a template sensor to duplicate an existing one.

Most of the solution came from somewhere else on the internet with a few additional improvements then suggested by jesserockz on Discord.

6

My solution for a time derivative, so far stable for me.

  - platform: copy
    source_id: wasser_boiler
    id: "wasser_boiler_derivative"
    unit_of_measurement: "°C/h"
    name: "Warmwasseränderung Boiler"
 
    filters:
      - lambda: |-
          static float last_value = 0;
          static float last_time = 0;
          float time   = (float) millis();
          if (last_time == 0){
            last_value = x;
            last_time  = time;
            return {};
          }                    
          float change = ( ( x - last_value ) / ( time - last_time ) ) *1000*60*60;
          last_value = x;
          last_time  = time;
          return change;
      - sliding_window_moving_average:
          window_size: 3
          send_every: 2          
      - or: 
        - delta: 0.01  
        - heartbeat: 120minutes            
      - throttle: 1minutes

Note: the delta is on the derivative, so compared to the source a acceleration filter.

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