How to programmatically change "unit_of_measurement" on the fly

ESPHome
,
1

I am making a refrigerator controller using a lily TTGO board (esp32, lcd) I want to configure my screen for either F or C temperature display.

I configure some Dallas temperature sensors. Then use a filter to do the conversion based on a flag.

I would like to change the “unit_of_measurement” as well, but that tag only accepts strings and, as much as I beat upon it, I could not figure out how to change that in yaml, or lambda per suggestions from ChatGPT (!). So I eventually rummaged around the sensor.h file and found a handy public method “set_unit_of_measurement()” which works fine.

So, is this a feature or a bug that !lambda cannot be used with the “unit_of_measurement” tag?

e.g. this works

    filters:
      - lambda:  if (id(bFarenheit)) return x * (9.0/5.0) + 32.0; else return x; 
      - lambda:  id(freezer_temp).set_unit_of_measurement(id(bFarenheit)?"°F":"°C"); return x;

This does not
unit_of_measurement: !lambda id(bFarenheit)?"°F":"°C");

Is there a better way to do this?

2

Believe the reason is that in HA the measurements are stored and used for historical data. If unit changes, would mess up sensor’s historical data. And yes, I know that you ask how to change, not why it doesn’t have native support for changing.
Not an option for you to add an additional sensor, so one for C and one for F and let your screen switch between those?

3

So a template sensor in F and leave the native in C

Sounds good to me. Something new to figure out :slight_smile:

Thx!

4

Can do something like this:

- platform: template
  id: freezer_temp_fahrenheit
  unit_of_measurement: "°F"
  lambda: |-
    return id(freezer_temp).state * 9/5. + 32;

Which updates every 60s (default), can use the update_interval to increase update speed.

Alternative is update the template sensor directly from your freezer sensor:

- platform: template
  id: freezer_temp_fahrenheit
  unit_of_measurement: "°F"
- platform: ...
  id: freezer_temp
  on_value:
    - sensor.template.publish:
        id: freezer_temp_fahrenheit
        state: !lambda 'return x * 9/5. + 32;'