Add device classes to `sensor` for weight

[nareddyt]

Context

Smart bathroom scales, load cells, etc. read dimensions that HA can handle only as generic. I would like to introduce a device class for weight-based sensors.

Proposal

Add 2 new device classes for weight-related items: force and mass

The original goal was to add a device class for weight. However, I found that the term weight is often misused. Weight is a force, but most people are really referring to mass when they measure their weight.

After further reading, the SI units clearly define the distinction:

• mass (kg, lbs for imperial conversion): This is what people commonly refer to as weight. Can be used for measurements by smart bathroom scales, bed occupancy sensors, etc.
• force (N): Can be used by any sensors that are not manufactured to take into account gravity. Anything that senses force horizontally.

So I added support for both. Users can choose between the two to refer to the correct scientific term when they want to measure weight.

PRs:

• Core PR: https://github.com/home-assistant/core/pull/45508
• home-assistant.io PR: https://github.com/home-assistant/home-assistant.io/pull/16301
• developers PR: https://github.com/home-assistant/developers.home-assistant/pull/779

Consequences

Sensors can use more specific device class for "weight" measurement.

[MartinHjelmare]

It's easier to accept changes to the entity model if we can see an overview of the current integrations and also possible devices/brands in the wild and how many would benefit from the added device classes. Can you make a table or summary of this?

[nareddyt]

Good point. Here is a summary of what I've found:

• The ESPHome integration allows connecting load cells to an ESP to measure mass (reference). Other sensors can be connected to measure force and/or pressure (example). I plan to implement https://github.com/esphome/feature-requests/issues/553 so device classes can be set for non-binary sensors directly in the ESP config. Both device classes will be useful here.
• The Fitbit integration calls the Fitbit API and retrieves body/weight. It is exposed as a sensor in HA. The mass device class will help here.
• Same for the Smartthings integration, it exposes the bodyWeightMeasurement sensor.
• Same for Withings. It has sensors for fat_mass_lbs, bone_mass_lbs, muscle_mass_lbs. Notice they all say mass in the name.

[blakadder]

Tasmota supports load cells.

Bluetooth scale data gathered with https://github.com/lolouk44/xiaomi_mi_scale