Predefined formula to calculate a dew point

Feature requests
1

Dew Point Calculation

I saw on the new page Rythme de développement | Gladys Assistant :slight_smile: that a PR is in progress to have more possibilities in the scenes to perform calculations (feat(scene): support more math functions in scene formulas by Pierre-Gilles · Pull Request #2882 · GladysAssistant/Gladys · GitHub).

I currently use the api https://api.mathjs.org/v4/ to find the dew point of a room based on its relative humidity and current temperature. (for automatic triggering of a dehumidifier).

I use the Magnus formula, and I’ll let you judge :sweat_smile:

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I think it would make sense to have a predefined formula where the user only needs to enter the 2x necessary variables (current temperature and humidity).

David.

2

I vote for it because in winter in Alsace, having a dew point is interesting in relation to frost in winter and having alerts :slight_smile:

3

I discovered Gladys after an article on Korben (in 2020… Already? :sweat_smile:) : https://korben.info/gladys-assistant.html

And I see that he just published an article about an HA installation following his move: https://korben.info/retour-domotique-home-assistant-mini-pc.html

Given the progress Gladys has made since 2020, and especially this year, I think he made a poor choice. He even did dew point comparisons to get ventilation advice (and HA doesn’t seem to have a native function for that). This reinforces my idea that this feature has an audience :stuck_out_tongue: .

4

I’m also interested because I have a calculation in Node-Red for the dew point for the next day and to have alerts for frost

5

Wouldn’t you rather get it from a weather source instead of calculating it?

6

Hello @cicoub13,

From my side, the goal is to calculate the dew point inside my room, which is obtained from the relative humidity and the current temperature, data provided by a sensor in a room. An external weather source will only give the dew point outside (which is also interesting data in itself).

7

For my part, I calculate the freezing point based on the dew point and the temperature.

To get the dew point and the outside temperature, I query a weather API.
Every evening at 6:00 p.m., I query the weather API to get the values for the next day at 6:00 a.m. That way, I can receive an alert if I need to cover my wife’s car to prevent the car from freezing before leaving for work.

8

I would also be interested in this dew point calculation inside a room, to be warned in the summer that my PaC setting, which sends cool water into the floor, is misadjusted and risks causing condensation on the ground.

9

Hello @all,

Risking coming across as the ignorant one, could you describe how you use the dew/frost points calculated, and especially in what context it is useful in your home? I asked Claude (Claude) and I gather its application in a cold or humid context?

Thanks for your usage examples,

Have a nice end of the day,

Jean

10

For my part, it’s already explained in my message: to know the freezing point and thus protect the car that’s parked outside so I don’t have to de-ice it when I leave for work at 6 a.m.

11

From my perspective, if the dew point in my home, calculated with the ambient temperature and relative humidity, is for example 20°C, I deduce that the floor temperature must not be below these 20°C, otherwise condensation and an ice rink are guaranteed. And so if my heat pump’s setpoint is at 19°C, I need to be alerted.

12

Hello @jean_bruder,

The dew point is the temperature at which, for a given temperature and humidity, the air reaches 100% relative humidity. From that point on, the air no longer has the capacity to hold more water vapor: it will therefore condense. And the warmer the air, the more water vapor it can hold — and this is not at all linear.

The most illustrative example I can think of is when you take a bottle out of the fridge. The ambient air comes into contact with a cold surface, whose temperature is below its dew point. The water present in the air will then condense on the bottle, making it « wet. »

I hadn’t thought about this use case at all in the summer, but @StephaneB does indeed describe exactly this phenomenon with his floor.

For me, it’s more of a winter problem. I have a specific spot that opens onto my kitchen, which is not insulated → thermal bridge. The surface at this point becomes cold enough to promote condensation.

  • Proper insulation → avoid having cold surfaces inside your home.

  • Proper ventilation → avoid having too high a humidity level in the air.

In my case, I have a few constraints that make the pragmatic solution to use a dehumidifier in this spot. This way, I reduce the humidity level of the air and prevent it from reaching its dew point upon contact with the cold surface, which limits the risk of condensation and thus mold in the long term. All of this controlled by Gladys :stuck_out_tongue:

In the blog post I shared, it’s another use case, this time in the summer. Comparing the dew point between the outside and the inside helps better assess whether ventilation will actually improve comfort.

Humans cool down mainly by sweating: when sweat evaporates, it cools us down. If the indoor air is already very humid, sweat evaporates less easily, and therefore we have more of that stifling heat sensation.

That’s why in the summer, even if the outside air is warmer in temperature, it can be interesting to ventilate if its dew point is lower than that of the inside: you let in drier air, which then allows for better evaporation of sweat.

13

Personally, I had made this request a million years ago, for my terrarium:

https://community.gladysassistant.com/t/dans-lintegration-tasmota-recuperer-la-donnee-point-de-rosee

14

Good evening,

Thank you @Will_71, @StephaneB, and @davidm50 for your responses and the examples given. Although I have underfloor heating that actually circulates cool water in the summer, I have never had any floor moisture. The advantage of historical Alsatian houses, no doubt :smiling_face_with_three_hearts:

Have a nice end of the day,

Jean