ventilation recommendation
Hello, I’m thinking about installing several T/RH sensors indoor and one outdoor.
So it should be possible to calc the amount of water in the outer and inner air and make a ventilation suggestion for a room. Like <red> inside too wet - open window, <green> humidity ok / in range.
Anybody already did such a device or rule?
@Michael-Scherr said:
Anybody already did such a device or rule?
See pimatic-dewpoint - http://pimatic.org/plugins/pimatic-dewpoint/
"It always takes longer than you expect, even when you take into account Hofstadter's Law.", Hofstadter's Law
Yes, I’m running it with a DHT11 on the Homeduino. But… How to get the absolute humidity, not the relative?
For a proper calculation you would also need to take the barometric pressure into account - well, it should be negligible in your case. So, maybe we can extend the dew point plugin to calc absolute humidity from RH and T - https://carnotcycle.wordpress.com/2012/08/04/how-to-convert-relative-humidity-to-absolute-humidity/
Question: Wouldn’t it be good enough to use the dew point for an ventilation alg? For example: If inner dew point is too high and outside dew point is lower then ventilate.
"It always takes longer than you expect, even when you take into account Hofstadter's Law.", Hofstadter's Law
OK, I got it. I’m not familar with coffee-script, but when I take a look at your dewpoint.coffee then almost everything needed is given and these lines seem to do the job…
globals:
dewPoint: 0.0
absHumidity: 0.0
attributes
absHumidity:
description: "Absolute Humidity"
type: types.number
unit: "g/m³"
acronym: "AH"
after your calc:
@_setAttribute 'dewPoint', td
ah = 2.16679 * ((100 * dd) / (273.15 + @temperature))
env.logger.debug 'absHumidity', ah
@_setAttribute 'absHumidity', ah
getters:
getDewPoint: -> Promise.resolve(@dewPoint)
getAbsHumidity: -> Promise.resolve(@absHumidity)
Reference:
http://www.vaisala.com/Vaisala Documents/Application notes/Humidity_Conversion_Formulas_B210973EN-F.pdf (Page 12)
This only uses other consts then you do:
a=7.591386 and b=240.7263 for -20…+50°C
Verify:
http://www.humidity-calculator.com/index.php
I’ve no github account, but maybe you like to add this?
Michael
@mscherr Will do.
"It always takes longer than you expect, even when you take into account Hofstadter's Law.", Hofstadter's Law
@mscherr Implemented. If you are familiar with installing from git you can give a try (see below). I’ll also file a pull request later on but it may take a few days to get the new package released.
To install from git:
sudo service pimatic stop
cd /home/pi/pimatic-app/node_modules/
sudo rm -rf ./pimatic-dewpoint
git clone https://github.com/mwittig/dev-pimatic-dewpoint.git pimatic-dewpoint
cd pimatic-dewpoint
npm install
sudo service pimatic start
To rollback to released version at a later stage
sudo service pimatic stop
cd /home/pi/pimatic-app/node_modules/
sudo rm -rf ./pimatic-dewpoint
npm install pimatic-dewpoint
sudo service pimatic start
"It always takes longer than you expect, even when you take into account Hofstadter's Law.", Hofstadter's Law
@mscherr Regarding the constants used as part of the dew point calculation my understanding is different valuations (see Wikipedia) have been defined over time by various organizations and researchers. However, I could not find a proper reference for the valuations @flybysun have used except for http://www.wetterochs.de/wetter/feuchte.html and http://x11-basic.sourceforge.net/examples/calculation/taupunkt.bas The latter is referring to FAQs.org, however, the FAQs.org article is suggesting the constants for vapor pressure over ice if T<0. I am not sure which constants provide the best results. Would be greate to have a proper reference for this explaining the valuations in some more detail.
Beyond this maybe the following link is interesting for http://www.hygrothermik.de Do you have some literature or HowTo on the ventilation algorithm? This would be great.
"It always takes longer than you expect, even when you take into account Hofstadter's Law.", Hofstadter's Law
I’m not a specialist for humidity, but beside the consts, the formular for sdd and dd, used in the coffee file, is the same as in my referenced PDF file in page 12 and page 6. There is also an calc example. I verified some calculations to the online humidity calculator and for me the results were exact enough.
@mscherr and @mwittig I added the reference [wetterochs.de] to the github project. When working on the plugin I took the formula from the online calculator on this site.
I just compared it with the equations and constants from the pdf of vaisala. If you take 20°C and RH50% then the deriviation of the two is 0,0087°C. Table 1 of the pdf has the constants for the calculation of the saturation vapor pressure. Indeed those values are different from the ones in the online calculator, but very litte. For the comparison I took the average of the value from (-20°C to +50°C) and (0°C to 200°C).
Something I found out when searching the web is that there are no uniform constants for calculation the saturation vapor pressure. But all the constants are very close to each other and base on several organizations doing research on this.
The absolute humidity seems to be right. The only thing is, that from the equation on wetterochs, I couldn’t find the same result, but if you use the calculator the results are nearly the same. Good work on that, @mscherr!
On question I still have: How does the plugin description on the pimatic.org website get updated? Is there something I need to do? I’m trying to get into this github, npm, open-source thing. I never did this before 
@mscherr How did you implement the ventilation suggestion?
@flybysun said:
How does the plugin description on the pimatic.org website get updated
I think this has to be done by the site maintainer on request. I wanted discuss the matter with @sweetpi occasionally as a lot of plugin descriptions appear to be updated these day. So, I believe this should be automatized.
"It always takes longer than you expect, even when you take into account Hofstadter's Law.", Hofstadter's Law
@flybysun said:
Something I found out when searching the web is that there are no uniform constants for calculation the saturation vapor pressure.
Yes, this matches pretty much with my findings (see also #7). May be we can consult a domain expert one day to help us out.
"It always takes longer than you expect, even when you take into account Hofstadter's Law.", Hofstadter's Law
@mwittig said:
Yes, this matches pretty much with my findings (see also #7). May be we can consult a domain expert one day to help us out.
I was thinking of getting into contact with the guy from wetterochs, but he already states on his site, that he can not give support to the formula as he is short in time.
@mwittig said:
I think this has to be done by the site maintainer on request. I wanted discuss the matter with @sweetpi occasionally as a lot of plugin descriptions appear to be updated these day. So, I believe this should be automatized.
This is an great idea!