SHT_75 with picbasic pro

[Dave]

iw2fvo , I have no idea where you got the calculation you are using but the calculation is different for relative humidity ABOVE ICE.. The calculation you are using is for ABOVE WATER. Please read this artical from the people that make the sensor you are trying to use....

http://www.sensirion.com/en/pdf/prod...Humidity_E.pdf

Dave Purola,
N8NTA

[iw2fvo]

dave,
thanks for the documentation : I red this documents but I still do not undrstand the reason for which rh is normally distribuited even under 0 degC.
Please see this US weather condition in Alaska:
Anchorage, Alaska, United States

* Weather overview
* Two-week forecast
* Hour-by-hour
* Past week
* °F
* °C

Current Time marted 30 novembre 2010, 12.29.06 AKST
Current conditions
Sunny. Cold. -6 °C
Sunny. Cold.
Change to Fahrenheit
Sunny. Cold.
Location: Merrill Field
Temperature: -6 °C
Comfort level: -8 °C
Dew point: -15 °C
Barometer: 1005 millibars
Humidity: 48%
Visibility: 16 km
Wind: 6 km/h from 300° West-northwestDirection East-southeast
Last update: mar 11.53 AKST

They are saying -6°C 48%rh and -15°C dew point.

Could you please spend few more words in order for me to understand this point ?
Thanks
Regards,
Ambrogio
IW2FVO
73!

iw2fvo , I have no idea where you got the calculation you are using but the calculation is different for relative humidity ABOVE ICE.. The calculation you are using is for ABOVE WATER. Please read this artical from the people that make the sensor you are trying to use....

http://www.sensirion.com/en/pdf/prod...Humidity_E.pdf

Dave Purola,
N8NTA

[Dave]

iw2fvo, As long as the atmospheric temperature is above the dew point one, moisture will stay in a gaseous form. But there is more to it in the world of thermodynamics. First, water can be found in a gaseous, liquid or solid form. When it is still gaseous in an environment where it should be liquid, it is then called vapour, and when it is still liquid where it should be solid, it is called under-cooled. But why is that? Well, first we must differentiate between absolute humidity and relative humidity. Water molecules tend to mix with air molecules all the time; even ice evaporates as gas. This is called sublimation. Absolute humidity is the volume of water that is contained in a volume of air. The higher the temperature, the greater the volume of water it can contain. For example, at the average temperature at sea level and average pressure, of 15 C, a cubic meter of air can't contain more than 13 grams of water. When that happens, the air is saturated with moisture and the relative humidity is 100 percent. When the temperature goes down to 0 C (freezing point) the air can sustain only 5 gr/m3. The absolute humidity is then 5 gr/m3 but the relative one is still 100 percent because at saturation point, a.k.a. dew point temperature. What happens when the temperature sinks below dew point? Well, the parcel of air has to get rid of some of its moisture by condensing it into tiny water droplets. But that can only happen by giving away energy in form of heat and the tiny molecules have virtually no mass to dissipate that heat energy. They have to touch anything like a dust, smoke particle, pollen or the bonnet of your car, to condense. Likewise, it must give heat energy to go from liquid to solid and that's why you can get tiny drops of water, in the clouds, that are under-cooled all the way down to -40 C! The type of sensor you are using does not have the capability of measuring the Dew Point below 0C. therefor the calculation you are using is not valid below 0C.....

Dave Purola,
N8NTA

[iw2fvo]

Hi Dave,
I red your explanation carefully: it is very clear to me now. I did not get those points before.
I thank you very much for your help on the matter.
Bye
73!
Ambrogio
IW2FVO
North italy

iw2fvo, As long as the atmospheric temperature is above the dew point one, moisture will stay in a gaseous form. But there is more to it in the world of thermodynamics. First, water can be found in a gaseous, liquid or solid form. When it is still gaseous in an environment where it should be liquid, it is then called vapour, and when it is still liquid where it should be solid, it is called under-cooled. But why is that? Well, first we must differentiate between absolute humidity and relative humidity. Water molecules tend to mix with air molecules all the time; even ice evaporates as gas. This is called sublimation. Absolute humidity is the volume of water that is contained in a volume of air. The higher the temperature, the greater the volume of water it can contain. For example, at the average temperature at sea level and average pressure, of 15 C, a cubic meter of air can't contain more than 13 grams of water. When that happens, the air is saturated with moisture and the relative humidity is 100 percent. When the temperature goes down to 0 C (freezing point) the air can sustain only 5 gr/m3. The absolute humidity is then 5 gr/m3 but the relative one is still 100 percent because at saturation point, a.k.a. dew point temperature. What happens when the temperature sinks below dew point? Well, the parcel of air has to get rid of some of its moisture by condensing it into tiny water droplets. But that can only happen by giving away energy in form of heat and the tiny molecules have virtually no mass to dissipate that heat energy. They have to touch anything like a dust, smoke particle, pollen or the bonnet of your car, to condense. Likewise, it must give heat energy to go from liquid to solid and that's why you can get tiny drops of water, in the clouds, that are under-cooled all the way down to -40 C! The type of sensor you are using does not have the capability of measuring the Dew Point below 0C. therefor the calculation you are using is not valid below 0C.....

Dave Purola,
N8NTA

[Dave]

Ambrogio, Thats OK, as I am no expert myself... I personally don't bother with the humidity information here in Michigan when the temp's are below freezing.. Only the wind chill factor... There is a lot of information on Dew Points and Humidity on the web. I only hope I was able to help out... 73's for now...

Dave Purola,
N8NTA

Hi Dave,
I red your explanation carefully: it is very clear to me now. I did not get those points before.
I thank you very much for your help on the matter.
Bye
73!
Ambrogio
IW2FVO
North italy

[iw2fvo]

Another item for the RH sensor...
This is the explanation provided by the sensor manufacturer ( Sensirion ):

Quote:

Thank you for your e-mail. The SHT75 can measure RH below 0°C. However as cold air below 0°C contains only a small amount of water molecules it is very difficult to measure them accurately. Therefore tolerances increase.

Hope this answers your question.

Best regards

Reto Kleiner
_____________________________________

Reto Kleiner
Technical Customer Support

SENSIRION – the sensor company
Laubisruetistrasse 50
CH-8712 Staefa ZH
Switzerland

phone: +41 44 306 40 00
direct: +41 44 927 11 44
fax: +41 44 306 40 30
mailto:[email protected]
www.sensirion.com

Von: Ambro_IW2FVO [mailto:[email protected]]
Gesendet: Samstag, 4. Dezember 2010 15:27
An: Info
Betreff: sht_75

unquote .




QUOTE=Dave;96720]Ambrogio, Thats OK, as I am no expert myself... I personally don't bother with the humidity information here in Michigan when the temp's are below freezing.. Only the wind chill factor... There is a lot of information on Dew Points and Humidity on the web. I only hope I was able to help out... 73's for now...

Dave Purola,
N8NTA[/QUOTE]

[iw2fvo]

Hi to all in the forum !

Is there any additional comment on this subject ?

Thanks

Ambrogio
IW2FVO

Another item for the RH sensor...
This is the explanation provided by the sensor manufacturer ( Sensirion ):

Quote:

Thank you for your e-mail. The SHT75 can measure RH below 0°C. However as cold air below 0°C contains only a small amount of water molecules it is very difficult to measure them accurately. Therefore tolerances increase.

Hope this answers your question.

Best regards

Reto Kleiner
_____________________________________

Reto Kleiner
Technical Customer Support

SENSIRION – the sensor company
Laubisruetistrasse 50
CH-8712 Staefa ZH
Switzerland

phone: +41 44 306 40 00
direct: +41 44 927 11 44
fax: +41 44 306 40 30
mailto:[email protected]
www.sensirion.com

Von: Ambro_IW2FVO [mailto:[email protected]]
Gesendet: Samstag, 4. Dezember 2010 15:27
An: Info
Betreff: sht_75

unquote .




QUOTE=Dave;96720]Ambrogio, Thats OK, as I am no expert myself... I personally don't bother with the humidity information here in Michigan when the temp's are below freezing.. Only the wind chill factor... There is a lot of information on Dew Points and Humidity on the web. I only hope I was able to help out... 73's for now...

Dave Purola,
N8NTA

[/QUOTE]