SHT_75 with picbasic pro

[iw2fvo]

Hi Dave,

Thanks a lot for your reply on the matter.
I will try to better clarify the problem that I have using the SHT15.bs2 ver.C program from Tracy Allen ( emesystems).

Point _1.
The program reads the raw temperature and store it on the " result " word variable.
The temperature is obtained using the following : degC=result+5/10-400.
This means that if the result is 5000 we have +10°C, if the result is 4000 temp is 0°C
and if 3000 the temp is -10°C. I modified the program in order to "display" the minus sign
when result is less then 4000. Result is the 14 bit raw temperature delivered by the sensor.
Up to here everythings is correct and my system shows the correct temperature. I did some comparison with the airport control tower close to my home : it is consistent.

Point _2
the program then reads the raw RH (12 bit ) and stores that value on the "result" word variable.
RH is then obtained as follows: RH=(26542-(54772**RESULT+RESULT))**RESULT-40.
At this point everything is ok to me : my PIC micro gives me the correct RH value.

Point_3
The program computes now the temperature compensated RH in two steps:
First step > RHtc=655+(RESULT*5)+(RESULT**15917)
Second Step > RHtc=(RHtc**(degC+2480))-(RHtc**2730)+RH
>>> At this point the RHtc is correct only if the temperature is above 0°C or, if the raw temperature data from the sensor is more than 4000 !.
If the temperature is negative e.g. -1.7 °C the RHtc is not correct. It is about 172%.
The control tower gives me T=-1.7°C , rh=85% and Dew point of -3.9°C.
>>> This is my problem.

I will appreciate very much if someone can give me an help to solve the troble.
May be that some other programmer ( I am not a programmer ! ) have already solved it !
Thanks in advance

Regards,
73!
Ambrogio
IW2FVO

P.s.: English is not my first language and I am not sure to have express my problem correctly: I am Italian !

iw2fvo , I believe that would be 0% RH. (freezing) Check out this site for more info...
http://www.techtoys.com.hk/8051/AT89...SHTxx_demo.pdf

Dave Purola,
N8NTA

[Dave]

iw2fvo, It is not possible to have this sensor or that equation to give you acurate results below freezing. Please read this artical: http://www.rhsystems.net/papers/RH_WMO.pdf. I have used a chilled mirror in the past for this type of measurement sampling air from a test chanber at - 20 F. However the type of sensor you are using is limited as far as humidity measurements and that is why the formula is not acurate below 0C.

Dave Purola,
N8NTA

[iw2fvo]

Dave,
thanks a lot for the document you have indicated to me.
I will read it carefully.
I noted that all the weather station here on the web and the control towers broadcast RH even if the temperature is below 0°C. All of them diclare a consistent RH. At this point I do not understand well. I must study a little bit more.
At the same time I am notice that my Oregon weather station gives a correct RH for negative temperature too.
The SHT15.BS2 program has a bug somewhere : I would like to know if someone solved it !
Please let me know... just in case you know a fix.
Thanks again Dave for the help.
73 !
Ambrogio
IW2FVO
North Italy

iw2fvo, It is not possible to have this sensor or that equation to give you acurate results below freezing. Please read this artical: http://www.rhsystems.net/papers/RH_WMO.pdf. I have used a chilled mirror in the past for this type of measurement sampling air from a test chanber at - 20 F. However the type of sensor you are using is limited as far as humidity measurements and that is why the formula is not acurate below 0C.

Dave Purola,
N8NTA

[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