SHT_75 with picbasic pro
Good day to all of you !
I am going to interface the temperature _ humidity sensor type SHT_75 fron Sensirion to the Pic micro.
I will like to know if there is a pic basic pro code to read this sensor and the routines to compute:
> temperature in C° having + and - signs
> temperature compensated RH ( + lin too !) in %
> dew point in C° having + and - signs
Thanks in advance for any assistance.
Best regards,
Ambrogio
|
Never used on, but a quick lppk at the data sheet makes me think I2C or SHIFTIN might work.
Here is a little about both for working with an EEPROM, maybe it will get you started?
http://www.picbasic.co.uk/forum/cont...-EEPROM-Part-1
Dave
Always wear safety glasses while programming.
|
SHT_75 with picbasic pro
Good day to all of you ,
I am going to interface the Sensirion SHT_75 Temperature_Humidity sensor to my pic micro.
I would like to know how to compute the RH termally compensate and how to compute the dew point. I rerad Srh @12 bit and St @14 bit.
I expect that the RHtc will range from 0 to 99% RH
The dew point temperature in C° will have the sign ( + or - ).
I thank you in advance for any assistance on the coding .
regards,
Ambrogio
IW2FVO
|
A quick google search got me to here for some basic stamp code. It may help. You could also search sht75 on this site for some more hits.
http://www.emesystems.com/OL2sht1x.htm
Please don't double post. I moved your post to this thread, even though it is pretty much the same as the first post.
</pre>Code:'{$STAMP BS2} ' SHT15.bs2 version C ' (c) 2002, 2004 Tracy Allen, http://www.emesystems.com ' access the Sensirion model sht11 or sht15 ' humidity and temperature chip ' temperature to xx.x degrees Celsius, RH to xx.x % ' Repeatedly shows raw data and converted values on debug screen. ' version B, corrects glaring error in humidity computation ' also adjusts timeout to allow use with faster stamps too. ' version C updates the math for temperature compensation of %RH ' and enables display of negative temperatures ' hookup sht11 or sht15 as follows for this program ' ' STAMP SENSIRION ' 220 ' p1 ----o-----/\/\---o--pin 3 sck, clock ' | ' ;---/\/\-----' pull-down ' | 4.7k ' Vss--o-o---------------pin 1 common ' | ' === 0.1uf ' | ' Vdd--o-o---------------pin 4 +5 volts ' | 4.7k ' '---/\/\-----; pull-up ' | ' 220 | ' p0 ----------/\/\---o--pin 2 dta, data ' ' ' The following code does not implement the CRC checking sck PIN 1 dta PIN 0 ' note, 5k-10k pull-up, also 330ohm between the dta on stamp to dta on sht dtain var in0 shtTR CON 3 ' read temperature shtRH CON 5 ' read humidity shtSW CON 6 ' status register write shtSR CON 7 ' status register read shtS0 CON 30 ' restore status register defaults (be sure to delay 11 milliseconds) cmd VAR Byte result VAR Word ' raw result from sht, also used as counter r0 VAR result.byte0 r1 VAR result.byte1 degC VAR Word ' degrees Celsius * 100 RH VAR Word ' %RH * 10 RHtc VAR Word ' for temperature compensation of RH initialize: outs=0 dirs=%1111111111111101 GOSUB shtrst ' reset communication with sht DO getTemperature: cmd=shtTR ' temperature command to sht GOSUB shtget16 degC=result+5/10-400 ' from 100ths to 10ths of a degree with rounding DEBUG tab,REP "-"\degC.bit15,DEC ABS degC/10,".",DEC1 ABS degC getHumidity: cmd=shtRH ' humidity command to sht GOSUB shtget16 RH=(26542-(54722**result+result))**result-40 ' temperature compensation follows: RHtc=655+(result*5)+(result**15917) ' intermediate factor RHtc=(RHtc**(degC+2480))-(RHtc**2730)+RH ' compensated value DEBUG tab, DEC result,tab,"%RH=",DEC RH/10,".",DEC1 RH DEBUG tab,"%RHtc=",DEC RHtc/10,".",DEC1 RHtc,cr PAUSE 1000 LOOP ' initializes communication with sht shtRst: SHIFTOUT dta,sck,lsbfirst,[$ffff\16] RETURN ' get 16 bits of data, enter with command in "cmd" shtget16: gosub shtcmd ' send the command "cmd" gosub shtwait ' wait for command to finish shiftin dta,sck,msbpre,[r1] ' msbyte low dta ' acknowledge pulsout sck,10 input dta shiftin dta,sck,msbpre,[r0] ' lsbyte input dta ' terminate communication pulsout sck,10 return ' send start sequence and command shtcmd: shtStart: ' send the start sequence ' dta: ~~~~~|_____|~~~~~~ ' sck: ___|~~~|_|~~~~|____ ' while dta is low, clock goes low and then high input dta ' pullup high high sck low dta low sck high sck input dta low sck shtcmd1: ' send the command shiftout dta,sck,msbfirst,[cmd] input dta ' allow acknowledge pulsout sck,10 return shtWait: ' wait for sht to pull data pin low ' or for time out result=4096 DO result=result-1 LOOP WHILE dta & result.bit11 RETURN
Last edited by ScaleRobotics; - 28th November 2010 at 15:02.
http://www.scalerobotics.com
|
SHT_75 with picbasic pro
Thanks mackrackit:
I do not have any problem to talk with the SHT_75.
I am able to get signed temperature tto.
My problem is to compute RHtc with picbasic pro and to compute DEW POINT with its sign.
Thanks for help
Ambrogio
IW2FVO
Originally Posted by mackrackit
|
SHT_75 with picbasic pro
Thanks very much for the code.
I am searching now for the dew point computation in picbasic pro !
Thanks a lot for the help
Ambrogio
IW2FVO
|
Here is some code for the SHT11 for calculating dewpoint. I have not compared datasheets to know how similar (or not) they are. But this may help:
http://www.picbasic.co.uk/forum/show...=3851#post3851
That is quite the equation! From NavMicroSystems post above:
And attached is an application note for dew point calculations.Code:'** Dewpoint Calculation ' See: SENSIRION Application Note "Dewpoint Calculation" ' logEW = (0.66077+7.5*T/(237.3+T)+(log(RH)-2) ' DP = ((0.66077-logEW)*237.3)/(logEW-8.16077) DewPoint: TempDP=Temp/10 logEW=6608 sign=TempDP.bit15 wz=ABS TempDP wx=(7*wz)+(wz/2) wy=2373+wz wj=wx/wy For ix=15 to 0 step -1 wx=(wx//wy)<<1 wz.bit0(ix)=wx/wy Next wx=(-sign^((wj*10000)+(wz**10000)))+sign logEW=wx+logEW wx=RHtc wj=(NCD wx) - 1 wx=wx<<(15-wj) wz=0 For ix=14 to 0 step -1 wy=wx**wx wz.bit0(ix)=wy.bit15 bt=~wy.bit15 wx=(wy< Next wx=((wj*4000)**49321)+(wz**6021) logEW=wx+logEW-30000 sign=logEW.bit15 logEW=(-sign^(((ABS logEW)+2)/4))+sign wx=1652-logEW sign=~wx.bit15 wx=ABS wx wy=20402-logEW wj=wx/wy For ix=15 to 0 step -1 wx=(wx//wy)<<1 wz.bit0(ix)=wx/wy Next DP=((23730**wz)+5)/10 DP=(-sign^DP)+sign
Last edited by ScaleRobotics; - 29th November 2010 at 15:21.
http://www.scalerobotics.com
|
sht_75 with picbasic pro
Hi scalerobotics,
yes, the sht_11 has the same interface characteristics of the sht_75 so the code would be OK,
Thanks for helping me,
Regards,
Ambrogio
IW2FVO
|
BS2 sht15.bs2 version C
Good day to all in the forum.
I am using the sht15.bs2 program together with the sht_75 sensor.
I found that the temperature compensated relative humidity RHtc is not correct for temperature below 0 degC.
I think that there is a problem in the code related to the use of the degC variable;
This variable becomes negative if the temperature result is less than 4000 ( this is equivalent to 0degC ): in the RHtc computation this negative value could generate problem...
I will appreciate very much any help or comment .
Thanks
Ambrogio
Iw2fvo
|
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
|
SHT_75 with picbasic pro
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, 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
|
SHT_75 with picbasic pro
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 , 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,
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, 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
|
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
|
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
|
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]
|
SHT_75 with picbasic pro
Hi to all in the forum !
Is there any additional comment on this subject ?
Thanks
Ambrogio
IW2FVO
[/QUOTE]
Members who have read this thread : 1You do not have permission to view the list of names.
Posting Permissions
Bookmarks