calculating Dewpoint

[cadillackid]

I am working on part of a script to calculate the dewpoint, both outside and inside the house. the formula is rather complex and always seems to return back a value much lower than say an online dewpoint calculater etc..

does VBscript have trouble handling exponents or complex numbers? ive tried relocating the paranthesis to see if I could change the order of manipulation but still cant get a good reading..

I use this as a means to determine whether my outside air intake is open or closed and also to determine the blower speed control on my air-conditioner. on super humid days (dewpoint 69 and above) its better to allow longer cooling cycles, move less air across the coil making the air colder as it will pull more humidity.. I attached the script below possibly someonecan help me out.

grabbing dewpoint from an XML feed is not an option here as my house typically has different humidity and temp readings than the NWS site.. plus I cant monitor interior dewpoints through that method.
-christopher

sub main() 
'***************************************
'* Script to Calculate Indoor Dewpoint *
'***************************************
dim tempf
dim tempc
dim rh
dim dewpointc
dim tdewpointc
dim dewpointf
dim x
tempf=cint(hs.devicevalue("^11"))    'Return air duct temp bobcat
msgbox tempf
tempc=cint((5/9)*(tempf-32))
rh=cint(hs.devicevalue("^8"))    'relative humidity from bobcat
msgbox rh
es=6.11*10^(7.5*tempc/(237.7+tempc))
E=(rh*Es)/100
tdewpointc=(-430.22+237.7*log(E))/(-log(E)+19.08)
dewpointf=cint((9/5)*tdewpointc+32)
msgbox dewpointf
end sub 

[jasv]

Not a vbscript expert but...

I am not a vbscript expert but wouldin't using the cint function cause you problems since the integer itself doesn't allow decimal places? Would using the CDbl function instead give you more accuracy?

[Michael McSharry]

prefix all your variables in the expressions with csng to assure the calculation is performed in floating point. For examle:

E = csng(rh) * csng(es) / 100

[olof]

I’m looking for a plug-in, script or event calculation to determine indoor dew point and write it to a virtual device. Does anybody has a solution?

[bandres]

Here is the .vb script I use. When run it takes the temp and humidity from from my attic multi sensor and writes the dew point to a status only device I created. Then I can use that in events to do other stuff. There is another post somewhere with dew point scripts and formulas but those all gave me erroneous results. It's been awhile since you posted so hopefully this is useful to somebody.

Sub Main(parm as object)

DIM AtticT As Integer
DIM AtticTC as Integer
DIM AtticH As Integer
DIM AtticDC as integer
DIM AtticD as integer

AtticH=hs.DeviceValue(4253)
AtticT=hs.DeviceValue(4251)

AtticTC=(AtticT-32)/(1.8) 'convert to celsius

AtticDC = (243.04 * ((math.log(AtticH/100)) + (17.625 * AtticTC)/(243.04 + AtticTC))/(17.625 - (math.log(AtticH/100))-((17.625*AtticTC)/(243.04 + AtticTC))))

AtticD=((AtticDC * 1.8)+32) 'convert to fahrenheit

hs.writeLog("msg","Calculating Dew point")
hs.writeLog("msg",AtticT & " Attic temp")
hs.writeLog("msg",AtticH & " Attic humidity")
hs.writelog("msg",AtticTC & " Attic Temp in Celsius")
hs.writelog("msg",AtticDC & " dew point in Celsius")
hs.writelog("msg",AtticD & " dew point")
hs.setDeviceValueByRef(4230, AtticD, true)

End Sub