Hi,
I'm trying to calculate WetBulb Temperature using an Aspirated Temperature Sensor (aka. Dry Bulb Temperature in Celsius), Relative Humidity (%) & Barometric Pressure (hPa).
Has anyone here done this before and know the correct equation to use?
I've spent hours searching for the equation but I could only find ones which don't include Barometric Pressure, which makes them inaccurate above certain temperatures.
If you are using a CR1000, CR3000, or CR800 datalogger, it has a built-in function that includes barometric pressure.
The help also includes the equation used by the function. This is from the help file:
The WetDryBulb instruction uses an equation given by List, Robert. J.: 1966, Smithsonian Meteorological Tables, Sixth Revised Edition, Smithsonian Institution, Washington, D.C., page 365. This equation is also used by the National Weather Service. The results are in kPa.
where:
VPW = Saturation vapor pressure at the web bulb temperature
TW = Wet bulb temperature, degrees C
TA = Ambient air temperature, degrees C
P = Pressure, kilopascals
A0 = 0.000660
A1 = 0.00115
See WebDryBulb instruction.
Dana W.
One of my co-workers pointed out I answered the wrong question :), and he provided the following information:
There is reference to a simpler way of estimating Wetbulb in the article here:
http://stuff.mit.edu/afs/athena/project/weather/INFO/Frequently_Asked_Questions-temp-dewpoint
This does allow you to largely compensate for the affects of atmospheric pressure but it is an approximation.
The specific section is:
Using the methods from Jensen et
al. (1990) ASCE Manual No. 70 (see pages 176 & 177) using the
following steps and equations:
1) compute e as [es(T)*rH/100]
where es(T) = 0.611*EXP(17.27*T/(T+237.3)) in kPa
T is drybulb temp in C
e = (rH/100)* 0.611*EXP(17.27*T/(T+237.3))
where e is ambient vapor pressure in kPa
2) compute dewpoint temperature (Td)
Td = [116.9+237.3ln(e)]/[16.78-ln(e)] in C
3) compute wet bulb temperature (Tw)
Tw = [(GAMMA*T)+(DELTA*Td)]/(GAMMA+DELTA)
GAMMA = 0.00066*P where P is ambient barometric pressure in kPa
DELTA = 4098*e/(Td+237.3)^2
Thanks Dana,
I've put together a basic program to calculate wetbulb temperature with error correction as per the web link you provided.
However I'm not sure on the accuracy of this program. I have found websites that claim to calculate wetbulb and my results seem do differ by <1 Deg Celsius.
Can you see any flaw in my code?
--------------------------------------------------
'Declare Public Variables
Public T 'Drybulb Temperature (DegC)
Public e 'Ambient Vapor Pressure (kPa)
Public rH 'Relative Humidity (%)
Public Td 'Dewpoint Temperature (DegC)
Public P 'Ambient Barometric Pressure (kPa)
Public Confidence 'Error between calculations
Public WetBulb 'Wetbulb Temperature (DegC)
Dim Tw 'First Iteration of Wet Bulb Temperature (DegC)
Dim GAMMA '
Dim DELTA '
Dim calculated_e 'Theoretical vapor pressure (kPa)
Dim EW_TW '
Dim New_TW 'Compensated Wetbulb Temperature (DegC)
'Main Program
BeginProg
T = 25
rH = 50
P = 100
Scan (10,Sec,0,0)
'Calculate ambient vapor pressure (kPa)
VaporPressure (e, T, rH)
'Calculate dewpoint temperature (DegC)
DewPoint (Td, T, rH)
'Calculate wet bulb temperature
GAMMA = 0.00066*P
DELTA = 4098*e/(Td+237.3)^2
Tw = ((GAMMA*T)+(DELTA*Td))/(GAMMA+DELTA)
'Wet Bulb Error Correction
New_Tw = Tw
Confidence = 0
Do While Confidence < 0.99
New_Tw = New_Tw-0.1
EW_TW = 0.611*EXP(17.27*New_Tw/(New_Tw+237.3))
calculated_e = EW_TW - GAMMA * (T-New_Tw)
Confidence = e/calculated_e
Loop
If Confidence > 1.01 Then
Do While Confidence > 1.01
New_Tw = New_Tw+0.1
EW_TW = 0.611*EXP(17.27*New_Tw/(New_Tw+237.3))
calculated_e = EW_TW - GAMMA * (T-New_Tw)
Confidence = e/calculated_e
Loop
EndIf
WetBulb = New_Tw
NextScan
EndProg
--------------------------------------------------
Thanks
CJ
* Last updated by: CJ on 1/8/2013 @ 4:47 PM *
i have a slightly unrelated question... sorry because i am writing in this topic...
i am searching for an equation for calculating entropy of moist air? anyone know something useful?
Sorry krneki I don't know the answer to that one.
Just an update on the above program though, it does seem to work and is within +/- 1 degree celsius of a few online calculators I found... however I'm a bit dubious to say the online calculator are scientifically accurate.