Wet Bulb Temperature

Issue

How to calculate wet bulb temperature from a temperature sensor and humidity sensor

Product Line

Satchwell Sigma

Environment

Sigma

BAS

Cause

How to calculate wet bulb temperature from a temperature sensor and humidity sensor?

Resolution

For an exact answer, the dew point and barometric pressure are also required.

However a reasonably accurate answer can be derived from a temperature sensor and humidity sensor, as follows:

In the 1st example, the pressure can either be taken as an input, or a constant can be used. the constant will normally be derived locally or the 1006mBars can be used. 

Example 1.

tw = t * (0.45 + 0.006 * Rh * sqr(p/1060))

Where

tw = wet bulb temperature (Celsius degree)
t = dry bulb temperature (Celsius degree)
Rh = relative humidity (%)

The BAS or Sigma programmable object would be:

10 XFlo = 0.000000
20 YFlo = 0.000000
30 XFlo = XFlo + ( SQRT ( 1006.00 / 1060.00 ) )
40 YFlo = YFlo + ( 0.00600000 * POINT 0|50 * XFlo )
50 YFlo = 0.450000 + YFlo
60 YFlo = POINT 0|49 * YFlo
70 RETURN VIA TEXT 101 VALUE YFlo

P0|49 is the temperature sensor (Celsius degree)
P0|50 is the humidity sensor
1006 mbars sample pressure which is the standard used in the formula below. This can be substituted for either a sensor or another reading.

Example 2.

In the range Tdry bulb = 15..40[deg C] and rh = 10...90[%] the Twet bulb[degC] can be calculated at P=1006[mbar] with max relative error ~ 6% . For lower Tdry and rh the relative error will be higher.

Twet bulb= (A * Tdry) + (B * rh) + (C * Tdry^2) + (D * rh^2) + (E * Tdry * rh) + F

where

A=5.391260E-01
B=1.047837E-01
C=-7.493556E-04
D=-1.077432E-03
E=6.414631E-03
F=-5.151526E+00
^2=squared

The BAS or Sigma programmable object would be:

10 XFlo = 0.000000
20 XFlo = XFlo + ( 0.539126 * POINT 0|49 )
30 XFlo = XFlo + ( 0.104784 * POINT 0|50 )
40 XFlo = XFlo + ( - 0.000749356 * POINT 0|49 * POINT 0|49 )
50 XFlo = XFlo + ( - 0.00107743 * POINT 0|50 * POINT 0|50 )
60 XFlo = XFlo + ( 0.00641463 * POINT 0|49 * POINT 0|50 )
70 XFlo = XFlo + - 5.15153
80 RETURN VIA TEXT 101 VALUE XFlo

P0|49 is the temperature sensor (Celsius degree)
P0|50 is the humidity sensor