Campbell Scientific BlackGlobe Temperature Sensor for Heat Stress - Manual (Page 15)

BlackGlobe Temperature Sensor for Heat Stress
The equation is an inverse of a version of Teten’s equation (Tetens, 1930),
optimized for dewpoints in the range –35° to 50°C, and is accurate to within
plus or minus 0.1°C within that range.
7.2.3 Vapor Pressure
Vapor pressure is calculated by the datalogger using Equation 3.
P = RH*P
sw
/100 (3)
where
RH = relative humidity (%)
P
sw
= saturation vapor pressure (kPa) over water
7.2.4 Saturated Vapor Pressure
Saturation vapor pressure over water is calculated by the datalogger using
Equation 4.
P
sw
(kPa) = 0.1*(6.107799961 + T(4.436518521 × 10 –1 + T(1.428945805 ×
10 –2 + T(2.650648471 × 10 –4 + T(3.031240396 × 10 -6 + T(2.034080948 Ч 10 –8
+ 6.136820929 Ч 10 –11 Ч T)))))) (4)
where
T = air temperature (dry-bulb temperature) (°C)
7.2.5 Wet-Bulb
Wet-bulb is derived using an iterative process. The wet-bulb temperature lies
somewhere between the dry-bulb temperature (air temperature) and the
dewpoint temperature. The datalogger uses Equation 5 to calculate vapor
pressure using the dry-bulb temperature and a wet-bulb temperature estimate:
P = P
w
–(0.000660*(1+0.00115*T
w
)*(T–T
w
)*SP) (5)
where
P
w
= saturation vapor pressure (kPa) at the wet-bulb temperature (°C)
T
w
= wet-bulb temperature (°C)
T = air temperature (dry-bulb temperature) (°C)
SP = standard air pressure (kPa) at the user entered elevation
The resulting vapor pressure is compared to the true vapor pressure (see above)
and the difference determines the next wet-bulb temperature estimate. The
process repeats until the difference between the current wet-bulb temperature
estimate and the previous wet-bulb temperature estimate is only plus or minus
0.01°C. The datalogger thus derives the wet-bulb temperature.
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