Casella CEL Software for Heat Stress Monitor HSM100 - Manual (Page 14)

HEAT STRESS MONITOR
Handbook No. HB3215-05
Page 14 of 16
APPENDIX A - FORMULAE.
The following formulae are used to derive Humidity and Dew point values based upon measured wet and
dry temperatures (Tw and Td).
The saturated vapour pressures are calculated using the MAGNUS formula (Ref. BS1339:1981-
Definitions, formulae and constants relating to the humidity of air)
Where:
ew is the saturation vapour pressure of the air (hPa).
Tw is the air temperature (
°
C).
The remaining constants can have different values depending on the temperature of the wet sensor. If the
sensor is below 0
°
C then it is deemed to be 'Over Ice' else 'Over Water'. The following constants are
automatically selected by the program during operation:
Over Water Over Ice
G 7.5 9.5
H 237.3 265.5
I 0.78571 0.78571
The actual Vapour pressure (Regnault and August and Apjohn) is calculated as;-
e = e - A P(td - tw)
s ( w )
×
Where es(w) is the saturated vapour pressure based on the wet bulb temperature(calculated using the
Magnus formular), 'A' is the Ventilation constant and is chosen to suit the instrument type and ventilation
rate of the wet sensor. A=7.99x10-4 is appropriate to a ventilation rate of 1-1.5m/S assuming the wet bulb
>0°.(Ref: MET.O.265b Hygrometric Tables)
'P' is the air pressure measured in mBar, usually this is considered to be a mean value of 1000mB.
Humidity is calculated according to;-
Where 'e' is the vapour pressure and 'es(t)' is the saturated vapour pressure calculated using the Magnus
equation for the dry bulb temperature.
Dew Point;
The Magnus formula is used to evaluate the dew point temperature assuming a saturated vapour pressure
equal to the actual vapour pressure;-
Dew Pt C =
[Log10 (Vapour) 237.3] - 186.44898
[8.2857 - Log10 (Vapour)]
°
×
WBGT CALCULATION
log e =
GT
H + T
+ I
10 w
w
w
RH% =
e
e
100
s ( t )
×
log e =
GT
H + T
+ I
10 w
dew
dew