ROTRONIC HygroLab - Manual (Page 36)

IN-E-HyLab-V4_11
Rotronic AG
Bassersdorf, Switzerland
Document code Unit
Instruction Manual
Document Type
HygroLab bench top indicator version 4:
instruction manual
Document title
Page 36 of 51
© 2006; Rotronic AG IN-E-HyLab-V4_11
Enthalpy
The enthalpy (or energy content) of moist air at pressure P
b
, temperature t (°C) and mixing ratio r
(g/kg) is defined by:
h [kJ / kg moist] = 1.00464 t + 0.001846 r x t + 2.5 r
Note: by convention, the enthalpy of dry air (r = 0) at 0°C is equal to zero. Negative values of
enthalpy are possible and indicate that the energy content of the air / vapor mixture is less than the
energy content of dry air at 0°C
1 lb = 0.4536 kg
1 BTU = 1.05507 kJ
h [BTU / lb] = 0.4299 x h [kJ / kg] + 7.68
The value 7.68 is added to make the temperature of 0°F the reference for enthalpy expressed in
BTU / lb
15 Dew point accuracy
The HygroLab 2 and 3 use the relative humidity and temperature measurements to compute
another parameter such as dew point, mixing ratio, enthalpy, etc. The accuracy of this conversion
varies, depending on the humidity and temperature conditions. Typical accuracy for the dew point
conversion is provided in the graph below:
Dew Point Accuracy
-60
-50
-40
-30
-20
-10
0
-40 -20 0 20 40 60 80 100 120
Temperature °C
D
e
w Point
D
e
pression °C
±1 °C
±1.5 °C
±2 °C
±2.5 °C