IN-E-HyFlex-V4_10
Rotronic AG
Bassersdorf, Switzerland
Document code
Unit
Instruction Manual
Document Type
HygroFlex transmitter version 4: instruction
manual
Document title
Page
52
of 61
© 2006; Rotronic AG IN-E-HyFlex-V2_10.doc
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
Both the HygroFlex 2 and the HygroFlex 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
The accuracy of the dew point conversion is less than the accuracy that is normally achieved with a
chilled mirror instrument. This is especially true at low dew point values (dew point depression of
more than -40…-50
°
C) and at low temperatures. In many applications, repeatability is more critical
than accuracy. Repeatability of the dew point conversion is typically 1/3 of the accuracy shown
above.
Using a dew point conversion permits to measure conditions that cannot be measured with a
chilled mirror instrument. Typically, the measuring head of a chilled mirror instrument is limited to a
maximum of 70
°
C and, therefore, cannot measure a dew point temperature above 70
°
C. In
addition to the ability to operate at high temperature, the relative humidity probe offers significant
advantages for industrial applications: less maintenance and higher tolerance to contaminants, no
sampling system, no uncertainty between dew and frost and better response to fast changing
conditions.
