Section 3
Operation
3-11
Sample Calculations
Example 1. Assume you wish to calibrate a relative humidity sensor in a
leaf chamber where the air temperature is 23.0 °C. The LI-610 dew
point temperature is to be set so that the chamber relative humidity will
be 40%. We shall neglect flow-dependent pressure changes, for the
moment. The steps to solution are: (1) compute the saturation vapor
pressure at 23.0 °C using equation 3-4; (2) solve equation 3-7 for the
required chamber vapor pressure; and (3), solve equation 3-4 for the
required dew point temperature setpoint for the LI-610.
1. The saturation vapor pressure at 23 °C is
e(23 )
0.61365e
kPa
17.502 23
240.97 23
° =
=
×
+
2 820
.
2. The chamber vapor pressure must be
40%
e
2.82 kPa
100%
=
×
e = 1.128 kPa
3. The LI-610 setpoint temperature necessary to give a vapor pressure of
1.128 kPa is equivalent to the dew point temperature corresponding to
a vapor pressure of 1.128 kPa. Solving equation 3-4 for T,
T
240.97z
17.502
z
, where z = ln
e
0.61365
=
−
z
ln
1.128
0.61365
0.6088
=
=
T = 8.7 °C, the LI-610 setpoint.
The psychrometric chart in Figure 3-2 can also be used to find an
approximate solution. This is especially valuable when trying to quickly
find the LI-610 dew point temperature necessary to give a desired
incoming humidity while making photosynthesis measurements in the
field. With reference to Figure 3-2, find the chamber or device
temperature on the x-axis, read up to the desired humidity line and note
the vapor pressure on the y-axis. Then, at constant vapor pressure, read
across the chart to the left until the 100% RH curve is reached. Finally,
