Campbell Q-7.1, Instruction Manual

Page: 5 / 12

Q-7.1 NET RADIOMETER
3
FIGURE 2. Correction Functions as a Function of Wind Speed
The inverse of this data gives the required
correction factors to give the correct reading as
a function of wind speed (u). Figure 2 shows
the manufacturer’s data plus correction curves
fitted by Campbell Scientific, using functions
which can be easily encoded in a datalogger.
Note that the functions fitted and shown in both
graphs have been amended slightly to allow for
a small under-estimate of the error (about 2%
of the error) which is the result of a known error
of the calibration technique in the wind tunnel.
The correction function for positive fluxes is:
Correction factor = 1 +
( )
( )
( . . )
. .
0 066 0 2
0 066 0 2
× ×
+ ×
u
u
For negative fluxes a linear function is used:
Correction factor =
( )
0 00174 .
×
u + 0.99755
If the wind speed is not measured it is often
sufficient to correct the original calibration
constant using a fixed correction factor. The
manufacturer suggests using fixed correction
factors of 1.045 for positive fluxes and 1.000
for negative fluxes.
If the wind speed is measured, the functions
above can be programmed into the datalogger
to give a real-time correction of net radiation
for the effects of wind speed. Note that the
wind speed is expressed in units of meters per
second. If the wind speed is measured in units
other than meters per second, convert the wind
speed to meters per second before inputting it
into the above correction functions.
If AC power is available, a special ventilator
(RV2) is available for the Q-7.1 which gives a
typical wind speed of 3.3 ms
-1
. Positive and
negative fixed corrections are 1.06 and 1.0033
respectively. The fan is also useful for
reducing the risk of external condensation
(dew) on the windshields.
The above corrections were calculated at sea
level under conditions close to standard
temperature and pressure. The magnitude of
the wind speed error will vary in relation to the
air density and the heat capacity of air. These
changes are generally small enough to be
ignored unless extreme temperatures or
atmospheric pressures are experienced.