Campbell Scientific Q7_1-L REBS Net Radiometer - Manual (Page 4)

Q-7.1 NET RADIOMETER
2
2.2 WIRING
If a differential measurement is made, connect
the red (+) lead to the high side (e.g. 1H) of any
datalogger differential channel and the black (-)
lead to the low side (e.g. 1L). Also, connect a
jumper wire between the low channel and
analog ground to prevent common mode
errors. Connect the clear (shield) lead to
ground (G on the CR10(X), ground on the 21X
and CR7).
If a single-ended measurement is made,
connect the red lead to any datalogger single-
ended channel (H or L) and the black lead to
ground (AG on the CR10(X), ground on the 21X
and CR7).
The black lead is negative with respect to the
red lead when the net radiometer is mounted
with the level up and there is more incoming
radiation than outgoing.
3. PROGRAMMING
Measure the output of the Q-7.1 with either
Instruction 2 (Differential Volts) or Instruction 1
(Single-Ended Volts). Use the 250 mV range
for the CR10(X) and the 500 mV range for the
21X or CR7. The slow integration with 60 Hz
rejection yields a more noise-free reading.
For still air, net radiation (Q*) can be computed
from the thermopile voltage (V
t ) by:
If V t > 0, Q*(Wm -2 ) = V t (mV) x F p
If V
t
< 0, Q*(Wm -2 ) = V
t
(mV) x F
n
where F p and F n are the positive and negative
calibration factors respectively.
The calibration factors and serial number are
given on a label under the sensor head.
The above formula give the correct readings
for conditions of zero wind speed. Additional
corrections can be applied to reduce errors for
non-zero wind speeds.
3.1 CORRECTING ERRORS CAUSED BY WIND
EFFECT
Most sensors which measure long wave
radiation are subject to some degree of error
caused by convective cooling as air moves
past the sensors. The response of the Q-7.1
sensor has been determined by the
manufacturer using a specially constructed
wind tunnel. Curves fitted to experimental data
by the manufacturer are given in Figure 1.
These functions show the change in reading in
response to increasing wind speed.
FIGURE 1. Percentage Change in Reading as a Function of Wind Speed