Campbell Scientific CMP6-L, CMP11-L, and CMP21-L Pyranometers - Manual (Page 24)

CMP6-L, CMP11-L, and CMP21-L Pyranometers
7.3.1.3 Offset
The offset will normally be fixed at zero as the sensor should output no
significant signal in dark conditions. In practice, because of the nature of
thermopile detector sensors, there will be some offset in dark conditions;
sometimes this offset can give negative light readings. This offset varies with
several factors (e.g., rate of change of sensor temperature), so it cannot be
removed with a fixed offset. Some users may wish to remove small negative
readings by including code after the measurement instructions that sets
negative readings to zero.
7.3.1.4 Output Format Considerations
Over-ranging may be an issue if the measurement values are totalized. Over-
ranging can be prevented when using CRBasic by storing the data in the IEEE4
format.
When using Edlog, the largest number the datalogger can store in final storage
is 6999 in low resolution mode (FP2) and 99999 in high resolution mode (if
available). The following example shows how over-ranging can be a problem
for Edlog dataloggers.
Example
Assume that daily total flux is desired, and that the Edlog datalogger scan rate
is 1 second. With a multiplier that converts the readings to units of kJ•m
–2 and
an average irradiance of 0.5 kW•m –2 , the maximum low resolution output limit
will be exceeded in less than four hours.
Solution 1 – Change the multiplier in the instruction to (m * 0.001). This will
totalize MJ•m
–2 instead of kJ•m –2 .
Solution 2 – Record the average flux density and later multiply the result by the
number of seconds in the output interval to arrive at total flux.
Solution 3 – Record the total flux using the high resolution format. The draw
back to high resolution is that it requires four bytes of memory per data point,
consuming twice as much memory as low resolution. Instruction 78 is used to
switch to high resolution in the Edlog dataloggers.
7.3.2 CMP21 Internal Thermistor Measurement
The thermistor is measured using a half bridge measurement instruction
(BrHalf instruction in CRBasic or Instruction 5 (P5) in Edlog). The value
provided by the half bridge instruction needs to be converted to resistance and
then converted to temperature.
The following equation is used to convert to resistance:
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
−
=
x
x
V 1
V
1000 Res.
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