Kippzonen BSRN Scientific Solar Monitoring System Manual de usuario descargar pdf (Pagina 77)

to guard against performance degradation between international comparisons. One means of monitoring

performance is the use of the reference instrument in WMO Regional Pyrheliometer Intercomparisons.

It should be cautioned that an instrument’s calibration coefficients should not be changed unless a

confirmed shift in the instrument properties has occurred.

The reference instrument will be used to monitor the responsivity of the field instrument on an ongoing

basis depending upon weather. This procedure should occur at least quarterly, if weather permits.

While frequent comparisons between the reference instrument and the field instrument are beneficial,

it is not the goal of the procedure to compare the instruments at every favourable opportunity. Therefore,

it is not necessary for the reference instrument to remain permanently at the station. At sites where

a cavity radiometer is being used continuously, normal incidence pyrheliometers should only be used

to fill in data gaps when the cavity radiometer is in calibration mode. The output of these thermopile

instruments should be correlated with the output of the cavity radiometer obtained immediately before

and after the calibration cycle of the field cavity radiometer.

In the special cases where two thermopile pyrheliometers are used to measure the direct beam radiation

at a station, only one cavity radiometer is necessary. In this case, the single instrument will follow the

procedures set down by the reference instrument in the preceding paragraphs with the exception that

the normal incidence pyrheliometers be compared against the reference instrument as frequently

as possible. This latter procedure, while more economical, will not provide the same overall quality

of data.

In all cases, the link between the WRR and the BSRN observatory instruments should be through a

cavity radiometer.

8.2.2 Detailed procedures

As all solar radiation measurements are linked to the output of the working cavity radiometer or working

pyrheliometers, great care must be taken to maintain the highest standards in comparing the working

instruments with the reference radiometer. This comparison is the prime link between individual BSRN

stations and the WRR and thus all other BSRN stations. The following procedures are presented as

a means of ensuring that this prime link is maintained with the lowest uncertainty possible.

(1) The reference and field radiometers should be co-located (within metres). If possible, a

permanent mount should be constructed on the same optical axis as the field radiometer

for the reference radiometer. If this is not possible, extreme care must be taken to ensure

the accuracy of the pointing of both instruments during the comparison.

(2) To reduce uncertainty, both instruments should be connected to the same data acquisition

system. All measurements should be differential. Extreme care must be taken to eliminate

unnecessary noise and ground loops. When a single digital multimeter (or equivalent) cannot

be used, a reference voltage and reference resistance should be measured by the different

multimeters and any discrepancies corrected. The references used should be of the same

magnitude as the signals being measured.

(3) The ampling frequency should be the same as used during normal operations.

(4) The averaging period should be a minimum of 10 minutes and a maximum of 25 minutes.

Following each averaging period those instrum ents operated in a passive mode (e.g., Hickey-

Frieden) must be zeroed and calibrated. The minimum time allotted for each of these functions

should be one minute.

(5) All calibration activities must be conducted on days in which the cloud cover is less than 4/8's

and the cloud is greater than 15° distant from the solar disk. As a quantitative check of stability,

all averaging periods must have a standard deviation of less than 0.1% of the mean solar

signal during the averaging period or 0.3 Wm (whichever is greater).

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(6) The irradiance levels should be between 400 and 1100 W m during the comparison (the

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maximum value is dependent upon season, latitude and altitude).

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