Kippzonen BSRN Scientific Solar Monitoring System Manual de usuario descargar pdf (Pagina 58)

to a fault in the system by performing the same zero test with the resistor attached

directly to the input terminal of the unit. Servicing by authorized personnel is required

if the data acquisition unit fails. If the unit does require servicing it is also a good

opportunity to have the unit calibrated, a procedure that should be repeated every two

years. If the problem is not found in the data acquisition unit, it must be assumed that

local conditions are causing electrical interference. The cabling should be rerouted

and the test repeated. Interference can be reduced by keeping signal cables away

from power cables. It is good practice to avoid parallel routing and to intersect all cables

at 90 degrees whenever possible.

(ii) Measure the lead resistance. Short the resistor and measure the resistance of the leads

as seen by the data acquisition system (a bridge circuit may have to be built for this

test depending upon the capabilities of the system). The resistance should be less

than 10 ohms. If the resistance is satisfactory, the resistor (and bridge) can be removed

from the circuit and the pyranom eter returned. If the value is excessive, determine if

this is a result of the length of the cabling. This can be accomplished by calculating

the overall resistance of the cable by either measuring a short length of similar cable

or obtaining the specification of resistance per unit length from the manufacturer. Once

the unit length is obtained, an approximate value of the entire length can be calculated.

If the resistance can be attributed to the length of the cable one can account for the

commensurate loss in voltage when the measurement is converted into engineering

units. If the resistance is greater than indicated by the length of the cable, it is caused

by a fault within the cable. This must then be repaired or replaced.

(iii) Test the complete system. Measure the resistance of the pyranometer as installed

with the data acquisition system and check that it is approximately within the

manufacturer's specification. This has to be done at night or with the dome covered

unless the resistance measurement is in the offset compensation mode in which case

it would be unaffected by the pyranometer signal voltage.

(2) Installation of constant signals

For greater assurance in the reliability of the data, channels should be set aside to be used

with known signals in the same range as those signals being measured. For example, on

a system where both resistance and voltages are being measured, a fixed resistor and a

known voltage should be included as part of the sampling sequence. These provide a means

of rapidly flagging any changes in the quality of the measurements. Obviously, in a multiplexed

system, the potential exists for scanning problems being missed.

(3) Instrument resistance checks

An easy means of determining a fault in cabling or a sensor which outputs a voltage is testing

for changes in the resistance. On programmable systems, it is encouraged to test the resistance

of each sensor on a daily basis to determine if any significant changes have occurred in the

overall resistance of each channel. While a trained operator may observe changes in a signal

associated with a broken wire or instrument, floating channels are not necessarily easy to

detect immediately if they are not well defined (e.g., pyrgeometer voltages) or adjacent to

a channel with a similar signal.

(4) Programmable flagging

If the system is capable of automatic data quality checks, it should be programmed to set

flags when:

(i) any irradiance values fall outside the range -10 <E< 1200 Wm . The program should

-2

not be designed to delete any data autom atically.

(ii) any air temperature value (air, case, dome, etc.) is outside the normal climatological

range of the station by greater than ± 10 °C. The program should not be designed to

delete any data automatically.

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