Appendix I: Failure Modes

The following is a list of possible problems that have been compiled from past experiences, mainly from the Atmospheric Radiation Measurement (ARM) research facilities and the experiences from developing and operating a solar measurement station at our respective institutions.

Some key points before addressing possible equipment failures:

  • Proper maintenance is the final step in quality control of measurements. After a measurement is recorded, only data quality assessment is possible.

  • A situation with no data is often better than one that produces bad data. Unless bad data is identified and flagged, it can destroy confidence in and value of the set data.

The first section will cover observed problems, and possible causes are noted. Next the corrective maintenance actions will be listed. Because all ARM and UO solar monitoring network sites use Campbell data loggers for solar and infrared measurements, comments on data loggers will be specific for Campbell data loggers, but the gist applies to all data loggers.

I.1 Observed Problems

I.1.1 NO SIGNAL Possible Sources of Problem

  1. An open circuit can be the result of a damaged signal cable, loose signal cable conductor connections at the data logger input terminal, improperly seated connector at the radiometer body, or broken thermopile winding (typically due to lightning strike).

  2. The data logger system program may also have been changed or lost.

  3. The data logger internal battery voltage may be below nominal limits.

I.1.2 UNSTABLE SIGNAL Possible Sources of Problem

  1. Loss of proper electrical grounding and shielding of the low-level direct current (DC) signal cable may cause erratic voltage readings.

  2. The radiometers have a time-constant on the order of 1 sec to changes in irradiance (voltage) due to clouds. Electrical noise will generally have a higher frequency than this natural variation.

  3. Data logger system has failed.

    1. The data logger system program may have been changed or lost

    2. The data logger internal battery voltage may be below nominal limits

  4. Other possible causes include

    1. Moisture inside the radiometer case or signal cable connector

    2. An improperly seated connector at the radiometer body

    3. Loose signal cable connections at the data logger input terminal

    4. Cold solder joint inside the signal cable connector

I.1.3 SIGNAL GREATER THAN PHYSICAL LIMITS Possible Sources of Problem

  1. The GHI pyranometer can, with unique reflections from properly positioned towering cumulus clouds, exceed the solar constant for periods of less than tens of minutes.

  2. Moisture inside the pyranometer dome or the pyrheliometer window can form small lenses that could increase the radiation on the radiometer detector.

  3. Half-melted frost or snow on the dome can reflect sunlight onto the sensor receiver.

  4. More often, high signals are the result of an open circuit causing the data logger to over range (6999), a ground loop introducing a spurious voltage, or an incorrect radiometer calibration factor or logger program.

I.1.4 SIGNAL LESS THAN PHYSICAL LIMITS

  1. Signal has been shorted by damaged cable.

  2. Corrosion at signal cable connections has caused increased resistance.

  3. Wrong calibration factor or application associated with instrument.

  4. Moisture inside the pyranometer dome or behind the pyrheliometer window.

I.1.5 GHI, DNI, AND DHI NOT INTERNALLY CONSISTENT

This is most easily determined from Equation 2.1. In addition to a bad radiometer calibration factors (Cf), possible causes are

  1. DNI low

    1. Incorrect tracker alignment

    2. Dirty water, ice on NIP window

    3. Nearby obstructions (trees, poles, structures) that shade or sometimes reflect sunlight onto the detector

  2. DNI high

    1. Suspect electrical problem first (see the section on signal greater than physical limit)

    2. Not many physical reasons for NIP to have prolonged increased output— can have temporary increase when sun is next to the edge of a cloud

    3. Contaminated NIP window, especially due to frost or dew, typically reduces reading, but may cause increased signal with specific solar geometry

  3. GHI low

    1. Dirty/iced dome

    2. Possibly bad electrical connections

    3. Nearby obstructions, trees, poles, or structures shade the pyranometer

    4. Alignment problem—pyranometer tilted to north

  4. GHI high

    1. If persistent, suspect electrical problem first

    2. Could be leveling problem, tilted toward sun

    3. Water droplets (dew) on dome can focus sun’s rays

    4. Ice or frost on dome opposite the sun’s location can reflect sun’s rays onto detector

    5. Nearby reflective surfaces, such as buildings and poles

    6. Artificial lights (at night)

  5. DHI low

    1. Dirty/iced dome

    2. Possibly bad electrical connections

    3. Correct PSP for thermal-offset problem—Model 8–48, B/W, will not exhibit this behavior

    4. Pyranometer not level, tilted to north

  6. DHI high

    1. Suspect misaligned solar tracker (dome must be in full shade of ball)

    2. PSP and PIR not coplanar

    3. Pyranometer not level, tilted to south

    4. Possible ground loop or other electrical problems (see the section on no signal or unstable signal)

    5. Nearby reflective surfaces, such as buildings and poles

    6. Artificial lights (at night)

  7. Upwelling GHI low

    1. Dirty/iced dome

    2. Possibly bad electrical connections

    3. Field of view of local terrain biased by dark surfaces—water pooling?

    4. Unlevel mounting platform

  8. Upwelling GHI high

    1. Fresh snow cover can reflect up to 98% of the GHI

    2. Frost on dome can reflect more radiation onto pyranometer detector

    3. Possible ground loop or other electrical problems

    4. Field of view of local terrain biased by bright surfaces—water pooling?

    5. Unlevel mounting platform

I.1.6 SHORTWAVE ELEMENTS (DNI, GHI, AND DHI) INTERNALLY CONSISTENT BUT WRONG (K-SPACE)

  1. Tracker failure (if pyrheliometer and shaded pyranometers are mounted on the same tracker, then DNI = 0 and DHI = GHI, where GHI is indicative of clear sky irradiance levels)

  2. Tracker alignment problem?

  3. Uniform soiling of optics

I.1.7 NO DATA COLLECTED BY SITE DATA SYSTEM

  1. Suspect communications failure

  2. Modem failure

  3. Loss of electrical power to data logging system

I.1.8 NO DATA COLLECTED BY CARD STORAGE MODULE

  1. Card not initialized at last site visit (card was full)

  2. Card battery failure

  3. Failed connection between logger and card storage module

  4. Loss of electrical power to data logger system for more than 10 days

I.1.9 ASYMMETRIC DIURNAL PROFILES (SYMMETRY WITH SOLAR NOON, NOT CLOCK NOON)

  1. Incorrect data logger time (clock drift? improper switch to daylight time?)

  2. Incorrect time zone

  3. Incorrect longitude

  4. Assignment of hours (0–23 vs 1–24)

  5. Instrument alignment (pyranometer not level?)

I.1.10 BAD DATA TIME SEQUENCE

  1. Data buffer overrun due to corrupt memory?

I.2 Corrective Maintenance Action Options

Based on past experience and an understanding of the irradiance measurement processes, the following actions are suggested for the previous failure modes. The following information should complement the existing instructions for routine maintenance

I.3 Corrective Maintenance Actions By-The-Number:

I.3.1 CLEAN OPTICS

Note: Excessive dew, frost, snow, or dust on the PSP or PIR can indicate ventilator failure.

  1. Contamination of optical surface

    1. Wash dome or window with distilled water, wipe dry

    2. Wash dome or window with alcohol, wipe dry

    3. Warm iced dome or window with palm of hand if necessary—do not scrape ice from dome or window

    4. Moisture inside the dome or window?

    5. Check PSP desiccant canister seal is tight

    6. Check PSP desiccant canister window is not cracked

    7. Remove PSP sunshade and check dome collar screws are tight

    8. Check NIP window screws are tight

    9. Change desiccant if granule colors are changed showing moisture saturation.

I.3.2 CHECK/ADJUST ORIENTATION

  1. Align Solar Tracker (north/south and base is level)

  2. Adjust shading balls to shade PSP and PIR domes

  3. PSP/PIR is level using circular spirit level

  4. Signal cables not caught on tracker or mounting fixture

I.3.3 EXAMINE INSTANTANEOUS DATA

  1. For Campbell Scientific Data Loggers that use Loggernet program in monitor mode to view all data channels

    1. Compare with expected readings

    2. Open channel value = “6999”

    3. Note lightning can burn out delicate thermopile windings

I.3.4 CONFIRM VENTILATOR IS FUNCTIONING

  1. Confirm electrical power available

  2. Sun shade must be level with black detector or base of PIR dome

    1. The sun shade can shadow or reflect radiation onto detector if not properly positioned at or slightly below the base of the outer dome

I.3.5 CHECK SIGNAL CABLE AND CONNECTIONS

  1. Confirm cable is not cut, stretched, or kinked

  2. Check electrical connection at data logger wire panel and at radiometer connector

    1. Hot and cold cycling loosens some wire connections

I.3.6 CHECK DATA ACQUISITION SYSTEM GROUND CONNECTIONS

  1. Check electrical connection at data logger, enclosure, and ground rod for corrosion or loose fittings

I.3.7 CONFIRM SERIAL NUMBERS WITH INSTRUMENT LOCATION

  1. Visually inspect radiometer serial numbers at each mounting location and compare with data logger program locations (Monitor mode) or current inventory listing

I.3.8 CONFIRM CALIBRATION FACTORS WITH INSTRUMENT SERIAL NUMBER

  1. Compare calibration sticker information with data logger program locations (Monitor mode) or current inventory listing

I.3.9 DATA LOGGER POWER

  1. The red LED indicator on power supply should be illuminated

  2. Check logger battery voltage is at least 9.2 Vdc using Monitor mode

  3. Check the “on–off” switch is in the “on” position

    1. Note confusing label on CR10X–1M wiring panel—switch is pointing away from “on” label when in operating position

I.3.10 DATA LOGGER CLOCK

  1. Confirm CR10X–1M logger clock is within 3 sec of GMT

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