TWP/VCEIL/C3 - low receiver sensitivity

The ceilometer reported frequent receiver alarms. There are periods when the receiver 
sensitivity dropped to about 50%. The problem was corrected by replacing the instrument with 
a spare.

• Lowest cloud base height detected.(first_cbh)
• Vertical visibility(vertical_visibility)
• Aerosol backscatter coefficient at 355 nm(backscatter)
• Backscatter sum(sum_backscatter)
• Second lowest cloud base height(second_cbh)
• Third cloud base height(third_cbh)
• Percentage of nominal factory setting (0 to 999%)(receiver_sensitivity)
• Altitude of highest signal(alt_highest_signal)
• Millivolts at internal ADC input (0 to 2500)(background_light)

TWP/VCEIL/C3 - standard instrument installed

A spare ceilometer without the tropics window option was installed. The manufacturer 
recommends the tropics window be used at +/- 25 deg lattitude to prevent direct solar 
radiation into the laser optics when the instrument pointed at zenith. No significant signal 
degradation is apparent during the autumnal equinox but, as a precaution, the unit was 
replaced with the original with a tropics window.

• Percentage of nominal factory setting (0 to 999%)(laser_pulse_energy)
• Vertical visibility(vertical_visibility)
• Aerosol backscatter coefficient at 355 nm(backscatter)
• Backscatter sum(sum_backscatter)
• Laser temperature(laser_temperature)
• Millivolts at internal ADC input (0 to 2500)(background_light)