By way of example please see this post from the Little Cayman station log (including photos), which concludes that a "catastrophic power loss" was caused by "condensation" within the "solar panel junction box." To my knowledge there are no diagnostic RH sensors deployed in the solar panel junction boxes at any CREWS/CCCCC station but this serves as an important lesson about the damage that moisture incursion can have on station operations. In this case the Cayman station was nonoperational for 73 days and when redeployed it was found that communications with the WXT (Vaisala's 'Weather Transmitter') had failed, which may indicate another yet-undiagnosed effect of junction box condensation at that buoy.
The following graph shows the Speyside / Angel's Reef (ARTO1) diagnostic RH values plotted over the buoy's deployment lifetime to date (through June 9th, 2015). The red line is RH maxima as measured within the Main junction box and the green line is RH maxima as measured within the Met junction box.
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| Please click on this image to see it in larger form. |
That first, lone spike above 75% in the Main RH (red line) occurred on October 23rd, 2014. Not long after that the Main RH values climb above 50% and stay there for life, with the final <50% reading occurring on November 17th, 2014, just after this station's November 10-13 recovery to land and redeployment. The dataset's only spike above 90% humidity occurred on January 13th, 2015.
Note that these data report only the maximum RH seen in a ten-minute period of those raw values collected every five seconds.
A natural question is how humid is too humid? I have heard it suggested that these junction box humidity maxima should not exceed 20%, and the lifetime of Met junction box RH data from the Buccoo Reef, Tobago CREWS/CCCCC buoy shows that this is an entirely attainable goal and can be regarded as a reasonable target. However, at what point should overly-high RH values prompt remedial intervention? I have for many years run CREWS programming tests inside my office which has had the side-effect of collecting a long-term dataset of indoor RH values, in an environment that is dry enough to prevent any damage from moisture or condensation. Based on these somewhat accidental datasets I would suggest that RH values up to 50% may be considered tolerable, but that prolonged measurements of diagnostic humidity in excess of 50% should be considered cause for immediate reparative action.
The story told by these data, then, is twofold: the Met junction box (green line) remains largely below 20% humidity throughout the buoy's lifetime (with 99.7% of readings falling below this mark), although there are isolated >20% spikes and midway through the dataset there begins an obvious though gradual trend of increasing humidity. The buoy's original non-increasing (almost exclusively below 6%) pattern seems to end on September 18th, 2014, and December 25th, 2014 is the last reported Met RH measurement to fall below 10% apart from two isolated reading during a maintenance operation on February 4-5, 2015. There is no immediate cause for alarm regarding Met RH levels but this parameter's increasing trend should be closely monitored.
On the other hand the Main RH numbers start low but show a much more quickly increasing trend. Our targeted 20% level is first exceeded on March 19th, 2014 but thereafter 97.7% of readings fall above the 20% humidity level. Our tolerable 50% level is first exceeded on October 23rd, 2014 but thereafter 94.5% of readings fall above the 50% level. Therefore this station can be said to have a persistent and long-lasting problem with moisture incursion into the Main junction box which should be attended to at the earliest opportunity.
Similar analyses have been conducted at this station's sister buoys located at Buccoo Reef, Tobago (BUTO1) and at Little Cayman, Cayman Islands (CCMI2). A pattern that is common to all three of these buoys is that the Main RH levels are all presently at alarming levels, after starting out acceptably low during initial deployment and increasing much more quickly than the Met RH levels do. This might suggest a design or construction problem with the moisture seals on the Main junction box, or a lack of clear deployment instructions regarding proper sealing of the junction boxes and the use of fresh desiccant.
The Met RH patterns at the three buoys range from BUTO1, where Met RH levels start low and stay low throughout the buoy's entire lifetime, to ARTO1, showing a mildly-increasing trend of Met RH levels that is not yet any cause for alarm, to CCMI2, where Met RH levels began low but increased quickly and are presently at levels that are alarmingly high. There does not seem to be any reason to suspect a systemic problem with the Met junction box design, construction, or deployment practices as there is in the case of the Main junction boxes.
The complete analyses for the other RH diagnostics, including graphs, may be found at this link for BUTO1 and at this link for CCMI2.
(signed)
Mike Jankulak