This post is part of a series of posts to share the results
of my recent evaluation of data produced by all of the CREWS/CCCCC buoys
over their lifetimes, from 2013 to the present. This post will discuss
the diagnostic relative humidity (RH) data collected from inside two of
the buoy's junction boxes: the 'Main' and 'Met' junction boxes which
house the Main and Met dataloggers, respectively. Overly high
humidities within either of these junction boxes could lead to a failure
of the buoy's controlling electronics and lengthy interruptions in
the data stream.
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
