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Performance of APEX Floats S. Riser University of Washington. Problems corrected Present reliability statistics Remaining problems and possible solutions.
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Performance of APEX Floats S. Riser University of Washington • Problems corrected • Present reliability statistics • Remaining problems and possible solutions
[Note: the results quoted herein are based on an analysis of the performance of 489 Argo and Argo equivalent floats that have been deployed by 9 countries participating in Argo. The Argo committees in these countries have graciously made their raw Argos messages available to UW in real-time so that engineering data and other parameters can be examined for Apex floats as a group.] The most important problem inherent to APEX that has been fixed in the past year is the motor backspin problem. This problem was discussed in detail in the report from last year’s meeting. The problem was corrected with the APF-8C version of the Apex controller. No floats deployed in the past year with the APF-8C have shown the motor backspin problem.This is encouraging, since most of the floats that showed the problem failed within 10 profiles after deployment.
(no motor backspin) (contains motor backspin problem) Present APEX reliability statistics based on 489 Argo and Argo equivalent floats. Note that the overall reliability includes results from floats that have already ceased operation due to known problems. Floats with the APF-8C are more indicative of present reliability. These are a mixture of UW floats that have been in the water for over a year and others that are newer. Results are encouraging. The table shows results from floats parking at 1000 m and profiling to 2000 m; floats parked permanently at 2000 m yield similar results after the motor backspin problem was eliminated.
Comments on reliability statistics…. • The statistics for APF-8C controllers were not kept separately until recently. However, UW APF-8C (parking < 1400 m) have had no failures due to the motor backspin problem. Many of these floats have completed more than 50 profiles with no cases of backspin. A check of floats parking at deeper levels (ie, 2000 m) also shows no failures due to backspin after use of the APF-8C controller was initiated. • Early reliability of APEX was considerably lower than at present. The APF-7 and early versions of the APF-8 controller were only roughly 50% reliable. In practice, this meant that many floats failed during their first year in the water. Present reliability of all 427 floats with APF-8 controllers (including the original version and variants A, B, and C) show 83.5% reliability. With 24638 profiles executed by 427 floats, this yields an average time in the water at present of about 58 profiles, or over 18 months. This is good news: we are beginning to approach the 90% number that is the stated goal of Argo.
Remaining problems with APEX…. (a) Alkaline batteries. We have found a large number of floats that show evidence of rapid battery discharge after about 100 profiles. In several cases these floats were recovered, and in several other cases Apex battery packs were studied in long-term pulse-discharge tests. The problem was found to be due to poor-quality construction of the alkaline cells; when one cell fails, it often causes the entire battery pack to fail, causing the failure of the float. note rapid discharge
Remaining problems with APEX (continued)…. • Possible solutions to the battery problem: • Add diodes between cells in the battery packs, so that if a single cell in the pack fails it will not drain the power from all other cells [this is the failure mode that has caused the problems]. This fix is presently being instituted by Webb on all new floats. • Change from alkaline batteries to lithium batteries. This will definitely fix the problem, but it introduces new problems. The added cost of lithium batteries is about US$500 per float. There are limitations to shipping lithium batteries that will become more stringent before the end of 2004. Webb has opted not to supply floats with lithium batteries at the present time. At UW we have designed our own lithium battery packs and have now deployed 5 floats using these. We intend to eventually change to lithium on all floats. We will use diode-protected alkaline batteries only where shipping restrictions prevent us from using lithiums.
Remaining problems with APEX (continued)…. (b) Grounding of floats. A number of APEX floats have failed when they grounded on the bottom of the ocean. Grounding is not unlikely during the lifetime of a float and floats should be able to survive this. In the case of APEX, the problem stems from the design of the cowling that covers the bladders on the lower end of the float. When a float grounds, it is inevitable that some sediment or other bottom material gets inside the cowling and cannot get out. This increases the mass (ie, weight) of the float. If this happens repeatedly (as when a float is grounded over the course of several profiles), the increase in mass can be large enough that the float is no longer capable of reaching the sea surface, which effectively renders the float useless. Possible solution: decrease the number of holes in the cowling, so that it is more difficult for sediment to get inside. We are now doing this at UW by filling most of the holes in the cowling with rubber plugs. So far, we have not lost any floats with these plugs by grounding.
Hole filled by large rubber plug (this hole is normally filled by a plug supplied by Webb; the plug has been removed here as part of the construction process at UW and will be reinstalled prior to ballasting) Holes in the cowling (8) Holes filled with rubber plugs (7) (one hole is left open so that water can escape when the bladder inflates)
