Present Status of APEX Float Technology S. Riser, University of Washington

1. Present Status of APEX Float Technology S. Riser, University of Washington • Lithium batteries. WRC continues to refuse to sell floats with Li batteries, so users must do their own replacement. More groups have decided to change to Li batteries in the past year. • APF8 controller likely to be discontinued, to be replaced by APF9 and APF10. The APF10 will have 2 additional serial ports and extra flash memory (64 Mb compared to 8 Mb for the APF9). • Pressure offset errors on APF8. This effect can be assessed using data from APF9-equipped floats. The problem remains unfixed but will go away when the APF8 is discontinued. • Bladder leaks. This was a problem on a number of APEX floats in the period 2006-2008. The manufacturing process of the bladder was changed from a single weld to a double-sided weld, with a change in bladder vendors. It appears that this problem has been solved; for now, the air bladders on all UW-built APEX floats continue to be tested under pressure during construction, until it is clear that the problem has been solved. • New capability: near surface sampling of temperature.

2. Measuring SST (or near-surface T ) from ARGOS floats: 9 points above 4 m Negligible energy cost: this requires 1 extra ARGOS message Data from UW float 6023 (WMO 5902077) in the western Equatorial Pacific  [ potentially a useful addition to the Argo data stream ]

4. Pressure sensor problems on Argo floats [prepared for AST-10] Stephen C. Riser Dana Swift Annie Wong

5. Specifications for SeaBird’s SBE 41 and 41CP CTD modules (essentially 100% of present Argo floats including Apex, SOLO, Provor, and others use the SBE sensor module) [http://www.seabird.com]

6. p sensor problems and corrections... • As the number of profiles has increased and most Argo groups have begun DM in earnest, several problems with p sensors have been found and have been addressed • p is being corrected in both RT and DM • This issue was discussed in detail at DMQC-3 at UW in September 2008 (http://www.coriolis.eu.org/cdc/argo_rfc.htm)

7. Results from 674 UW floats with 3 sensor types: Ametek, Paine, Druck Ametek & Paine (< 2004) are known to drift positive. Druck (> 2003) ~5% have the oil microleak problem.

8. Druck microleaks: Oil leak leads to an internal volume loss, which then is manifested as an increasingly negative offset at all pressures (i.e. a slow negative drift).Unfortunately these negative pressure offsets are truncated in APEX floats with the old APF-8 controller. 15% Statistics from UW floats with APF-9 controller show that about 5% of Druck pressure sensors are affected by the microleak problem. 214 UW Druck floats with APF-9

9. For Druck pressure sensors, surface pressure readings are often bound within the noise level of 1 dbar. About 5% of them show a slow negative drift (the microleak problem). 214 UW Druck floats with APF-9 Most of the remaining 95% have surface pressure readings that do not follow systematic trends. Problem: most Apex floats (APF-8) truncate negative surface pressures to zero

10. When the pathological cases of microleaks are taken out, statistics show that the Druck sensor tends to start out its life at zero mean, then drifts a consistent positive or negative amount and after about 40 profiles stabilizes at +0.2 dbar mean (red line), bounded within  1 dbar (green lines) of the original calibration. This +0.2 dbar mean surface offset can be due to a combination of environmental and instrument factors, such as surface waves, changes in atmospheric pressure, non-linear hysteresis, etc. It does not necessarily indicate depth-constant pressure calibration drift. The p drift can be removed during the DM step.

11. UW 0015 (WMO 5900961) Float 0015 measured p at 15 minute intervals while on the surface. Results show that these samples are not representative of the low-frequency pressure variation. mean of 15 min. surface p samples  range of 15 min. surface p samples

12. well-behaved UW Druck UW problem Druck BSH Druck Data from CSIRO, JAMSTEC, BSH, PMEL, and UW hint that the microleak problem is increasing in frequency….possibly affecting 15% of floats deployed since late 2006.

13. Conclusions… • There are small, systematic p errors (drifts) that occur on SBE CTD units that are common to all Argo floats, although they might appear in different forms on different float types The errors are generally well within the manufacturer’s specs at the sea surface (where we know the expected p  0) but are more difficult to determine at depth, where the errors could be different.  There are other types of p errors that are still being examined. These include the microleak error and the p spot sample error. • The required p corrections at the sea surface can be made in both RT and DM in a straightforward manner; it is essential to examine the entire ensemble of p measurements during DM • Future work: Work with SBE to fix these problems! Examine pressure/temperature dependence of the pressure error. At UW we are now testing p on every CTD with a dead-weight tester at several temperatures and pressures.