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Notes From the Field Part 1

Notes From the Field: Welcome!


By Richard L. Pyle and Sonia J. Rowley



Welcome to the first installment of a new, ongoing series of articles about Poseidon rebreathers. As the title of this series suggests, our intention is to report on how we use Poseidon rebreathers in our scientific field projects. But more than just field reports with pretty pictures of undersea life, our aim is to use this forum to discuss a variety of topics, ranging from the scientific work that we do, to the lessons we’ve learned while using and testing these rebreathers, in addition to sharing interesting, exciting, and sometimes funny stories of events that happen along the way.  If we are successful, we will simultaneously entertain and educate you about our research, and provide you with insightful information about how Poseidon Rebreathers work, including tips and tricks to improve your diving experience, as well as explanations of how the system works, and why it was designed to work the way that it does.


Before we dive into this first missive, we should probably take a moment to explain who we are, what we do, and what makes us qualified to share our experiences. 


Richard-Pyle-Photo-by-Robert-KRichard is a self-described “fish nerd”, and has been for his entire conscious life. Born and raised in Hawai‘i, he started scuba diving at the age of 12 in 1979, and by the late 1980s was using open-circuit mixed-gas diving techniques to discover new species of coral-reef fishes in remote locations around the tropical Indo-Pacific. After being trained on the Cis-Lunar MKIV rebreather by Bill Stone in 1994, he joined the development team in designing and testing the Cis-Lunar MK-VP and its successors, the Poseidon MKVI Discovery and SE7EN Rebreathers. He continues to lead and participate in scientific exploratory expeditions throughout the tropical seas, with a primary focus on deep coral reefs (aka, “Mesophotic Coral Ecosystems”). He works as an ichthyologist and database developer at the Bishop Museum in Honolulu, where he has been employed since 1986.



Sonia-Rowley-Image-by-Robert-KSonia began diving in 1984 at the age of 11, introduced by her parents Mike & Penny Rowley, both keen divers since 1976 and later owners of the renowned British diving vessel Maureen of Dart. Instantly fascinated, Sonia embarked on a life-long love of the ocean, initiated years previous by watching her parents hunt for unknown shipwrecks around the UK coast. At the age of 30 she decided to combine her passion for diving and natural history, and become a Marine Biologist. Originally trained on the Evolution by her father Mike, and having a wealth of diving experience behind her, Sonia integrates diving and coral-reef research; particularly on deep reefs around the Indo-Pacific. She now works as a research affiliate specialising on multiple aspects of gorgonian (sea fan) coral biology and evolution, at the Bishop Museum and University of Hawai‘i in Honolulu.    




Richard-Pyle-With-Fish-School-Photo-by-Sonia-KBoth of us are members of the official Poseidon test-dive team. In that capacity, it is our job to test BETA versions of new rebreather “firmware” (i.e., the embedded software within the rebreather itself) and prototypes of new designs, features, and accessories; before these are released to the rest of the diving community. We also meticulously take notes about our experiences using the rebreathers in various circumstances, and provide detailed reports to Poseidon on any problems we encounter, with suggestions for improvements and ideas for new features. This means that we need to be intimately familiar with exactly how the system is designed to work. It also means that we sometimes use the system at (or even beyond) its rated limits; something that is a necessary final-stage, real-world test of the system before it is released. We make this point here because, through this series of articles, you will occasionally see references to dives we have done that exceed the stated manufacturer’s limits.  DO NOT MISINTERPRET THIS AS AN ENDORSEMENT TO EXCEED POSEIDON’S STATED LIMITS FOR USE OF ITS EQUIPMENT!  Whenever we make such references, we will include a clear statement that we are doing so with specific intent, and only when extraordinary safety measures have been implemented, and only in the context of our roles as official Poseidon test divers.



Sonia-Rowley-Collecting-Samples-Image-by-Robert-KFuture installments to this series will have specific themes, usually focused on a particular aspect of our research, or of an important lesson learned concerning rebreather diving, or sometimes just an interesting, exciting or funny story. Some of the articles will be written by both of us; some by just one of us, and some by other members of the Poseidon test dive team. Our overarching goal in this series is to illustrate how Poseidon rebreathers allow us to conduct research that would have been much more difficult (or impossible) to do otherwise. Along the way, we hope to sprinkle insights regarding our research as well as tips to improve your own diving experience using Poseidon Rebreathers. In that vein, we will close this first introductory article with an overview of the Poseidon Rebreather predive system, including our top tips for making sure you pass the test consistently.


SE7EN-predive-checklistOne of the most innovative features introduced with the Poseidon MKVI Rebreather (and continued in the next-generation SE7EN Rebreather) is the robust automated pre-dive test system that runs automatically whenever the rebreather is powered up. This feature represents a significant advancement of the manual pre-dive checklist that was included within the menu system of the Cis-Lunar MK-VP rebreather. These tests are described in more detail in Chapter 2 of the MKVI User Manual and the POSEIDON SE7EN User Manual (both available in multiple languages), which you should read and understand thoroughly. You can also click on the image here to enlarge it and see the SE7EN pre-dive checklist. This pre-dive test system is extremely important, because it ensures the proper functionality of the entire rebreather system prior to starting a dive. The tests are rigorous (as they should be!), and involve many sophisticated analyses that are not immediately obvious. However, if you’re like us, then you may sometimes get frustrated when the tests fail, and thereby force you to restart the test from the beginning.  Over more than five years of experience developing and testing the pre-dive test routine, we’ve come up with a list of what we think are the ten most important tips for getting past the pre-dive test routine consistently.


Top Ten Tips for Passing Pre-dive


1.    Make sure your battery is fully charged the night before the dive. Nothing is as frustrating as kitting up in the boat almost ready to dive, only to discover that the rebreather fails test 48 (insufficient battery charge).  Unless you have a charged, spare battery on hand at the dive site, there is really no way around the test. We always make sure to put the battery on a charger after every dive, even if the battery still has most of its capacity remaining.  Although it’s fine to leave the battery in the charger indefinitely, if there is a power failure it may lead to a discharge of the battery, so Poseidon does not recommend leaving the battery in the charger all the time. In fact, leaving it in the rebreather itself helps ensure that you don’t forget to grab the battery as you rush out to go to the dive site -- the only thing worse than having an insufficiently charged battery is to have no battery at all!


2.    Make sure to follow the battery learn cycle recommendations. This is really an extension of the first tip. The reason why the battery must periodically undergo a “learn cycle”, is to make sure that the system will accurately predict the remaining battery life.  A learn cycle consists of a full discharge of the battery, followed by a full, uninterrupted charge.  The longer it has been since the last learn cycle, the more battery charge is required to pass test 48.  If the learn cycle was very recent, the test will pass with as little as 20% charge remaining. For each day since the last learn cycle, this minimum charge value increases by half a percent.  Thus, ten days after a learn cycle, 25% battery charge is required to pass test 48.  If 160 or more days has passed since the last learn cycle, even a battery that is 100% fully charged will fail the test. The reason for this stringent requirement is that as time has passed since the last learn cycle, the system has less and less confidence that it can accurately predict remaining battery life; and as such, it increases the amount of charge required to allow a dive.  One important note: DO NOT RUN A LEARN CYCLE WHEN NOT SUGGESTED BY THE CHARGER. This will shorten the life of the battery unnecessarily.  Initiate a learn cycle whenever the charger recommends it, but never do it when it is not recommended.


3.    Pay attention to the full Pre-Dive test. It’s easy to get distracted while waiting for the pre-dive tests to complete; however, it’s important to pay attention to the rebreather throughout the entire pre-dive process.  Not only is this important for the obvious reasons; but some of the tests require action on your part (confirming something via the wet switch, changing the mouthpiece position, turning on the gas cylinders, etc.). Each of these tests sets a finite time limit for you to perform the necessary task.  We can’t begin to tell you how many times we’ve become distracted by something during the pre-dive test, only to have a test fail because we missed the prompt to perform the necessary action.  As with all test failures, this means the entire pre-dive test procedure needs to be re-started from the beginning.


4.    Keep the over-pressure relief valve on the counterlung clean and secured all the way down in the clockwise direction. In our experience, the most common test failure is test 49:  the positive pressure loop test. This is actually one of the most sophisticated tests in the entire pre-dive routine.  In addition to the obvious task of ensuring that the breathing loop holds positive pressure, this test also ensures that none of the four solenoid valves is leaking gas, ensures that both metabolic oxygen solenoid valves are functioning properly (i.e., actually injecting gas when they should), ensures that the depth sensor is working with sufficient sensitivity, and several other functions as well.  There are five ways this test may fail, but by far the most common two are error codes 48 (positive pressure not achieved) and 49 (loop leaking). Sometimes there is a genuine leak in the loop, such as a hole in a counterlung or breathing hose, or a badly seated or sealed O-ring. But the most common source of this test failing is when the over-pressure relief valve (OPV) on the counterlung is either dirty, or not fully closed in the clockwise direction.  Ever since we have been careful to rinse the OPV thoroughly after every dive (opening it all the way counterclockwise and flushing fresh water through to wash out any sand, dirt, or salt crystals on the seal), and also to routinely keep it stored in the closed (clockwise) position, we almost never fail test 49.


5.    Change oxygen sensors when they have expired (even if they still seem to work). The second most common test to fail is test 53: oxygen sensor calibration. Although there are several (potentially field-correctable) reasons why this test might fail, the most common reason is that the sensor is genuinely not up to par. Poseidon Rebreathers hold oxygen sensors to a very high standard -- with good reason. Oxygen sensors are perhaps the most critical component of any rebreather system, as their proper function is vital to ensuring your good health. Like test 49, there are actually several things (besides oxygen sensor calibration) being tested during test 53, including proper function of both the oxygen and diluent calibration solenoid valves, ensuring that the oxygen really is oxygen and the diluent really contains the fraction of oxygen it is supposed to, in addition to several other things. The primary oxygen sensor (the one directly beneath the two gas supply hoses connected to the pneumatics block on the electronics module) is the sensor that is most carefully scrutinized for proper function, and is usually the sensor responsible for failing test 53.  Some people attempt to solve a consistent failure of the primary oxygen sensor calibration by switching the primary and secondary sensors, with the hope that secondary will pass where the primary failed.  As tempting as this may be, it’s a bit of a fallacy, because it means you are using a known bad sensor as your backup (even if it does pass calibration) -- not good diving practice! Keeping a spare new sensor on hand is certainly a wise thing to do, but if you leave it sealed in the original bag, it will require up to 24 hours to “wake up” after removing it from the bag before it’s appropriate to use it for diving. If you keep a spare sensor on hand that is not sealed in the bag, then loses its shelf life, and is not going to be as fresh when you need it.  Thus, the best way to make sure you pass test 53 is to change the sensor after its indicated expiration date.


6.    Make sure the mouthpiece stays in the correct position. The next most common reason why both test 49 and test 53 may fail is that the mouthpiece is in the wrong position.  The mouthpiece must remain in the “open circuit” position during test 49 (to keep the loop sealed), and it must remain in the “closed circuit” position during test 53 (to make sure the loop stays at ambient pressure during calibration).  A well-lubricated mouthpiece is great because it’s easy to switch from one position to the other, but it can also cause you to fail one of these two tests if it gets bumped out of its appropriate position during either test 49 or 53.  Sometimes just the torque in the low-pressure hose supplying the integrated open-circuit regulator is enough to shift the mouthpiece out of the full open-circuit or closed-circuit position, causing a failure of either test 49 or 53 (respectively).  Thus, take special care to make sure that the mouthpiece is not bumped, and the supply hose is not applying torque to the mouthpiece, during both tests 49 and 53.


7.    Flush the loop with air before starting a pre-dive using hypoxic gas mixtures. This tip only applies to divers using one of the Poseidon Decompression-enabled batteries (48m or 60m). These batteries allow the oxygen content of the diluent to be set as low as 16%. Tests 34 and 35 of the pre-dive routine ensure that the oxygen sensors are installed, and are not reading unacceptably low in ambient air. Sometimes, after the rebreather is re-started following a calibration using low-fraction diluents, the loop is sufficiently flushed with this diluent gas that millivolt output from one or both sensors spoofs the system into failing test 34 or 35. Thus, when using diluent mixtures with low oxygen fraction, be sure to flush the loop with air before powering the unit up, to make sure the oxygen sensors are exposed to (at least) 21% oxygen.


8.    Run a pre-dive test before leaving the house. Although some test failures can be corrected out at the dive site, some can only be corrected with access to items often not brought along for the dive.  For example, if the gas mixtures (either oxygen or diluent) are not what the rebreather is configured to expect, then the system will likely not complete the pre-dive tests successfully. As already mentioned, sometimes the battery is not sufficiently charged. Sometimes you need to replace an O-ring  to pass positive pressure loop test. Sometimes you grab the wrong battery with a different version of firmware (which will cause a failure of test 15). In any case, we have saved many dives by simply running through the pre-dive once before we leave the house to go for a dive (or before we board the dive boat while still on shore), to make sure the system truly is ready to dive. Some people will leave their unit powered up continuously after passing pre-dive hours before the dive, but there are many reasons why this may not be a good idea (i.e., cylinders may get turned off or gas may leak if they’re left on; the battery may lose  sufficient charge, the mouthpiece may get bumped out of position leading to an alarm, etc.). So, the best thing to do is make sure the unit will pass pre-dive, then power it down completely before you head off to go diving.


9.    Make sure the real-time clock is properly set. Sometimes the real-time clock in the system will be set incorrectly.  In some cases, this can lead to various test failures during the dive (particularly test 9, error code 90). The clock on the rebreather system can be reset using the configuration utility, and we have found it to be good practice to always check the clock value every time you use the configuration utility, to make sure the clock on the rebreather is correctly set.


10.    Keep a printed copy of the test numbers and associated error codes handy. The pre-dive routine includes many tests, and each test may have multiple different associated error codes.  Even as designers of the system and official test divers, we don’t have all the test numbers and error codes memorized.  Some test failures cannot be fixed while in the field; but many of them can.  However, before you can know how to fix the problem, you first need to know what, exactly, the problem is.  This is why it’s so important to keep a printed version of the test numbers and error codes (i.e., Appendix 1 of the Manual) among your dive kit, so an unexpected error out in the boat or elsewhere in the field can be identified and (ideally) solved.


Of course, there are many other tips for consistently passing the pre-dive tests, but these are the ones that we consider to be the most important, and associated with the most commonly encountered pre-dive test failures.  Over the course of this ongoing series of articles, we will no-doubt include other such tips and pointers, as they relate to whatever theme or story that article is about. We hope that you will find these articles useful, entertaining and (most of all) educational.  Until next time, we hope your dives continue to be fun, safe, and bubble-free!



Please join us for our next installment on May 2nd: Into the Twilight Zone