The best twenty-five dollar investment you can make
How over-weighted is the average technical diver?
Well, the smart-assed answer goes something like: “How over-weighted does he want to be?” but let’s not go there right now.
One of the most common errors one sees among candidates for beginner tech classes is a truly poor understanding of just how much (or how little) ballast is needed to be “perfectly weighted.” Perhaps the problem starts with open water sport divers throwing on too much lead right from the get-go, but whatever the cause, the issue is real and unresolved when the average punter shows up for his intro-to-tech, cavern, or deco class.
For any diver, but a techie in particular, incorrect weighting can make getting full control of buoyancy a real challenge. Over-weighting will put any diver at risk of getting pinned to the bottom in the event of some sort of massive wing failure, but more often means that in real terms and on every dive they are forced to complete the shallower portions of their ascents (the staged decompression parts), with way too much gas moving around in their wing and or suit. This makes holding trim and water position a challenge even for a diver with lots of experience; for a newbie, it can be downright disheartening.
In some cases – much less common – the opposite is true and newly minted tech divers start their dives with enough ballast to descend but finish their final stops fighting to stay below the decompression ceiling, grabbing rocks, ascent lines and anything else they can lay hands on to prevent bobbing to the surface.
Unfortunately, one incident of finishing a decompression dive with too little ballast is an open invitation to most folks to opt for the quick fix and add several kilos of additional lead. From their next dive onwards this cultivates a habit of diving grossly over-weighted.
Now, let’s leave those general assumptions aside and ask another question: “When did you last do a buoyancy check?” It can have been a practical one – you know, floating at eye level, breathe out and so on – or arithmetical – manufacturer’s specs on your cylinders, guesstimates on the rest of the gear package and a fudge-factor for good measure. Either way, when was the last time you did it?
One more question for all you open-circuit techies doing dives that require staged decompression – promise the third-degree stops after this. “What is the buoyancy shift in your primary and deco cylinders when everything goes as planned, and how does that compare with the situation if you had to share air with a team member for the whole of the ascent and decompression?” (If you dive CCR, you’re not off the hook. What’s your situation if your unit partially floods and you have to finish the dive on your bailout gases?)
Just in case you are new to technical diving and are wondering what I mean by buoyancy shift, it is the difference between the buoyancy for a cylinder when it was full at the start of the dive and its buoyancy characteristics at the end of the dive when the gas it contained (or a portion of the gas it contained) has been consumed. To calculate this, we assume the mass of one litre of air to be 1.2 grams. (For those who strive for more accuracy a better figure to take would be the figure of 1.247 Kg/cubic metre for air at 10 degrees C, but the 1.2 grams per litre number works OK. (Imperial unit divers, sorry folks, don’t have a bloody clue but I think one cubic foot of air has a mass of 0.0807 lbs.)
Hate to say this because I know how unpopular it is with new tech divers when it’s brought to their attention, and certainly a fair percentage of “experienced” tech divers just don’t wish to hear it but… your risk management plan has a big hole in it if you do not have REAL figures to answer these questions.
Now at this point, you may be saying to yourself: “This guy is dreaming because I hate maths!” or “Good grief, he’s right and I need a smart phone with a better calculator.” As luck would have it, there is a solution that may be acceptable for both general schools of thought.
Visit your local outdoor/fishing store and find the digital fish scales. They will be right beside the old-fashioned traditional analog version consisting of a hook, spring scale and a ring to hold the whole contraption – and a fish – up for weighing. The beauty of a digital scale is accuracy – one gram increments – multi-functionality – one I use has a flashlight built-in and gives air temperature (not sure if there is a connection there) – and readings in Imperial measurements as well as SI units. FYI, all these features come cheap. I purchased mine in a Bass Pro Shops store (a large North America outdoor, hunting, fishing outlet) for less than $25.
The beauty of the whole operation is that you can forget about all the guesswork and fudge factors; and you can put your calculator back in your pocket. Simply assemble your kit, put it in the water and take a reading of its in-water weight with your new toy.
One thing to be careful of – did I tell you that my current digital fish scale is my second one – is putting too much weight on the scale and busting it. My old one had a maximum weight load of around 25 kilos (about 60 pounds), which by the way is not enough to support the weight of a fully charged rebreather and bailout bottle on dry land without breaking. I now understand this emphatically. The secret to longevity is to lower whatever is going to be weighed into the water and THEN hooking the scale into a DRing or bolt snap.
You may not feel that being over- or under-weighted is a problem, and that’s fine. However, if you agree that both states present us with unnecessary challenges, I can thoroughly recommend adding a digital fish scale to your dive kit. It may just be the best investment you can make for under $25!