Who looks out for you when you have diving equipment problems? Diver does. In a new quarterly series, John Bantin fields your queries about gear, and calls on the suppliers for help where appropriate. If you have a problem with kit, let him know.
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Forget the old negative stereotypes of neoprene and membrane drysuits - they no longer apply. |
I'm about to buy a new drysuit and am wondering which dump valve is the best option: a cuff dump, or a shoulder auto-dump? Most members of my club say auto-dumps are an accident waiting to happen, because you can close them at depth and forget to open them again. My local dive shop, however, recommends the auto-dump. If I ever want to sell the suit, I don't want to reduce my market by making the wrong choice. Also, the option to have both a cuff dump and an auto-dump is available for £20 extra. Could I close the auto-dump if I didn't want to use it?
Mark Gifford
Auto-dumps (I prefer to call them "constant-volume" dump valves) are not "an accident waiting to happen". They are mechanically very simple and now the most commonly supplied form of drysuit dump-valve.
You still have to know how to use one. Read the manual! The air will dump properly only if it is installed at the highest point in the suit, on the shoulder, and is not obstructed on the inside by an inappropriate undergarment. With some suits you have to raise your shoulder to get the last bit of air out.
Manufacturers usually recommend that you close the valve by only a quarter of a turn. If you close it down all the way it can't release air when you ascend. Also avoid making any fast ascents, thereby causing the valve to fail to dump the expanding air fast enough (and for lots of other reasons too).
Cuff dumps are even simpler, and rely on the relevant cuff being kept at the lowest point at all times during the dive except when you want to dump air.
However, if you want to raise that hand for any reason, such as holding a camera, looking at a wrist gauge, or even winding a reel, you will lose both air and buoyancy.
There seems little point in fitting both types of dump-valve, because you can close down the auto-dump but not the cuff dump, so will be unable to close the cuff dump if you don't want to use it.
How do I know if my tanks are still in test? There are lots of details stamped on them, but all I want to know is whether I can get them filled without a hitch. I purchased them secondhand and they are marked for use with nitrox even though I am only certified to use air. However, I do normally pay the extra to have them filled with double-filtered clean air.
Chris Smith
Dive tanks are safe to fill within 30 months of manufacture or of their last test |
I asked John Stanton of G&H Engineering & Diving to help me with the answer to this. If your tanks were tested while in the possession of their previous owner, they will have a date stamp on them. Otherwise you must look around the neck for the date (year and month) of their manufacture. Tanks are safe to fill 30 months from the date of manufacture or from the last test.
A properly managed compressor should filter out hydrocarbons but double-filtering ensures that no moisture passes into the tank. Otherwise elevated levels of oxygen (in nitrox) can cause premature rusting inside steel tanks.
Aluminium tanks are now subject to a special test procedure to identify cracking. All tanks must now have their contents identified by the recently introduced standardised label.
I've talked to plenty of people and everyone seems to have a different opinion about drysuits. So which is better, neoprene or membrane?
Richard Halifax
This is a complicated question and the answer is to choose the one that works better for you!
It used to be that neoprene suits were damp but hard-wearing, while membrane suits were dry but fragile. All that has changed. Some membrane suits are extremely durable and modern neoprene suits can keep you extremely dry.
Even the problems of buoyancy change at depth with neoprene have been overcome by pre-compressed neoprene, while crushed neoprene is used in some of the toughest membrane suits. Take a look at our comparison test of drysuits (www.divernet.com/equipment/dsuits2003/dsuittests1.htm) and be sure to get a suit that fits you!
What are your recommendations for a back-up computer? I use a Suunto Vyper for main, but reckon it's worth having another just in case. Is it worth getting one secondhand, if I can find one?
(From Divernet Forum)
Many divers now carry a back-up computer, but it is often the older purchase that they have replaced with a new one. If you carry more than one computer, it makes sense to ensure that they both work with the same algorithm, or mathematical model. Otherwise each might require you to manage your dive in different ways.
If you want to pair your Vyper with another Suunto computer, it should be either a modern Stinger, Mosquito, Gekko, Cobra or Vytec, and not an earlier model. For those who use a different make altogether, check our most recent computer comparison on Divernet to get an idea of which will make a useful pairing. Naturally, it's best to have two the same.
I am in the process of buying twin 12s and was wondering how much extra lead I'll need. I understand that everyone is different and will be doing a buoyancy-check prior to diving. My question is, what sort of weight is going to be a good starting point?
Glynn Jones
Alas, wearing twins means more weight on your back before diving and possibly more weight on your belt, too. How much depends on the type of cylinders you use. Assuming that you already know how much weight you are comfortable with in your suit and the rest of your kit when using one tank, you will need to add weight to counteract the extra 12kg-plus of lift provided by the displacement of the additional tank, less the weight of that tank and its air. The 2400 litres of air in a typically full tank weighs around 2kg.
Getting your weight right with a twin-set relies on buoyancy checks. |
It is always surprising how much extra weight you need when using aluminium twins, and how little when using steel. Steel cylinders also vary in weight according to their manner of manufacture.
As I am constantly changing my equipment configuration, I need to do a buoyancy check at the beginning of every series of dives. A buoyancy check with full cylinders and no air in BC or suit should leave you with your chin above water when you inhale, and just below the surface when you exhale.
Once you have achieved correct weighting for that, add just sufficient lead to counteract the amount of gas you will use during the dive, so that you can stop comfortably at 3 to 6m depth. Avoid being over-weighted.
I am thinking about buying an air-integrated computer but I am told that the diver's breathing-rate affects its decompression calculation. If you needed to share air, wouldn't this mean that the extra inhalations of your sharer would increase deco-times just when you probably didn't need it?
(From Divernet Forum)
An air-integrated computer normally calculates a "remaining air time" based on a calculation of your current depth and previous breathing rate recorded during that dive. This can be compared with the time to surface at the prescribed rate of ascent, taking account of any deco-stops you might need to make.
It is a good idea to keep the first figure larger than the second! Normally the remaining air time reaches zero when your tank pressure reaches the predetermined reserve pressure, which might be 30 or 50 bar.
I'm not sure how or if the rate of breathing affects your decompression requirement, and I know of no computer which takes your heart-rate into consideration, which might be more relevant.
It seems that many technical divers argue about failure points in equipment. What could fail and how far should we go in avoiding these failure points?
(From Divernet forum)
Anything mechanical or electrical can fail, and once you reach depths from which an emergency swimming ascent is impractical or unsafe, you need to consider what might go wrong and what to do about it. So the ethos of technical diving is about redundancy and self-sufficiency.
For example, O-rings are a possible point of failure, so many deep divers try to avoid relying on them. Most are agreed that DIN regulator connections are less likely to cause problems, but O-rings are used throughout the mechanical parts of scuba equipment.
Some divers prefer to use regulator first stages with no more gas ports than they actually need, citing the danger of additional O-rings in the blanking plugs. Some divers believe in keeping it simple with independent twin tanks, while others prefer the option of manifolds with or without isolator valves, despite the increase in O-rings in that equation.
It's down to the individual to make an informed personal decision. However, well-maintained diving equipment fails less often than you might expect.
How long will it take for a nitrox mix in a manifolded twin-set to be fully balanced, so that exactly the same mix is in both sides of the twin-set?
Bryan Salt
Oxygen and air can take longer than you think to blend in a twin-set, so check your nitrox mix with an analyser just before diving |
Assuming partial-pressure blending in your own tank, the oxygen goes in first, and will fill each cylinder with the same pressure, as will the following top-up of air.
Assuming that for some reason the isolator-valve was closed, and only one tank was filled, it will take time for the gas to move across and equalise when you open the valve. If you leave the manifold isolator-valve of your twin-set fully open, the gases should be mixed after an overnight interval.
There will be a degree of turbulence within the gases as this happens but gas-analysis expert David Tracy tells me that if you analyse a nitrox mix immediately after partial-pressure blending, you will get a richer mix recorded than if you leave it for at least eight hours.
He says it could take more than 24 hours, depending on such things as the percentage of O2 in the mix, the ambient temperature, and whether the tank was agitated or not. So always analyse your gas immediately before diving.
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