 |
 |
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.
|
|
We bought two diving cylinders at the Dive Show but cannot get them filled at our local dive shop because we are told that the cylinder-valves are made to EU standards, and that they need to meet current British standards. I thought the UK was part of Europe. What's this all about?
Janet Rutters
Cylinder valve thread specifications are causing almighty confusion in the UK |
There has been turmoil in the diving industry recently because Britain's Health & Safety Executive has elected to select a different standard (the Transportable Pressure Equipment Directive, or TPED) to that used by the rest of the European Union (the Pressure Equipment Directive, or PED).
Mainland Europe is where most diving cylinders and valves are made, but the TPED standard has been adopted by British IDEST cylinder-testing stations, causing something of a conflict. It's a question of unacceptable tolerances in the DIN thread when measured independently, but not the tolerance when mated to a regulator or built-in international A-clamp adaptor.
Some unscrupulous diving retailers have used this confusion as an excuse to retain tanks unless the owners buy expensive replacement cylinder-valves of a type made in the UK - but these too have since been found not to meet the TPED standard. In fact, few valves will do so once they have been in use for some time, and the threads start to wear.
Recently, some European manufacturers have solved the problem by making valves specifically for the UK market - which of course is in itself a breach of European law!
There are many European-made cylinders being used safely throughout the world that conform to the EU standard. It does seem strange that British bureaucracy should be out of step. That said, every compressor-operator has the right to refuse to fill any cylinder for whatever reason, because he or she is the one who might get injured if anything goes wrong. If your local dive shop refuses to fill your cylinders, you'll just have to go to one that does not.
If you have any further problems, go back to the retailer from which you bought the cylinders or to the distributor of that equipment, and avoid dealing with any other retailer who might be trying to take advantage.
I have a Nikonos camera with a 15mm lens that will focus only under water. I am told that this is because it has its own dome-port. However, if I use my digital SLR with a housing and dome-port, why will it focus in air as well as under water? Also, can I use a macro lens behind a dome-port?
How you focus a lens under water when using a dome-port depends on the size of the port |
The lower the lens-to-subject distance, the greater the lens-to-film-plane distance, so lenses come with an adjustment to allow for this.
The air in a dome-port works with the water beyond it to produce a virtual image apparently very close. The lens must be able to focus on this.
The distance of this virtual image depends on the dome-port's diameter. That's why some small ports require a wide-angle lens to be fitted with a dioptre lens to reduce their focal length, and so effectively increase the distance from the film-plane to the back of the lens relative to the focal length. It is because they cannot be racked out far enough otherwise.
Bigger domes have a smaller degree of curvature and put the virtual image further away, often within the normal focusing range of the lens, so no dioptre is needed.
Your Nikonos 15mm lens has a very small-diameter dome-port built in, so the virtual image is very close. The manufacturer provides a focusing range to accommodate this.
Without the air/water interface, no refraction occurs, and the lens sees straight through as normal. Your lens will not focus in air unless it is extremely close to a subject, as it is already racked out so far from the film plane.
However, if your SLR lens does not need a supplementary dioptre lens to focus on the close virtual image, it will retain its ability to focus at infinity through air as well as water.
The big focusing ranges of macro lenses allow them to focus both closely and at infinity when the lens-to-film-plane distance (usually) equals the focal length of the lens.
As to using your dome port with a macro lens, this is a big secret of the underwater-photography world, and it usually works! However, the diameter of a big dome usually proves to be inconvenient when getting close to subjects, and there may be distortion towards the edges.
I have just obtained a secondhand set of regs. They have an isolator valve on the octopus to cut air flow to the second stage. I can see merits in this but I am also concerned that, if a buddy needed to use the octopus, it is one more thing that may freak someone out, if the valve is off and they get an octopus with no air. What is your opinion?
Andrew Diggins
I guess you're talking about the Apeks Freeflow Control Device. This sits inline between the regulator and medium pressure hose and allows you to cut off the supply of air to a regulator.
However, bear in mind that, should a first stage stick open (as downstream valves tend to do if there is a problem), the build-up of pressure in the hoses must be allowed to escape and it will take the route of least resistance.
This is normally via the octopus rig, but it could be the direct-feed to a drysuit or a BC if that path is blocked - which could have disastrous results.
I recommend that you use this device only on your primary first stage, and use it for what it was designed to do, to momentarily cut off the air should you experience a free-flow. If your octopus is inclined to free-flow, get its demand valve adjusted.
Perhaps this was fitted to the octopus you bought because the previous owner was always planning to donate the second stage in his mouth.
I have noted in that configuring an octopus so that it is on a diver's left is popular. Is this only possible with regs that can be attached with the hose to the left-hand side of the DV? I have found that the AAS hangs upside-down, the face against me. This reduces the usefulness of the fluorescent front. Any tips?
Also, I have read about using exhaust bubbles when inflating delayed SMBs, to avoid temperature drop at the first stage and free flows. How does this work when using twins and a reg at ambient temperature rather than the chilly one being breathed on?
Toby Prosser
Rigging an alternative second stage with a standard-length hose from a port on the left side of your regulator first stage leaves most types inconveniently the wrong way round for you, but ideal for someone else who might need it.
The fluorescent cover can still be rigged outwards with the second stage inverted and is less likely to free-flow by accident, as it can rotate about its hose.
You're right that filling a DSMB using an alternative air source can precipitate an unwanted free-flow, but only in cold fresh water. The sea round the UK is rarely cold enough to cause icing. Big airflows cause a drop in temperature in the vicinity of the first stage.
Using exhaled air to fill a DSMB may be a good party trick, but it's difficult and time-consuming.
Why are fins called fins, and not flippers?
Bill Quinn
A diver flippers over the sand - it doesn't sound good, does it? |
My dictionary calls a fin "a thin wing-like projection of a fish's body, used for propelling a fish through water, balancing and steering". The same dictionary defines a "flipper" as "a limb adapted for swimming or a rubber foot-covering imitating an animal flipper, worn for underwater swimming".
So who said they shouldn't be called flippers, as indeed many Americans still call them? Perhaps the rest of us use the noun "fins" because we are uneducated and illiterate, but it does work better than "flippers" as a verb!
|
KEEP YOUR QUERIES COMING
Submit them, marked "Kit Q&A":
by letter, addressed to DIVER, or by fax on 020 8943 4312
by e-mail by john@divermag.co.uk
on DIVERNET's Equipment Talk page. This can also bring you rapid responses from other readers (though these should of course be treated with caution).
We regret that questions cannot be answered on the telephone or, generally speaking, replied to individually.
|
|
