Monthly Archives: March 2016

  • What Causes a Regulator to Free-flow?

    It’s very annoying, isn’t it? You jump into the water and your regulator starts to free-flow. It could even be life-threatening if that happened at depth. It’s as if someone has pushed the purge valve of the regulator in and held it there, losing you precious air. What causes it to happen?Dive2005B

    Modern day regulator manufacturers compete with each other to give the diver the most efficient and natural way of breathing. When you inhale from your regulator, the drop in pressure inside the body of the second-stage of the regulator drops and allows the second-stage valve to open, supplying you with air. Regulator designers try to make the valve as finely balanced as possible so that it takes the minimum amount of effort to pull it open against its closing spring, the spring that holds it shut and stops the gas escaping from your tank. Modern regulators can be so finely balanced in this way that it is often more effort to force the exhaust port open when you exhale than inhaling. (ANSTI breathing machines prove that!) So why do they sometimes free-flow?

    If you pull very gently on a regulator, the second-stage valve only opens a little to let air pass. The more you suck or the deeper you are the more it has to open to satisfy your needs.

    The depth the diver is at affects the pressure-sensing diaphragm. It operates a lever that pulls open the second-stage valve and also doubles as a ‘purge valve’. If for some reason the purge valve gets pushed in for a moment as it might when passing from air at the surface to water (a sudden increase in hydrostatic pressure) the valve opens and lets a whoosh of air past the back of the pressure sensing diaphragm. This fast moving air, just like the air moving fast over the top of an aircraft wing, causes a drop in pressure directly behind the diaphragm. This causes the valve to open even more and – viola! It’s exponential. The more the valve opens the greater the drop of pressure behind it and that leads to it opening the valve even more, resulting in that annoying rush of lost air.

    Luckily, that only normally happens at the cusp between air and water; that is to say at the surface. Putting your thumb over the mouthpiece is usually sufficient to cause a momentary increase in pressure inside the second-stage body to stop it. It’s annoying when it happens at the surface but it could be more than annoying if it happened at depth. Alas, in water that is colder than 10°C, it can happen at any depth if the mechanism of the valve is iced up or affected by ice. This is when it gets more serious than just annoying.

    Why is there ice? When air (or any other gas) is depressurised, it experiences a drop in temperature alongside the drop in pressure. The converse is also true. When a tank has been freshly pumped full, it feels hot to the touch.

    The water you are in may be at 10°C together with the air in your tank but that air in the tank might be as pressurised as much as 200bar. The first-stage drops it down to eight or ten bar more than the pressure of the water it is surrounded by. That’s a huge drop.

    It could easily cause a drop in temperature as much as 20°C and if you are in cold fresh water at 10°C you realise that it equates to minus 10°C for the air passing through the regulator’s first stage. This causes the water in its immediate proximity to freeze.

    Luckily, seawater rarely gets colder than 10°C around our temperate coasts but it’s a fresh water inland sites you might experience this problem – and it can be life threatening.

    You should have been taught how to breathe from a free-flowing regulator on your first diver-training course. The remaining air in your tank will give you time to get to the safety of the surface. Every diver should know how to do that.

    If you are in the habit of diving at inland sites, get a regulator designed for the job. They usually have first-stages that are environmentally sealed, with no working part coming into contact with the water, and they include extra metal to act as a heat sink to transfer what little warmth there might be in the water to the much colder air coming from the tank. Ask about that when you next buy a regulator. Ocean Leisure stocks cold water approved regulators by both Apeks and Scubapro.

  • Dome Ports for Underwater Cameras

    Serious underwater photographers shoot their wide-angle pictures from behind dome ports. What’s that all about?nauticam4.33domeport

    A dome port has no effect when viewing through it with the same medium (air) on both sides but once you put the outer surface in contact with water the refraction between that and the air in front of the camera lens comes into play. What happens is that a virtual image is formed ahead of the dome port and the camera lens is allowed to focus on that instead of the real subject some distance ahead. The effect is to produce an image that is more saturated in colour. The problem comes when you realise that this virtual image is curved and the distance in front of the lens is quite close.

    Some photographers get disappointed when they find that their expensive wide-angle lenses are no longer giving images that are sharp from side to side and resort to fitting them with high-strength dioptre close-up lenses in order to get them to focus close enough to get this virtual image sharp.

    That’s because most expensive wide-angle lenses are rectilinear designs that have a very flat field-of-view, something that is admirable when using them solely in the medium of air. At the same time, few focus close enough.

    That’s why you’ll see top underwater photographers using full-frame fish-eye lenses behind dome ports. Often these give very disappointing results in air but in conjunction with a dome port their aberrations actually become an advantage. A curved field-of-focus is a positive bonus where trying to make a sharp record of a curved virtual image.

    Dome ports come in different diameters with a different radius to their curve. The bigger the dome port the further in front of it the virtual image is formed and the easier it is to get the camera lens positioned at the right point behind it. That said, big dome ports can be unwieldy to use, hence the popularity of mini-domes. The smaller domes produce their virtual image much closer to the front of the port and it’s really important that the front node of the lens is positioned in the correct place relative to it.

    The front node is not something you can see. It’s an optical term. Camera housing manufacturers have done empirical tests with most popular lenses to confirm what spacer ring might be needed to allow the dome to be positioned in the right place relative to the camera. They provide lens/port charts for this purpose and your underwater photography equipment dealer will have that information if you cannot find it on-line.9inchzendome

    Dome ports can be made of acrylic material, polycarbonate or glass. Glass is the most expensive and the most hard wearing but if you are unlucky enough to scratch or chip it, there is nothing you can do apart from clone out the unwanted mark in your pictures, later with software on your computer.

    Polycarbonate is inexpensive and lightweight but the same applies as glass should you damage it. Acrylic ports have an advantage in that the material has the same refractive index as water so minor scratches become invisible in your shots underwater unless you happen to take a picture into the sun. Acrylic is very easily scratched but in the same way it is very easily polished.

    Simply take a piece of fine grade abrasive as used in finishing the paintwork of cars and gently cut back the scratch until they are has become an evenly matte surface. Then polish it back to clear acrylic using some proprietary silver polish wadding. It takes some elbow grease but you will be rewarded with a dome port that is immaculately clear of marks.

    Some say that glass ports are optically superior to acrylic ports. I have owned and used both including an optically coated glass port that I imported specially from Japan and can tell you that the pictures taken with both this and a top quality acrylic dome are indistinguishable.

    A manufacturer like AOI makes a range of glass and acrylic dome ports for Olympus system compact camera housings so the choice is yours.

    You may find that a large glass port is easier to use for those over-and-under shots taken at the surface because droplets of water are less likely to cling to the glass. A large dome port certainly helps get those type of pictures because, remember, the lens has to focus on a nearby virtual image for the under part and the over part is in air, probably at infinity. You need to use very small lens apertures to get the huge depth-of-field needed or a split close-up lens that affects only the bottom part of the lens. These rarely fit on fish-eye lenses.

    Some underwater photographers report good results using smaller domes for these over-and-under pictures but invariably they are in bright sunshine that allows them to use the smallest lens aperture with perfectly calm water, but they normally need to make an exposure adjustment in digital post-processing to get both halves of the picture in balance.

    The important thing to remember is that when buying a port, you will need the right extension ring to position it correctly relative to the lens. Alas, it’s not something you can confirm by taking a picture when in the equipment sales room and not underwater.977685_632320780114194_1262934048_o

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