STRONG OPINIONS IN PRINT sometimes interfere with a company's ability to sell its products. When that happens, protests are vigorous.
     Scubapro took exception to some words I used recently in an otherwise glowing report of its MK20/S550 regulator in Diver Tests (March).
     The folk from Scubapro objected to me saying that "the S550 is unashamedly a warmwater regulator" and "unsuitable for use in Stoney Cove in winter".
     Gilbert de Coriolis, the manufacturer's "Directeur Technique", says I am wrong, and that Scubapro piston-design regulators are perfect for use in cold, fresh water. Their design allows them to get as much water as possible pumped past the working parts, which he says helps stop them freezing.
     He also tells me that divers are not to be trusted in the judgment of these matters, and that only a regulator-testing machine such as Scubapro's, built by ANSTI, gives a true test result.
     De Coriolis says all Scubapro regulators have TIS (Thermal Insulating System), and that this virtually eliminates the potential for freezing. He also claims that only Scubapro regulators have achieved EN250 Cold Water certification at 300 bar, and that the technopolymer and carbon fibre body and internal components of the S550 help prevent icing.

HE MIGHT WELL BE RIGHT. He is an expert in these matters; I am merely a witness. But I do a lot of diving in a wide range of conditions, with many experienced divers. When relevant I refer to their input as well as my own experiences in Diver Tests. So is de Coriolis right, and to what extent is the EN250 Cold Water test realistic?
     When the air pressure in your cylinder is reduced, as it is when it leaves the cylinder and passes through the first stage, it loses heat. It loses more heat when pressure is reduced again at the second stage. The air becomes very cold indeed.
     Britain's seas rarely become cold enough to cause a regulator freeze-up, but if you dive in water at less than 5°C, as it could be in a freshwater inland site in winter, cold air could cause water to turn to ice at those points where it comes into contact with the chilled regulator.

ICE FORMS AT 0°C, SO A TEMPERATURE DROP of only a few degrees could cause freezing. Even the moisture in the air you exhale can form ice. Ice crystals could cause the mechanical parts of your regulator to jam, and if modern regulators jam it's in the open position, increasing the air flow and making matters worse.
     The effect is an unstoppable free-flow, with disastrous consequences for your remaining air supply. You can still breathe from a free-flowing regulator, and every diver should have been taught how to do so.
     Air-sharing from a buddy's octopus doubles the air flow on that regulator. That can cause the octopus, presumably also close to freezing, to go into an unstoppable free-flow too.
     The only answer is to head for the surface, which is why when diving in such conditions you should not undertake dive plans requiring decompression stops. You must also be sure the air in your tank is moisture-free and that it has not been chilled by, for instance, being left in your car overnight. Don't exhale into your regulator before submerging. Avoid circumstances where you demand large quantities of air.
     There is also the question of using the right regulator for the job. Scubapro apart, most regulator manufacturers seem to believe that a diaphragm-design first stage, insulated or kept completely dry by an environmental sealing kit, is best for use in cold water.
     They use non-stick parts and add metal as a heat-sink to their second stages. The metal transfers whatever heat there is in the water to those parts of the regulator chilled by the much colder air.
     Brian Bickell of Poseidon says that this manufacturer came first, second and third in US Navy Experimental Diving Unit tests. "All first stages had environmental seals fitted. Metal second stages act as heat-exchangers between warmer ambient water and cold incoming air from cylinder and first stage."

ANDY DAVIS, DESIGN MANAGER OF APEKS, tells me: "It's important that the first stage medium-pressure spring and mechanism is kept dry. We chose a dry-sealed chamber system."
     Aqua-Lung makes the Cousteau D Glacia, Titan LX Supreme and Titan D Impulse Cryo, all of which have environmentally sealed diaphragm-type first stages and heat-exchangers on the second stages. "The use of metal heat-exchangers on the second stage will help the regulator resist freezing," says the company's Roger Poll. "The environmental dry-seal reduces the likelihood of freezing on coldwater dives."
     Beuchat states in its brochure that its VX10 Iceberg first stage has "an original patented revolutionary environmental thermal isolation system" and its second stage "a unique thermal radiator".
     Mares (presumably encompassing stablemate Dacor) makes diaphragm-type regulators. "All-metal second stages and technopolymer second stages with metal inserts, thanks to their higher thermal conductivity, help to reduce freezing," says the manufacturer. "For particularly demanding conditions, first stages can be fitted with a 'CWD' kit which completely isolates all the moving parts from contact with the water."
     Sherwood, promoting its Blizzard, says it has "a heat-sink to retain the warmth of your exhaled breath, helping prevent freeze-up."
     Oceanic SW supplies its DX4 and CDX first stages in environmentally sealed configurations because "our product testing and independent CE testing has shown that properly environmentally sealing a regulator makes the product more reliable in coldwater conditions."
     James Munt, formerly of Apeks and now with Cressi-sub, says: "I believe a dry-sealed first stage and heat exchangers on the second stage do help a regulator resist freezing."
     Have they all got it wrong? Scubapro seems to think so, although even it makes a balanced-diaphragm first stage, the MK18, which it says is "completely sealed to provide unparalleled performance in challenging environments".

SCUBAPRO IS NOT ALONE, HOWEVER, in having faith in an exposed piston design. Martin Parker, MD of AP Valves, tells me his company is developing a regulator that will expose as many of the working parts of the first stage to a flow of water as possible to discourage freezing, while adding that he has yet to test the theory.
     What happens in practice? In Britain, it is only freshwater inland sites that get cold enough to cause free-flow problems. Martin Woodward, director of Stoney Cove Marine Trials, tells me he has seen many cases of unstoppably free-flowing regulators over the years.
     In pursuit of better test results using machines, he reckons that at one time manufacturers lost sight of the problem of human divers in cold fresh water. For some time he recorded all free-flows that occurred at Stoney Cove, though he has not done so for the past year.
     He tells me that things are much better nowadays, but when I push him to reveal whether any brand of regulator cropped up particularly often in relation to unstoppable free-flows (presumably due to freezing) at the Cove, he immediately suggests a piston design.
     Woodward is also a director of a company that imports Dacor regulators, so it could be said that he is not an impartial witness. However, this anecdotal evidence might lead us to question the credibility of the EN250 Cold Water test.
     "We try to be as realistic as possible," says Ian Himmens of ANSTI Test Systems. "These tests are not bullet-proof, but they form part of the overall jigsaw that makes up a regulator's performance.
     "The EN250 Cold Water criteria did not stipulate any humidity level for the exhaled air, and this omission has recently been rectified with the new EN250:2000 Cold Water standard. Even these new criteria allow a regulator to free-flow for a short time during testing."

IT SHOULD BE NOTED THAT COLDWATER TESTING for EN250:2000 Cold Water allows for the water to be at +4°C, and that water at Stoney Cove in winter is often at that temperature or even less.
     Also, a regulator needs only to perform under test conditions for a limited amount of time to meet the criteria.
     Machine-testing is all very well, but human divers often do everything wrong, and I have never seen an ANSTI machine in a drysuit going diving at Stoney Cove.
     How dry is the air in the cylinders of divers who dive in that cold fresh water? How damp is their exhaled air? How long are their dives? Some divers do leave their cylinders in their cars overnight, take anxious breaths before ducking under the water, or purge their regulators before replacing them under water.
     Death from drowning due to loss of air through an unstoppable free-flow is never put down to regulator design, as it is impossible to prove after the event.
     When the air flow stops, the problem ice melts. Like decompression illness, a regulator freeze-up cannot be detected until it happens. You will never know how close you come to disaster.