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July 2005 Vol. 31, No. 7   RSS Feed for Undercurrent Issues
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Is Your First Stage Up to the Job?

some may not deliver

from the July, 2005 issue of Undercurrent   Subscribe Now

When most people think about two-stage regulators, they consider the second stage the “business end.” After all, that’s where the mouthpiece delivers air to the diver, where breathing ease may be adjusted (on some models), and where the most common kind of regulator failure -- free flow -- occurs.

But the lowly first stage is also a sophisticated engineering marvel. It’s here that breathing gas is reduced from tank pressure to an intermediate pressure around 140 psi before being fed to a diver at ambient water pressure. Coping with pressures of three thousand psi or more makes special demands on a first stage, and problems could potentially lead to catastrophic failures.

Modern first stages are service rated, just like scuba tanks, usually from 3,000 to as high as 4500 psi, according to Mike Ward, President of Dive Lab in Panama City Beach, Florida, which conducts performance and engineering studies on diverworn life support equipment like scuba regulators. Look for the service rating on the first stage yoke, the dust cap retainer, or in the owner’s manual. Most “A” yokes are only rated to a maximum of 3500 psi and even the European DIN fitting can cause problems because not all DIN fittings are rated for pressures above 3625 psi. For higher pressures, you have to switch to the 300 bar DIN fitting. It’s unsafe to use a tank that is filled above the pressure rating of your first stage (some technical divers now use 4500 psi tanks, but only with compatible regulators).

As we mentioned in the March Undercurrent, when opening a tank valve, it’s best to crack it slightly at first, then stop and allow your regulator to pressurize slowly, to avoid unnecessary pneumatic shock and adiabatic heating. “When opening a valve quickly the temperatures inside the first stage can momentarily reach more than 800 ??F,” says Ward, hot enough to melt the seat or even cause a regulator fire. Once your gauge shows your system is pressurized, open the valve the rest of the way.

Octopus Performance Anxiety

Some first stages may not be up to the task of delivering air to two divers at once, especially if tank pressure is low, you are in deep water, or your breathing rates are in sync or elevated due to stress (all reasonable scenarios for an air-sharing situation).

The English have a government agency, the Health and Safety Executive (HSE), which is responsible for the regulating risks to work health and safety, similar to OSHA in the U.S. The HSE recently issued a report that concluded “reduced breathing performance was experienced when using low cost/performance first-stage regulators compared to high cost/performance models.” Entitled Breathing Performance of ‘Octopus’ Demand Diving Regulator Systems, the report is available at

Mike Ward agrees. “It’s possible two divers could over-breathe a primary and octopus.” European standards call for a second stage to provide air to a diver at a breathing rate of 62.5 liters per minute. At this rate, a diver should be able to breathe comfortably down to sport diving limits (130 fsw), and with 725 psi remaining can probably complete a 30 foot per minute nondecompression ascent from 130 feet. The U.S. has no such standards, but Ward says, “Most good first stages flow in excess of 1100 liters/min. at 725 psi,” which is plenty to allow for a 62.5l l/m breathing rate. Some can put out as much as 1700 l/m, but a poor performer may flow only 6-700 l/m, according to Ward. A dive shop equipped with a flow meter can measure the flow coming from the hose to the second stage, but, says Ward, “there’s much more to it, and knowing exactly how the first stage reacts can only be determined on a breathing simulator under various conditions. Dive shops are not equipped to properly measure or evaluate regulators for actual breathing performance.” Another problem is that no one has set up U.S. testing protocols to determine the proper first stage output to drive low pressure inflators, an octopus and a primary second stage simultaneously.

It’s unsafe to use a tank that is filled above
the pressure rating of your first stage

The HSE recommends that “a high-performance first stage should be acquired if an octopus rig is to be based on it.” Plus, says HSE, “Any two demand valves set up as octopus partners should be of similar performance.” That goes for primary second stages as well; owners who may have attached a newer second stage to an old first stage should consider upgrading so both units are of equal performance. Finally, HSE cautions, “Older valves, or ones where performance may have degraded, should not be used.”

If you suspect your regulator cannot carry the load, upgrade now to compatible first and second stages, rather than become a test subject yourself.

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