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 www.hse.gov.uk/research/rrhtm/rr341.htm.
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.