There are various tech-diving disciplines. Broadly, they
can be categorized in terms of depth and distance.
Depth — Tech diving is pushing deeper and deeper:
200 feet is an intermediate training dive, and 300 feet is
not unusual. Very experienced tech divers now dive to
600 feet.
To about 300 feet, tech divers can use open-circuit
equipment — typically four to six cylinders with various
gas mixes to reduce oxygen toxicity risk, manage gas
narcosis and optimize decompression.
In open-circuit diving, deeper depths mean faster gas
consumption. Carrying enough gas becomes increasingly
cumbersome and expensive, so below 300 feet (and
increasingly, below 200 feet) closed-circuit rebreathers
(CCRs) have become the first choice. CCRs recycle
breathing gas, which saves a lot compared to open-
circuit, and they adjust the gas mix so it is optimized to
the diver’s present depth. A brief dive to 300 feet would
consume more than 140 cubic feet of gas on open-circuit
scuba but less than 5 cubic feet with a CCR.
With either technology, the main “deep gas” is trimix,
a blend of oxygen, helium and nitrogen. Helium is used
because it’s nonnarcotic and less dense than oxygen and
nitrogen, which means reduced breathing effort at depth.
A common trimix at 300 feet, for example, is TMx10.5/50
— 10.5 percent oxygen, 50 percent helium and the
balance nitrogen. This gas cannot support life at the
surface and thus requires special handling considerations.
For decompression, tech divers use trimix, enriched
air nitrox and/or pure oxygen. As they ascend, open-
circuit divers progressively switch to cylinders with higher
percentages of oxygen and lower percentages of helium
and/or nitrogen. CCRs raise the proportion of oxygen in
the breathing gas. Both of these methods make for more
efficient (i.e., quicker) decompression. High-oxygen gases
are toxic at depth, so tech divers follow special procedures
to prevent breathing the wrong gas, which can be, and has
been, fatal.
Distance — Tech diving also pushes limits horizontally,
mainly in cave diving. Tech divers go thousands of feet
into caves using open-circuit scuba. The technique they
use is to wear “stage” cylinders, use one-third of the gas in
each, switch to the next and then secure the used cylinder
for retrieval while exiting. This continues until the divers
leave their last stage behind and the team explores
using its main cylinders (at least two cylinders with two
independent regulators). With four stages (six cylinders
total) cave divers can get more than half a mile in before
turning for home.
As distances get longer and depths deeper, carrying
gas becomes a challenge, especially when decompression
is part of the dive. Leading-edge divers may use large
diver propulsion vehicles (DPVs, or scooters) to tow
them along with more cylinders than they could swim
with. Alternatively, support divers might place full stage
cylinders in the cave so “push” divers can exchange used
for fresh and go farther. Open-circuit will likely remain
dominant in cave diving, but CCRs are becoming popular
for accommodating distance as well as depth.
Differing depths, distances, technologies and
environments combine into different types of tech, such
as deep trimix wreck diving using open-circuit gear or
long-range exploratory cave diving with CCRs.
COMPARATIVE RISK
Is tech diving riskier than mainstream recreational diving?
Absolutely. If you have an hour of decompression or
you’re in a cave, you can’t ascend immediately in the
event of a problem. This alone adds risk. Tech diving also
has more inherent risk due to complexity of equipment
and procedures. The chain of errors leading to a serious
accident is much shorter than in recreational diving.
Tech divers manage this risk with added equipment
(at least two independent life-support systems at all
times) coupled with more stringent training and a deeper
understanding of the risks. Dive incident statistics suggest
that tech divers manage the added risk rather well.
But while the incident rate is low, it is not as low as in
recreational diving. If you take up tech diving, you must
accept that you are taking on more risk.
TECH CONFIGURATIONS
There are three basic tech-diving configurations.
BACKMOUNT OPEN CIRCUIT
:
The original tech setup;
the modern configuration consists of:
• Double cylinders with a valve that accepts two
independent DIN regulators. You can shut down
either and access the remaining gas from the other. In
deep diving, your doubles hold your main (“deep”) gas.
• One or more stage/deco cylinders. (Stage
cylinders extend bottom time; deco cylinders hold
decompression gas). All are marked with the gas they
contain and its maximum breathable depth.
• Harness and rear wing-style BCD
SIDEMOUNT OPEN CIRCUIT:
Originally for diving in
small caves, sidemount uses single cylinders mounted on
the diver’s sides. It is becoming more popular in open-
water because of some of its cylinder-handling benefits.
• Two main single cylinders, one per side. These stay
with you at all times.
• Stage/deco cylinders
• A special sidemount harness and BCD
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