TDI Decompression Procedures Diver

Gas volume is calculated by multiplying the RMV by time and absolute pressure (ATA). For 21m (3.1 ATA) it is 15 L/min * 15 min * 3.1 ATA = 697.5 L, and for 15m (2.5 ATA) it is 15 L/min * 20 min * 2.5 ATA = 750 L, totaling 1447.5 liters. The incorrect options fail to account for the ambient pressure at depth or mistakenly use only surface consumption rates.

If decompression gas is lost and a buddy is unavailable, the diver must use their bottom gas to complete the decompression. This requires extending the decompression schedule using contingency run times cut for the bottom gas. Ascending directly or using the original Nitrox schedule on bottom gas will lead to severe omitted decompression.

The M-value defines the maximum theoretical over-pressurization a tissue compartment can tolerate without forming symptomatic bubbles (DCS). Exceeding the M-value means the gradient between tissue tension and ambient pressure is too high. The other options describe MOD, ascent rates, or deep stop depths, not tissue overpressure limits.

TDI recommends maintaining conservative CNS limits, and approaching 85% leaves very little margin for error if a delay occurs during decompression. Adjusting the plan to lower the PO2 or extending the surface interval mitigates the risk of oxygen toxicity. Proceeding without changes or relying on medication are unsafe practices.

The breakaway connection on the lower bolt snap is an essential safety feature in technical diving, allowing the diver to cut the line and free themselves if the cylinder gets permanently entangled. Rigid mounting prevents this emergency release. The other options do not accurately describe the primary safety purpose of the breakaway rig.

In-water recompression is highly dangerous and explicitly discouraged by TDI and major training agencies due to the risks of oxygen toxicity, hypothermia, and worsening symptoms underwater. The standard protocol for omitted decompression is surface oxygen, monitoring, and contacting EMS.

The Rule of Thirds is a conservative gas management strategy designed to ensure sufficient gas to exit an overhead environment or return to an anchor line, including a reserve for a buddy’s emergency. Open water drift dives often use different gas management strategies (like ‘all usable minus reserve’), making the Rule of Thirds specific to certain dive profiles.

Maintaining a horizontal attitude at the same depth ensures all team members are executing the same decompression schedule and can quickly communicate or share gas if an emergency arises. Vertical stacking or single-file positioning breaks visual contact and complicates emergency responses.

In the GF model, GF Low dictates the percentage of the M-value allowed at the deepest part of the ascent, effectively controlling the depth of the first decompression stop. GF High controls the surfacing M-value percentage, dictating the length of the shallow stops.

Gas switching errors (breathing the wrong gas at the wrong depth) are a leading cause of technical diving fatalities due to oxygen toxicity. The strict protocol of verifying depth, identifying markings, confirming with a buddy, and tracing the hose ensures the correct gas is breathed. The incorrect options introduce severe risks, such as losing access to back gas or violating depth limits.

Entanglement is the most significant immediate physical risk when deploying a DSMB; if the line catches on the diver and the spool jams, the buoyant force of the bag can drag the diver to the surface, causing DCS or barotrauma. Proper technique (keeping the spool clear of the body) mitigates this. The other options exaggerate or misidentify the primary mechanical hazard.

Nitrogen narcosis is immediately reversible by ascending to a shallower depth. Making physical contact and leading the diver up is the safest response. Switching to a deco gas at 42m would cause fatal oxygen toxicity (PO2 well over 1.6 ATA), and waiting does not resolve narcosis.

Isobaric Counterdiffusion (ICD) occurs when a diver switches from a highly diffusible gas (like helium) to a slower diffusing gas (like nitrogen) at depth. Helium diffuses out of the tissues faster than nitrogen diffuses in, potentially causing a localized pressure increase and bubble formation without any change in ambient pressure.

Arresting the uncontrolled ascent is the absolute first priority to prevent barotrauma and omitted decompression. The diver must simultaneously dump gas while disconnecting the LPI hose. Shutting down a post takes too long and cuts off breathing gas unnecessarily, while delaying the dump will result in a rapid ascent.

OTUs/UPTDs are used to track long-term, low-level exposure to oxygen, which affects the lungs (pulmonary oxygen toxicity), typically becoming a concern during multi-day repetitive technical diving or extremely long single dives. CNS oxygen toxicity is tracked separately using the CNS clock percentage.

A diver must never compromise mandatory decompression to fight a current. The DSMB marks the diver’s position for the boat crew to follow. Swimming hard at depth or on the surface increases the risk of DCS (due to exertion and omitted deco). Drifting while completing the stops is the standard protocol.

If a second stage free-flows, the diver only needs to isolate the specific post supplying that regulator (the right post). By leaving the isolator open, the diver retains access to the gas in the right cylinder (which flows through the manifold to the left post). Closing the isolator unnecessarily traps gas in the right cylinder.

Standard contingency planning requires divers to carry ‘run time’ tables for their planned exposure, as well as tables for exceeding their planned depth (e.g., +3m) and exceeding their planned time (e.g., +5 mins) in case they accidentally go deeper or get delayed on the bottom.

Breathing oxygen-rich gas lowers the inspired partial pressure of nitrogen, creating a steep gradient between the nitrogen-loaded tissues and the lungs. This inherent unsaturation (the oxygen window) allows nitrogen to diffuse out of the tissues and into the blood much faster than if the diver were breathing air.

First, find the volume consumed: 80 bar * 24 liters (total internal volume of twin 12s) = 1920 liters. Next, divide by the time (20 minutes) to get the consumption rate at depth: 1920 / 20 = 96 liters/minute at 4 ATA. Finally, divide by the pressure (4 ATA) to find the surface equivalent (SAC): 96 / 4 = 24 liters/minute.