Thailand – Australia – United Kingdom

Trimix: What’s the plan?

It’s 7.30AM on a cool, clear summer morning. The sea around the boat is mirror-smooth, in stark contrast to the hive of activity on the deck. To the outsider, it’s chaos; in fact, every action has been carefully planned. In less than an hour, the first two of eight divers will be in the water, with the last pair climbing out a few hours later.

This is mixed-gas, or trimix, diving. Each diver has already made a heavy commitment in terms of training and equipment, and will make a more personal commitment as soon as they switch to the trimix carried on their backs. A mistake from this point onwards will be painful – or worse. Thorough planning, so important in recreational diving, is absolutely essential for safe mixed-gas diving.

Mixed-gas diving is all about establishing a purpose, and doing whatever needs to be done to safely achieve that aim. Planning for mixed-gas diving trips starts months in advance and can roughly be broken down into five main areas.

The Team
Most teams are carefully selected by word of mouth and by reputation, to suit the aims of the trip. Each team member needs to be skilled, reliable, easy to get on with, willing to do their share of the work, and prepared to do whatever it takes to be fit and ready for the dives.

The dive manager’s role is to co-ordinate the diving activities, liaise with the skipper and deal with any emergencies that may arise. The dive manager does not necessarily need to be a mixed-gas diver, but should have a thorough understanding of the implications.

The Boat
The type and size of boat depends on the distance offshore and likely weather conditions at the dive site, but the majority of mixed-gas diving is carried out off hardboats because of their greater stability, space and protection from the elements.

Particular features to look out for are a sturdy kitting-up bench, sufficient deck space for equipment, and unobstructed level decking that allows a diver in full kit and fins to walk to and from the entry and exit points. Some boats have a hydraulic lift, which is ideal. As important as the boat is a good skipper who is happy to work with mixed-gas divers, and who understands their needs and concerns.

Many mixed-gas divers prefer to use a double-bladder wing-style BC with a steel backplate, for the uncluttered chest space, redundant buoyancy and stability. The twin-set containing ‘back gas’ and two decompression stage cylinders, or ‘stages’, are each fitted with a high-quality regulator, with the decompression regs clearly marked to avoid confusion. A rule of thumb is that the richer nitrox mix for decompression is mounted on the diver’s right-hand side: ‘rich is right’. Kit, such as knives and line cutters, is doubled and mounted on the arms or harness. Each diver will have at least two reels that are capable of reaching the surface from maximum depth, and two delayed surface marker buoys (DSMBs) – one yellow and one red.

All mixed-gas dives involve mandatory decompression stops, which can be lengthy and are generally carried out on a deco station of some sort. A decision needs to be made about whether to leave the station attached to the main shot-line for the duration of the decompression, or to detach it to drift with the current. With a current running at up to 1.5 knots, decompressing on an attached station is excellent. The diver uses a Jon line to clip on and is pulled out level by the current.

Where currents or tidal flows are stronger, the deco station can be detached to drift free once all the divers are safely in position. This requires a tagging system, where the descending divers each clip a tag onto the point at which the transfer line meets the main shot-line, retrieving it again on the ascent and unclipping once the last tag has been removed. The boat then follows the drifting trapeze. A free-drifting decompression is extremely comfortable, but is not practicable in areas where the divers may drift into danger or obstruct shipping.

The Dive
Mixed-gas diving has a good safety record, and incidents are rare. However, due to the increased exposure of this type of diving, accidents are more likely to be serious and a minor complication can rapidly escalate into an out-of-control situation. All possible emergencies need to be thought through to their conclusion and a plan put in place – not only for prevention of the incident but also of actions to be taken should the worst happen. Find out the location of the nearest recompression facility and likely evacuation methods and times.

Once the evacuation and recompression situation in the pre-trip planning has been checked, the emergency kit should be kept together with telephone numbers, diver details and next-of-kin information. Emergency oxygen supplies should be plentiful, as should isotonic drinks and water.

As the divers may have extended decompression stop times, care needs to be taken to ensure that they will not drift into danger during a free-drifting decompression. They cannot be brought to the surface prematurely, so it is important to keep an eye on the weather forecast. If the weather is due to deteriorate before or close to when the divers are expected out of the water, it may be wiser to change plans or even abandon diving for the day.

Confirm the diver-to-surface signalling protocol with the dive team. A simple system, which has been in use for several years, is as follows: a red delayed SMB indicates that the diver is there and okay; a yellow delayed SMB indicates that there is a problem that requires immediate investigation. Both red and yellow delayed SMBs deployed simultaneously indicate that the diver is low or out of gas. Consider the best way to remove an unconscious diver from the water.

On average, each diver will be wearing some 45-60kg of equipment, and most will struggle to put on their own stage cylinders on board the boat. Trying to climb a ladder while wearing this weight at the end of a dive could trigger decompression illness (DCI), so it is normal for the divers to remove their stage bottles in the water. Surface support can greatly ease the burden of kitting and de-kitting, while shallow support divers waiting at the deco station to remove bulky items, such as cameras, will be appreciated.

Gas Blending
Commercially prepared trimix is expensive and not widely available, so significant savings are made by purchasing the ‘J’-size oxygen and helium cylinders directly from an industrial gas supplier and then blending with air. Eight divers, making only one dive in a day, will need 36 cylinders of mixed gas to be blended and filled, in addition to the emergency drop tanks and support divers’ tanks.

If blending on board, the boat will need a compressor with the capacity to support this volume of gas filling, in addition to having sufficient space for the large helium and oxygen cylinders. The gas must be ordered well in advance, and delivery or collection arranged. Basic equipment required for gas blending consists of oxygen-clean blending hoses and gauges, a compressor and gas analysers.

Mixed-gas planning tips
✓ Select the team carefully. No one wants to discover at 75m that the diver next to him doesn’t dive as well as he talks

✓ Select the boat and the skipper the same way you select the team – by reputation. A good skipper will be pleased to answer questions and discuss contingency plans for bad weather. Ensure that the skipper is happy to work with mixed-gas divers

✓ Prepare a rota for gas blending, support divers and dive manager

✓ Have a designated non-diving dive manager each day to co-ordinate the diving and ensure appropriate response to problems

✓ Prepare a list of required team equipment. Send it out and ask the team to fill in items they can bring. Anything still needed can be hired or bought and the expenses shared

✓ Send out a list of emergency signalling equipment in advance. If just one diver in the group has the wrong colour of emergency signalling equipment, you run the risk of an inappropriate response when seconds count

✓ Plan for the worst – have a list of emergency contact numbers and next-of-kin details at all times

✓ Contact your nearest recompression chamber and local coastguard to discuss your plans in advance

✓ Never be afraid to cancel a dive if the conditions aren’t right, irrespective of the financial cost

✓ Enforce a ‘no alcohol’ rule until all the trip’s diving is over


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