Maximum dive time for Public Safety Diving searches.
Maximum dive time for Public Safety Diving searches.
Part of the TeamLGS Public Safety Diving Standards and Guideline Series
Copyrighted Walt “Butch” Hendrick and Andrea Zaferes 1989, 2019
Why is 20 minutes with a possible 5 minute extension the maximum dive time for low to zero visibility diving? Ice diving has a 15 minute maximum with a possible 5 minute extension. Some divers may need to have their time reduced to 15 minutes at depths greater than 45 feet in order to exit the water with at least a third a third of their air.
- Air is an important issue. Divers need to be back on shore or the boat with a minimum of 1000 psi in an 80 cuft cylinder for all dives. We have found that the average diver we train breaths with a 20-35 psi/min Surface Air Consumption rate in a no current dive. A current of 1.5 knots can make that SAC rise by as much as 4-5 times. A rescue mode can double a SAC rate. So for most divers a 20-25 minute dive to a max depth of 50 feet plus a possible 5 foot extension means it is very possible to come back with 1,000 psi. Divers with SAC rates of 50 psi or more in still water may need to be designated with a shorter maximum dive time.
- If divers are allowed to search for 40, 50 or more minutes, then those divers are usually spent for at least a few hours due to mental and physical fatigue. They can not be put back in the water in an hour or two and be at their optimal search capability or level of safety. Divers who dive for 25 minutes or less can easily be put back in the water in 50-60 minutes with a new cylinder and proper hydration. This means that a response of 3 divers and two tenders can dive nonstop. After dive one, the primary diver moves to the 90%-ready slot, the backup diver moves to the primary diver slot, and the 90%-ready diver moves to the backup slot, while the primary and backup tenders switch places. Team with limited personnel on a shift can greatly benefit by this. Divers can have three peak searching dives a day safely, effectively, and efficiently in this manner. Of course this depends on weather, time of day/night, current, etc.
- What makes some divers magnets, which is defined as “if the item is in your search area you will find it the first time round”? Magnets have two qualities. They fully believe that if the item is there they will find it, and they have very a well developed mind’s eye. A mind’s eye is a part of your brain that visualizes everything that you touch. As your hands move across and through the bottom your mind says, “that is a pebble, that is a bottle top, this is a twig, this is a clam shell…..” If your mind wanders for even a few seconds you can easily miss your object. The question then becomes, how long can you fully concentrate your body’s tactile senses and your mind’s eye perception? After about 10 minutes a voice in your head at 2 am in the rain says, “why am I here?” After about 15 minutes a voice may say something like, “okay, let’s get this over with now.” Then around the 20 minute mark there is an inner voice that can be nothing more than a feeling that might say, “I’m not going to find it on this dive.” I think most of us can relate to this. The moment you lose the belief that you will find the body or the gun on that dive, your searching ability is no longer as good as it can be. So keeping dives 25 minutes or less keeps you in the optimal physical and mental searching capability.
- Tender concentration and profiler (backup tender as the record keeper of the diver’s breathing rates, estimated air left, every movement in the search pattern, etc.) concentration is critical do the diver’s safety and the accuracy and effectiveness of the search. How long can a tender keep their eyes continually on a diver’s bubbles without looking away for even a few seconds? How long can a backup tender stay fully alert of everything happening with the primary diver so that the backup tender will catch any small area the diver missed, any change in breathing rate or quality, etc… We have found that like divers, surface personnel can hold full, continuous attention and concentration for a maximum of 20-25 minutes.
- Backup and 90% divers also need to stay alert and healthy. If primary divers are allowed to do 40, 50 minute dives, then these two kinds of contingency divers are more apt to become cold, overheated, bored, and impatient as they sit through as much as 100 minutes before it is their turn to become a primary diver.
- How many tenders can work no-decompression tables? How many certified PSD divers could actually pass a quiz on dive tables right now without a quick review? Not enough. If you stay 25 minutes or less, in 60 feet or less, then divers can easily make 3 dives a day and stay well out of decompression modes.
Too often we see machismo take over safety and search effectiveness. We have watched divers severely shivering because they stayed in the water well past a 40 minute mark. Surface supply divers are made to be some of the worst offenders of too long search times. Longer does not mean better. Sure, you may come out of the water with 2200 psi at the end of a 25 minute dive. That’s great. Having lots of air left does not mean you have lots of attention, concentration, or energy left. And if you are not concerned about yourself in the primary diver mode, you should be concerned about your surface support and contingency divers.
We have been using the 20-25 minute maximum for over 30 years for thousands of PSD divers and it has proven very effective for all the above 6 reasons.
