현재 위치: 홈1/Teaching Neutrally Buoyant and Trimmed: How to weight properly, Part 3
Teaching Neutrally Buoyant and Trimmed: Part 3: How to weight properly
By Kosta Koeman
In the first blog post, I provided an overview of the importance and advantage of teaching open water courses neutrally buoyant and trimmed. This was followed by a set of exercises preparing students for scuba, including a series of mask and snorkel exercises, finning practice, and finally skin diving. In this next post, I will focus on proper weighting and the related topic of weight distribution, which are critical concepts for the open water diver to understand and know how to implement with changing equipment configurations. As a reminder from the first post, DAN listed correct weighting at the top of their Ten Most Wanted Improvements in Scuba list in their 2016 Annual Report.
At this time, I would like to repeat the definition of proper weighting from the first blog post: Proper weighting is the minimum amount of weight needed to keep a diver at their safety stop (5 m or 15 feet) with an empty BCD/wing, and a nearly empty cylinder (50 bar or 500 psi). If the diver is wearing a dry suit, then the dry suit has the bare minimum amount of gas to keep the diver sufficiently warm. And not a gram or ounce more. A diver then rises and falls with just normal breathing. While there are several methods of approximating proper weighting, none of these are as accurate as performing a weight check at the end of a dive with a nearly empty cylinder once you have completed your safety stop.
Secondly, there is the concept of a balanced rig. This means being able to swim up to the surface on full cylinders with an empty BCD and dry suit, without ditching weights. If a diver with no gas inside their BCD cannot swim to the surface without ditching weights, then they are certainly overweighted. In open water courses, students are taught to ditch weights. This should be reserved when at the surface to ensure positive buoyancy and lifting the head as far above the surface of the water as possible. After determining the exact weight students need, I recommend that instructors check that each of their students is diving a balanced rig. You can do this by going down with them to 15 feet / 5 meters and without any gas in their BCD or wing, allow the student to attempt to successfully kick up to the surface. I would not recommend this practice for Discover Scuba participants, but definitely teach it at the appropriate time in an Open Water course. Some may opt to dump weight while underwater if they find they are unable to ascend, but this is a scenario that should never happen with a properly trained, configured and fit diver. If divers do ditch weights underwater, they must never ditch too much weight at once, as this can result to an uncontrolled runaway ascent to the surface where divers risk death due to an arterial gas embolism.
Problems caused by overweighting
The reason why overweighting is such a problem with Open Water students is that in order to remain at a constant depth, you have to add additional gas into your BCD. This gas is wasted and could be saved with adopting a balanced rig. This was addressed in the part 1 [add link here].
The situation gets exacerbated when using a dry suit, as there will be an additional bubble that will grow/shrink in size as the depth and pressure changes. It is easier/faster to dump gas from a BCD/wing than it is from a dry suit. Another thing to keep in mind is that this concept also affects a diver using a wetsuit. When neoprene is manufactured, even in the newer styles of ‘crushed’ neoprene, air bubbles get caught in the material which will change as the depth varies.
Another negative impact of overweighting is increased gas consumption due to drag, even when trim. If divers are not trim while finning, they are wasting even more gas (and energy) to compensate for being negatively buoyant. If they are neutrally buoyant and start finning when their bodies are at an angle, they will swim towards the surface. I don’t mean to give the impression that divers must be trim all the time; that is definitely not the case, as I put my body in all sorts of positions when taking pictures to get the image I want or just to look at sea life. However, divers should be trim when finning through the water to minimize their gas consumption.
Real life application
When I became adamant about proper weighting, I saw a dramatic change in my students. Without exception, they stopped cratering and corking. I’ll admit to being frustrated when I overweighted my students because they would often cork. “I just went up” was the exact phrase they used, and it was completely my fault because of the excessive weight they needed to stay on their knees (which is how I was initially trained to teach.) Because I teach in an area with a silty bottom, cratering would result in a silt cloud and I’d have to move my class for safety. This is why I almost immediately started providing my open water students with my own dive lights so that I could always see them inside the silt cloud. Now, I still give my open water students dive lights, but the silt clouds are a thing of the past. The reason for the lights: even when diving during the day in the Puget Sound, they are necessary for communication and seeing things. I highly encourage all instructors to find a method for proper weighting in both the confined and open water portions of the Open Water course, as the exposure protection used can be dramatically different between them. Open Water courses taught in colder climates with temperate water typically have confined water sessions in indoor pools.
My first confined water weighting method
When I began weighting students individually, I started during the first confined water session with the float test and then the 200 yard/meter swim. Since students finished this swim at different times, it allowed me to work more efficiently, by determining the weight each student needed with minimal time wasted on students waiting their turn.
I followed these simple steps:
Have students lay flat face down in their scuba kits in chest deep water with no weights.
As they relax and are ready, hand weights to the students in small increments to hold in their hands at the locations in which weight will be stored (waist for weight belt or next to weight pockets for those in jacket style BCDs). As students are given weights gradually, they slowly sink more and more on exhale. And because they are holding weights against their body, they cannot be sculling with their hands. This gradual process made the transition from breathing at the surface to breathing underwater manageable even for students who had some reservations or fear of the water.
Repeat until students rise and fall in a fairly horizontal position in the water column with normal breathing.
During this process, if the students are foot heavy, move some weights to trim pockets to reestablish proper (horizontal) trim. You may have to attach weights to the shoulder straps using bungee cords. Using lighter, more buoyant fins is another available option.
In the event that they are head heavy (never seen this, but won’t rule it out), move the cylinder down slightly, use heavier fins, and move weight as far down as practical.
This method was successful on my first attempt. Though I did have one student in a pool session who required no weights at all.
Since I started using this method, I moved on to how to determine a good starting weight for students wearing thicker exposure protection. This is applicable to confined water sessions taught outdoors in water that meets the criteria for confined water: pool-like conditions in terms of clarity, depth, and calmness (Note: Any site other than pools must be approved in writing by the Training Department).
My current weighting method
I transitioned to a more mathematical formula to come to a better starting weight. Now before you roll your eyes about a “mathematical formula”, please hear me out. Remember that we want to determine the correct amount of weight for a diver at a nearly empty cylinder of 500 psi/50 bar at their safety stop of 15 feet / 5 meters.
One part of the formula is the buoyancy of the scuba kit with a nearly empty cylinder and any accessories. This number can be obtained by suspending the assembled scuba kit in the water and measuring the buoyancy with either a fish scale or luggage scale (just keep it dry). I measured a jacket style BCD with a low-pressure 85 cubic foot steel cylinder and regulator assembly at about 5 lbs. / 2.3 kg negative. This step is performed only once for each unique scuba kit configuration. When determining the buoyancy of the scuba kit, it is important to include all equipment provided, such as the primary and backup dive lights as in the scuba kit shown below (what I provide to my open water students). I do teach open water courses in a backplate and wing configuration with a Hogarthian harness. Mark Powell blogged about teaching in this configuration in his post “Can You Teach Open Water In Backplate, Harness, and Wing?”.
For wetsuit divers, the formula is:
buoyancy of wetsuit + buoyancy of student + buoyancy of scuba kit (discussed above).
I determined the buoyancy characteristics of wetsuits used by students prior to the start of class. This is a step that is performed once, and dive centers can record the buoyancy characteristics of their rental wetsuits to provide to instructors using this weighting method.
The wetsuits are placed inside a mesh bag and attach to a small dive weight bag. Weights are added until these items are neutral underwater. Now, I will admit that the number derived will be too high for a couple of reasons. First, a wetsuit worn is less buoyant as it is stretched when worn. Second, a wetsuit will compress slightly at even the shallow depth of a safety stop.
I’ll use one of my students as an example for numbers. She wore a woman’s small 7 mil wetsuit that needed about 22 lbs. / 10 kilo to become neutral. Now when worn and at the safety stop, this number is a little bit high, but it is close enough. This is fine-tuned later.
The third number is the amount to get the person to sink on exhale. This can be quickly determined during the float test at the start of the first confined water session. While the amount of weight needed in fresh and salt water are different, in this case it is insignificantly small. In the case of the student wearing the above-mentioned wetsuit, she needed 2 lbs. / 1 kg to sink when just under the surface and exhaling normally.
Using the student used as an example, the calculations is 22 lbs. + 2 lbs. – 5 lbs = 19 lbs. At the end of the first open water session, where her cylinder was drained to 500 psi after the safety stop was over, she was sinking slightly at her safety stop while breathing normally, so we removed 1 lb. On following dives, no further adjustments were needed.
This method works well for courses taught in wet suits in cold water, as you get close to the amount of weight needed without having the student get wet. After all, a big part of teaching in cold water is managing students’ comfort.
Dry suit divers
For dry suit divers, the formula changes slightly. The buoyancy of the diver and exposure protection (dry suit and undergarment) is combined. To determine this amount, students are given an insufficient amount of weight on a weight belt, about 10 lbs. We then go walk into the water from shore, put on our fins, and stop when we reach chest deep. I have the student breathe from a regulator with long hose attached to a scuba kit floating at the surface with the wing fully. The student then lays forward in the water at the surface and holds onto a weight bag. I gradually put weight in the weight bag (this is done as it can be cumbersome to hold weights wearing bulky gloves). They hold the weight bag in a location in front of them that allows them to remain horizontal once they have enough to start sinking on exhale. Once they are able to float horizontally, rising and falling with their breath in the water column, they exit the water and subtract the predetermined weight of the submerged scuba kit with a nearly empty cylinder. Next, we move to weight distribution.
Weight Distribution for Achieving Trim
It is not enough to be properly weighted. That weight must be properly distributed so that the diver is comfortably in the horizontal, or trim, position, with the lower leg bent for performing frog kicks and the fins flat and not sculling. Sculling is a symptom of improperly distributed weight.
As discussed in the first blog post, there are two centers to keep in mind: the center of mass (gravity) and the center of displacement (volume) The farther these two centers are apart, the more difficult it is to distribute weight. Typically, I can get a diver trimmed by sharing weight between the weight belt and two weight pockets attached to both sides of the two scuba cylinder tank straps. Choice of fins also has a significant impact on trim. In terms of buoyancy, fins can range from quite negative to slightly positive.
Here’s a tip if you have one cylinder strap or find that two cylinder straps don’t provide you sufficient granularity in weight distribution: affix a cylinder tank strap that has a buckle and has two weight pouches attached to it, as shown below to the left. For a BCD with a single cylinder strap, this allows sliding weights up or down the cylinder to help your student get trimmed. For BCDs with two cylinder straps, the added will likely be difficult to slide up and down, but gives you additional distribution points for finer granularity in placing weight. One important point for adding the third strap is whether or not this is easily done with a single tank adapter (STA). The Deep 6 STA, shown below to the right, has a design that accommodates a middle cylinder strap to be placed. This isn’t absolutely necessary, but is a nice to have for making finer adjustments.
Steel cylinders come in different shapes: a round-bottomed or flat-bottomed. A round-bottomed scuba cylinder’s center of mass changes as the cylinder is drained. Flat-bottomed steel cylinders do not have this problem or it is relatively insignificant when compared to their convex counterparts. Unfortunately, flat-bottomed steel cylinders are not available in the United States. You need to be able to adjust your scuba kit down as you drain your cylinder to maintain comfortably floating in the horizontal position. The adjustment you need to make are small, but they are necessary to avoid becoming head heavy.
Always evaluate this in your students when you have them hover motionless. Teach your students to make their own adjustments when they change configuration. When your student’s cylinders are half full, figure out the weight distribution to maintain trim. Fortunately, it does not take much time and your courses will go more smoothly.
Addressing concerns for instructors
A number of instructors are concerned about losing control of a student. This is not hampered by properly weighting students and having them neutrally buoyant and trim. Because the student is horizontal and properly weighted, if there is a panic (steps should be taken prior to going underwater for ensuring a student’s comfort), the student goes straight towards the instructor, not up. This is in contrast with a student on their knees who is vertical and can push off the bottom while heading to the surface. In addition, the properly weighted student does not have extra gas in their BCD or wing that will expand as they ascend, making them more buoyant and ascend to the surface faster.
My advice to other instructors is to start small: four students per class with an assistant, adding two students at a time with assistants until you reach comfort and experience when teaching neutrally buoyant and trim. In the end, you and your students will benefit from this method of instruction. Your retention rate will go up and more of your students will come back to you for further training. Your students are more likely to enjoy a lifetime of diving. It is a win-win, without any downsides.
Now the ball is in your court. I hope that you will find the ideas and techniques I offered in this series to be clear, informative, and useful. Use them as they are or make your own adjustments and derive new ones that are yours alone. The main thing I want you to take away from this series is the importance of proper weighting, which includes weight distribution, and to understand that proper weighting must be ultimately checked underwater. Also remember that you don’t have to place your students in scuba immediately. Some students need smaller steps and there is nothing wrong with that.
Finally, teach your students how to quickly and accurately weight themselves when they embark on dive vacations. While they may be using different gear each time, most BCDs are going to have similar weight characteristics. The same is true of wetsuits of the same thickness. Teach them to perform weight checks at the end of each dive with nearly empty cylinders after finishing their safety stops until no weight changes are required. While students should log every dive, I recommend consolidating the information they record on weighting on their smartphones. During their open water course, consistently check their ability to hover motionless horizontally. Good luck!
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