How I weight students in open water courses

These days, I only teach open water with students in dry suits (it is just torture learning in a wetsuit in Puget Sound – my opinion).

For wetsuit divers, there are 3 buoyancy numbers I need: the weight to get the student to sink in a pool in just a swim suit, the buoyancy of the wetsuit to be worn in open water, and the buoyancy of the scuba kit with a cylinder at nearly empty (500 psi or 50 bar).
The first number, the weight to get the student to sink, can be determined during the float test. Just have them exhale and see if they start to sink. This number will be slightly low due to salt water, but that gets more than compensated for in the next measurement. The second number requires a mesh bag with the wetsuit inside attached to a weight bag. Now the mesh and the weight bag are slightly positively buoyant. Also, a wetsuit is less buoyant when worn as then the material stretches, but it gets close. Finally, there is the buoyancy of the scuba kit with a nearly empty cylinder. Taking a rowboat, attaching the scuba kit to a luggage or fish scale, and you get a value on how buoyant it is. Two numbers will be negative, one will be positive. Add the two negative numbers with the positive number and you have the proper weight for your student.

Weighting properly for drysuit divers takes 2 numbers. Have students put on all their exposure protection: undergarment (for different undergarments, repeat this exercise and repeat), drysuit, gloves, hood, and mask, and fins. With a drysuit completely empty, have the student breath from a long hose attached to a scuba kit floating at the surface. Give them weight so that on exhale they start to sink. Add to that the buoyancy of the scuba kit with a nearly empty cylinder (a negative number), and you’ll have a good starting weight for your students.

Next regardless of wetsuit and dry suit, and this is really important: have students put on their entire scuba kit. They now will be negative at the surface. Have them float in the shallows horizontally. Distribute weight so that the student can float horizontally effortlessly. Check for this also when zeroing in on the correct weight at the end of the dive at the safety stop when the cylinder is drained close to 500 psi/35 bar. A lot of divers in BP/W configurations will shift their BP/W & cylinder to account for the changing center of mass of the cylinder as the breathing gas is consumed.

Whether you teach in dry suits or wetsuits, you can quickly weight students fairly accurately. Of course, you want to make adjustments at the safety stop: have them dump all gas from their BCD and dry suit (if worn). If they sink, remove a bit of weight. Of course, if they cannot maintain a safety stop with an empty BCD and dry suit, then they need more weight. But using this technique, I haven’t had to adjust for more than two lbs or one 1 kg. And again, distribute weight so that students can be horizontal effortlessly.