Eric Sedletzky
Contributor
Quite possibly so.
The weighting at the end of the dive and the stop at 15’ is the determiner. Add a few lbs to avoid shrink wrap and it’s there.
PADI OW and BPW
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The weighting at the end of the dive and the stop at 15’ is the determiner. Add a few lbs to avoid shrink wrap and it’s there.
Eric Sedletzky
Contributor
You think so?
It wasn’t intended to be taken personally.
There are work arounds a such as pressurizing on the surface and pressuring again at 10’ or so. diving overweighted to compensate for the inability to equalize out of an upright position is a fine line of compromise I suppose.
I knew ab divers that had to pressurize before they kipped and went down then roll vertical and pressurize again once under. Not unheard of.
In order to be correctly weighted, you will need to adjust your weighting to account for thermal protection, salinity, etc., so the definition does not change.
I agree that dry suit divers must determine how much gas is required at their safety stop to keep them comfortable (for both warmth and the shrink wrap effect as you eloquently put it). Now in reality, guessing how much gas a diver should have in their dry suit at the start of the dive, I'd argue that warmth is not yet an issue, and with a "slight" (a very subjective term) amount of uncomfortable shrink wrap shouldn't be an issue unless there's a long surface swim (for people local to me, I'm thinking Edmonds Underwater Park). But still, they should be able to dump out all the gas from their BCD and sufficient gas from their dry suit to descend feet first with a full cylinder. I'm willing to discuss how I'm wrong if I'm wrong.
Think it through. The amount of weight you need for one of your scenarios will be different for another, so for that dive under those circumstances, your weighing needs are different from your needs for a different dive in different circumstances.
Absolutely! The amount of weight is specific to particular configurations and salt vs freshwater. But the process for evaluating that is the same. The floating at eye level one is much more coarse (a possible starting point, but I never use it) than checking at the end of the safety stop with a nearly empty cylinder.
A number of years ago I did a day of diving in Puget Sound, using a combination of what I had brought on my trip and what I borrowed from some friends. I had at that time never used a drysuit in salt water. I was using my thickest undergarment, which I had never worn before, in fresh or salt water. All my other gear was borrowed.
Believe it or not, I did not bring along an almost empty tank and take it to safety stop depth to see if I could hold that stop. Believe it or not, I did a weight check on the surface before the dive.
I do the same working with students when they do their OW dives in 7mm suits for the first time. I do not send them to safety stop depth before the dives with an empty tank. I do a weight check on the surface. Scoff at me if you like, but I don't see a problem with that.
Believe it or not, I did not bring along an almost empty tank and take it to safety stop depth to see if I could hold that stop. Believe it or not, I did a weight check on the surface before the dive.
I do the same working with students when they do their OW dives in 7mm suits for the first time. I do not send them to safety stop depth before the dives with an empty tank. I do a weight check on the surface. Scoff at me if you like, but I don't see a problem with that.
I would never send a student down with a nearly empty tank. What I do at the end of a dive is to perform a check, and then we exit the water.
So how do you know what to do at the start of the dive?
From How I weight students in open water courses:
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.
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