Silly question perhaps....

[Bubbletrubble]

"Your drysuit is a buoyancy compensator device."
Bubbletrouble, this raises an interesting beginners' question:I'm still wet (till next weeks' course) but my buddy dives dry - is it safe to dive with a BC with 23 to 28 lb lift when you're wearing 30+ lb weights with your drysuit?! My buddy has a bag suit. Me, I'm looking at the Mares Ice, 'exclusive patented neoprene called High-Density." I don't think it's quite crushed, (CAN'T be, at the price) but neoprene anyway. Is there a difference in just how unsafe it is to dive with the warm water BC's with these suits? Or can I rely on the suit to take over where the BC gives out in its' lift capacity?

@seahorsey: Out of courtesy to the OP, it would usually be best to start a new thread with a question like this. Since on_two_wheels appears to be satisfied with the answers we've given him, I'll take a stab at answering your questions.

The short answer to your boldfaced question above is "maybe." For most single tank drysuit divers, 30 lbs. of lift is sufficient in a BCD for diving temperate waters. You mentioned having to wear 30 lbs. of lead. I'd recommend attaching a certain amount of that directly to you (10-14 lbs. weightbelt or weight harness) and putting the rest of it on your rig (16-20 lbs. in weight-integrated pockets, trim pockets, camband pockets). See below for an explanation of why...

The BCD must have enough lift to be able to do the greater of:

1. Supporting the complete rig (BCD, tank, weight attached to BCD; but without you in it) at the surface.
2. Compensating for a complete loss of buoyancy of your drysuit at depth.

To address #1, you need to know where your weight is (attached to you vs. attached to your rig) and the buoyancy characteristics of the tank you'll be using. Let's say you have a Worthington X7-100 steel tank (10 lbs. negative when full) and you have a weight-integrated BCD carrying 30 lbs. of lead. In this example, is a BCD with 30 lbs. of lift "big" enough? What will happen if a 30 lb. lift BCD is inflated fully at the surface with a full X7 tank and the diver separates herself from the rig? The rig will sink because the BCD lacks sufficient lift! In this scenario, the BCD would need at least 30 lbs. + 10 lbs. = 40 lbs. of lift in order to keep the rig afloat.
How does the scenario change if the drysuit diver splits up her weight between a 14 lb. weightbelt and 16 lbs. of lead in the BCD weight-integrated pockets? When the diver separates herself from the rig, she takes 14 lbs. with her (which is probably OK because her drysuit likely gives her 20+ lbs. of positive buoyancy). The BCD has 16 lbs. of lead and a full tank (10 lbs.) attached to it. A 30 lb. lift BCD will enable the rig (16 + 10 = 26) to float at the surface!

To address #2, you need to know what the buoyancy of your drysuit + undergarment system is. One way to do this is to jump into the water with just your drysuit + undergarment on and enough air inside your drysuit to be comfortable (no squeeze). Start adding weight until you are neutrally buoyant. That's how much positive buoyancy your exposure system has. In order to meet condition #2 above, your BCD needs enough lift to compensate for a total loss of positive buoyancy of your drysuit. Most shell drysuits + undergarment systems have 22-26 lbs. of positive buoyancy. If this is the case, then a 30 lbs. lift BCD would be sufficient.

Your drysuit instructor or an experienced drysuit diver should be able to explain this to you in person.

Hope this helps...

 

[Bubbletrubble]

I differ slightly with BubbleTrubble -- I DO weight myself slightly heavier in very cold water, to make sure I can carry enough gas in the suit to stay warm during the ascent. Weight checks are typically done while floating at the surface, where you are vertical, and the suit is largely empty; that's why I don't do them there, but rather in 4 or 5 feet of water, in a horizontal position, and with the amount of gas in the suit that I want to have there until the end of the dive.

To clarify, when I'm doing my weight check at the surface, my drysuit is not empty. My aim is to have enough air inside to maximally loft the undergarment, which is decidedly more than just to offset squeeze. In order to achieve this, I have to close off my exhaust valve, otherwise air inside the suit escapes.

Alternatively, I suppose a drysuit diver could do a weight check at the surface with a "largely empty" drysuit. Then he could add a few extra pounds (2-4?) to give himself the ability to carry more air inside the suit during ascent (and stay warmer).

My undergarment is made of Thinsulate. It's not super-fluffy, so it doesn't require a lot of extra air inside the suit to stay maximally lofted.

 

[seahorsey]

Thanks, that was quite the treatise! I'm sure I may have learned it in the course, but that's another good week and meantime I would have worried ..now, not so much anymore, because we DO have our weights split between belt and BC -and that 'weight check without BC' answers the question I had with the # 2 scenario, and with neoprene vs bagsuit, and different underwear. We just have to actually, physically check. Makes sense. Better than calculating or guessing. Thanks!