10% rule?

[fishoutawater]

I haven't used a full suit in a long time. I always use my shorty when I go to Florida, and I use two 3 lb weights with it in fresh water. In salt water, I add a couple pounds, and I can maintain neutral in any position at depth.
I went to Mermet Springs in Southern Illinois over the weekend, and I used a rental 5 mil Henderson and a hood. Wasn't sure how much weight I needed, so I asked. The guy asked me how much I weighed. I told him I weigh 190 lbs. He set me up with eighteen pounds worth of shot bags and a belt.
I sank like a rock. Maintaining horizontal? Fuggetabout it.
On the second dive, I dropped down to 12 lbs. Better, but I still couldn't hold a horizontal position to save my life, and I still needed a little air in my bc at depth to stay neutral. I tried pulling the belt up almost around my arm pits, but that didn't help either (kinda hard to breathe that way too).
I'm thinking I might be able to drop down to about ten lbs of lead next time.
So, now I'm wondering. Does that 10% rule actually work for anybody? Is that rule meant for salt water only? What's the deal?

 

[divad]

With a 5mil, I'm using the 5% rule.

 

[David P]

10%+5lbs in salt water with 7 mil

 

[dannobee]

It works for most people in salt water, but a rental 5mm suit has probably been crushed down to a 3mm suit. Who knows how many hundreds of dives were on that suit?

 

[Walter]

It doesn't work. There are way too many variables - % body fat, various tanks, what suit, etc. The only way to know it to actually do a weight check in the water.

 

[DA Aquamaster]

I agree, one of the local instructors uses the 10% rule plus another 5 lbs for the tank and most of his students are badly over weighted. He complains about their poor bouyancy skills and obviously does not realize how much harder it is to maintain precise bouyancy in the average rental BC when you are 10 or more pounds over weighted.

You are still best off doing an in water check. In just your wet suitm, mask fins and smorkel you should float at eyeball level with full lungs and sink when you exhale. That's a good starting point.

At the end of the first dive with about 500 psi in the tank, you should be back to floating at eyeball level with full lungs and a completely empty BC. If not, adjust your weight accordingly. That will leave you about neutral at 15'.

It helps to keep track of your weight requirements in a log, but be advised that a new wet suit, particularly a 7mm will have several more pounds of bouyancy than an older suit that has been compressed slightly. So the bouyancy of rental suits may vary and if you have your own new suit, expect to have to reduce your weight requirement by a few to several pounds after the first 20 or so dives.

 

[BKP]

...
I'm thinking I might be able to drop down to about ten lbs of lead next time.
So, now I'm wondering. Does that 10% rule actually work for anybody? Is that rule meant for salt water only? What's the deal?

I dive a 3mm in fresh with 6lbs. of shot, and a 5mm with 10lbs. (However, I also use a compact AL80 in fresh, which is about 1.5lbs. negative at 500psi).

I believe the 10% rule is just for salt... I don't think it has anything to do with your neoprene.

(I *do* agree with dannobee, that your rental suit probably had limited buoyancy left in it).

 

[fishoutawater]

Thanks for the responses guys. I hadn't thought about the rental suit crush factor. It was kinda worn out. I also didn't do a weight check on the first dive. I just took the guys advice. I did do one on the second dive though. I still sank some, but I went with it anyway. I'm going to end up buying my own five mil, because for a quarry, Mermet Springs is actually a really fun dive and I want to spend a lot more time in it. And it's only three hours from me. Hell, I might even take some advanced type courses there.
It has some really huge Paddlefish in it. Pretty close to six ft long. Kinda neat to look up and see them with their mouths wide open, feeding.

 

[K Mejean]

I had a negative experience last weekend. (get it) Anyway, I went with my wife for her first dive. A fresh water checkout. She told me that she was only going to use 9 pounds which seemed a little lite to me but I just agreed. I decided to put on nsome extra weight just in case she couldn't get down. I normally use 18 pounds with a shorty( I know it sounds like a lot but fat boy here bounces out of the water like a bobber). So I added more and went up to 23 with a shorty. Let me just say holy crap it about killed me. I lost total control under water. I couldn't get myself bouyant at all. IO normally don't have a problem staying bouyant but that extra weight was too much. The second dive I went back to normal weight and everything was fine. I don't know what rule you are talking about but 10% would sink my bad.

 

[Sideband]

The "Rule" is used to get you out of the shop with enough weight to be able to do your buoyancy check and not come up short. It is not a rule of how much to wear.
Just out of curiosity, if the guy had given you 50lbs of lead would you have worn it all then as well?

Joe