Weight guesstimation: change from al80 to hp100

[jazzymusicman]

Hey guys, I'm doing an advanced dive on Sunday on the Tenneco Towers off of Ft. Lauderdale. My dive buddies are diving HP100s and I don't want to limit their dives by bringing an al80. Since I've always wanted to try an HP100, that's what I reserved for this dive. I know how to do proper bouyancy checks, but probably won't get a chance to before the first dip. I wanted to get some feedback on my weight guesstimation for the first dive.

I weigh ~215 and dive with 10 pounds wearing a lycra or 11 pounds wearing a 1mm with an AL80. Assuming I'm diving with the 1mm because of the depth and probability of a thermocline, what should my new weight be?

AL80 is -1.4 full and 3.4 near empty
HP100 is -10 full and -2.5 near empty.

My guesstimate is that I should take off 4-5 pounds to compensate for the bouyancy shift at the end of the dive. My worry is the amount of air I would need in my BC at the start of the dive to stay neutral. Am I overthinking this? Whats your advice

 

[WetLens]

My log shows I went from 12# with a 5mm suit and an AL80 to 4# with an HP100 and the same suit. Pretty negative at the start of a dive, but very close to neutral at the end (still slightly neg).

 

[chrpai]

Lux and Cat 80's are about +4 - +4.4 empty. The PST E7-100 and PST 100 tanks are -1 - -1.3 empty.

That's 5 -6.3 pounds you can take off your belt. Assuming your current weighting is correct, you'd be looking at 5-6lbs wearing your 1mm.

Any chance that as a new diver you are overweighted? Odds are you are. I'd probably take 5lbs off my belt and wear 6lbs on my rig. I'd put the weight in pockets and try giving some away at the end of the dive to retest your weighting. I wouldn't be surprised if you could get rid of more. If you are overweighted by say 5lbs and you take 6lbs off for the steel tank, well that's no more weight belt.

 

[jazzymusicman]

My current weighting is based off floating at eye level with 500 psi in the tank. It seems to be working for me, though trim and bouyancy control is something I'm always trying to work on in the water. Some dives are much better than others.

 

[chrpai]

The weight has a way of falling off. I dove for 6 years (and really nailed my weighting )and then retired for a number of years. When I got back into diving I do some weight checks and decided I needed 6 pounds. After diving for a month I realized I needed 3lbs and after a few months I realized I needed 0.

Some of that was my new wetsuit breaking in but not all of it. Some simply is getting much more comfortable in the water and your breathing changing and things like that.

So you know how to check it .... just keep checking and checking and don't assume it's a static value.

 

[tracydr]

A lot of times when you do a single weight check, all of the air is not removed from the BC. It's hard to get all of the air completely out of a standard BC, they are notorious for trapping air.
Newer divers also don't get their lungs as empty and tend to kick while doing their weight checks, rather than keeping their feet still.
There are a number of reasons a one-time weight check might not be completely accurate. It's a good idea to do a bunch of weight checks to get the most accurate check.
When diving on your deeper dive with the HP 100, have you considered using a 3ml? This would add a bit more positive bouyancy, an added safety factor when diving a full,steel tank, and will be warmer on your deeper dive.

 

[Gachnar]

I pulled off 6lbs when I switched and didn't have any issues with being underweight, I think that is probably a good place to start. I've since dropped more, but as chrpai points out, that was front getting more comfortable in the water and breathing techniques.

 

[jazzymusicman]

A lot of times when you do a single weight check, all of the air is not removed from the BC. It's hard to get all of the air completely out of a standard BC, they are notorious for trapping air.
Newer divers also don't get their lungs as empty and tend to kick while doing their weight checks, rather than keeping their feet still.
There are a number of reasons a one-time weight check might not be completely accurate. It's a good idea to do a bunch of weight checks to get the most accurate check.
When diving on your deeper dive with the HP 100, have you considered using a 3ml? This would add a bit more positive bouyancy, an added safety factor when diving a full,steel tank, and will be warmer on your deeper dive.

Thanks for the suggestions tracydr. My BC is the back inflate Aqualung Balance (the wing is pretty darn near close to exactly what I've seen on the bp/w setups) and I've been pretty good about trying to make sure all the air out, and although I'm pretty sure I've kept my feet still, this'll be something I pay more attention to next time. I've tried to do buoyancy checks at the end of every dive. As far as exposure protection, I'd dive close to naked if it wasn't for jellies. I was in 111 feet in Key Largo last weekend and the temp difference was 2 degrees. A month ago I experienced a 10 degree thermocline off Miami and I honestly thought it sucked more going from cold to warm than warm to cold.

The one thing I'm still fighting over is whether or not to put any weight in the trim pockets if I'm only going to carry six pounds, or whether to put 2 pounds trim and 4 pounds ditchable

 

[spectrum]

chrpai gave the precise answer, this is physics based on empty buoyancy values, nor more, no less. If anyone thinks feels or has experieinced other wise the baseline was wrong, the new check was wrong or some other confusion is happening.

jazzymusicman, What you describe in post 4 is a real good starting point. I do find an even more objective answer can be attained doing it underwater at the end of the dive. More here. I like doing it this way as it is the real deal and saves you from dealing with waves, breathing patterns, fidgetnig feet and all that noise.

Pete

 

[claymore]

If you are leg heavy you may want some of your weight in the trim pockets to lower your center of gravity. If you have floaty feet you will want all of you weight farther down you body.
Using a HP 100 instead of an AL80 opens the door to the buoyancy and size characteristic of the different tank. If they are of different lengths the length will affect you trim. My LP95s don"t trim out as well as my LP108s for me because they are shorter