Weighing In Discrepencies...OVER WEIGHTED...UNDER WEIGHTED??? Pony Bottle

[Captain Mikey Likey]

Bringing it back to basics...I am 175 pounds diving a 7mm wetsuit in salt water. I dive a 100 steel. I got some goodies on my BCD but nothing to crazy. I was using 2+2(4) on my weight belt...2+2(4) in my trim pockets and 4+4(8) in my ditch pockets...totaling 16 pounds. Did a few dives recently in 25 feet of water and felt as if I was over weighted...like by a lot. I messed around a little bit and came to a happy median between 12-16lbs. I just bought a aluminum 40 pony bottle.

What should I expect in the additional weight of the pony bottle?

Is it true that my 100 steel is 7 pounds negative buoyancy?

The pony bottle is something like 4 pounds positive?

 

[TONY CHANEY]

I dive an Al 50 and a steel 45 (Al for all dives except for caves where the steel is used due to you do not want your tank going anywhere in the caves). IMHO you will not have any big weight changes with the Al. unless you suck it down and then you might feel a difference.

You are more than likely about right with the weight of the steel but it does depend on Mfg. etc.

The Al is not positive when it is full and with the weight of the reg and valve not very positive at 500 psi or even less.

 

[kelemvor]

Is it true that my 100 steel is 7 pounds negative buoyancy?

Worthington HP100's are 10 lbs negative filled to rated capacity (3442psi). They're only 2.5lbs negative when empty. I don't have numbers for any other HP100.

 

[Croomrider]

You should be a little over weighted at the beginning of the dive due to the gas you are going to breathe while down. Adjust you weight to where you are about neutral with no air in your BC at 15 feet, and 500 psi left in the tank.

Mike

 

[Landlocked123]

What Brand HP100?
You saw some gear on the belt..exactly what? Do you know the actual buoyancy of the gear?
Pony sounds about right as I have a 19 and its +2.5 excluding Regulator.
My Faber HP100 is -8.5 lbs negative
All I can tell you is whtaever you do do not switch to a Steel Backplate ! you will regret it… You should be able to work things out nicely given your options for moving and adding/removing weight. Keep experimenting. if you have access to a swimming pool it will be way easier.
Good luck

 

[Crass3000]

When adding my 40cf pony, I added 2lbs. Seemed to work fine. Probably not absolutely necessary unless you are really weighted perfectly before the pony IMHO but somebody may tell you different.

 

[spectrum]

Captain Mikey Likey

I'll begin with your weighting irrespective of a pony cylinder. You mention feeling heavy during your dive at 25 feet. At that point you suit is somewhat compressed and you may still have a large portion of your ear. While you may be able to recognize needing a gross amount of air in your BC to be neutral that usually only comes with considerable experience. Adjusting weights based on mid dive observation is a good way to become under weighted and unable to hold a stop or make a controlled ascent. Some methods of objective weight checking are discussed here.

Your AL40 filled with air will be several pounds negative, give or take depending on brand. Once adorned with a regulator and rigging it will be closer to 4 or 5 pounds.

Now comes the rub...... The buoyancy of the pony usually does not matter. If you are diving with others you may want to preserve the ability to hand it off to another diver in need. Under this scenario you will then need your normal weight to complete the dive. Likewise, if you are solo diving should you end up breathing the pony down you do not want to become under weighted. Some divers will write these scenarios off and resign themselves to muddling though, corking or just hope for the best. This is something you need to come to terms with. While this will technically have you over weighted the 4-5 pounds should not be an issue if you have the skills to be out there solo or making a dive that warrants a 40 CF pony.

Your HP100 is 9 pounds negative filled. Of this 8 pounds is air, about 7 of which you should plan on processing and not having at the end of the dive. There are so many factors that enter into correct weighting that cylinder specification only matter once you have a golden configuration and then want to use a different cylinder. For this you compare the EMPTY BUOYANCY specifications. Then it is a legitimate put or take. Otherwise perform a solid weight check to get a functional baseline.

Pete

 

[tdtaylor]

Here is a listing of the weights and capacities of most common tanks:

Scuba Cylinder Specifications from Tech Diving Limited - 928-855-9400

My 19sf al pony with the regs is 2 pounds negative, as I weighed in in a pool with a luggage scale.

Terry

 

[Michael Guerrero]

For the pony bottle, total the negative buoyancy of the bottle, valve, and regs, and then subtract the weight of air breathed down. It's about one pound for each 13 cu ft of air.

As an example, my Catalina 30 cu ft pony is -2.4lbs full. Add a pound each for the valve and the 1st/2nd stages (rough estimate) and my pony is -4.5lbs. If I breath it down to 500 psi (about 5 cu ft left), I will have consumed 2lbs of gas. So my bottle goes from -4.5lbs to -2.5lbs (roughly). Based on that I don't need any additional weight.

What I have found is that I need to redistribute some weight to my right side to offset the added weight on my left of the pony bottle. That keeps me in trim. That's diving wet, diving dry I just use the bubble.