How to figure out the best tank to buy?

[oxyhacker]

The HMR doesn't say anywhere that a tank must be continually plus rated from manufacture on to be eligible. It just says that any 3AA tank (and certain others) that meets the list of qualifications may be overfilled, as long as it met the requirements at the "last test or retest". So a 3AA tank can be plus marked at any time in its life, as long as it passes the expansion test and other requirements, regardless of its previous status.

I've confirmed this point in phone conversations with Hattie Mitchell at the DOT.

If they did NOT do the "+" test you cannot go back and get it again.

 

[NAM001]

big tuna

i use faber lp95's they are 95 cub at the 10% or 2640. i routine overfill them to 3000-3300 depending on the fill method (wet or dry). at 2400 they hold 86 cf
3000= 108 and 3300 is 119 cf. it is not hard to find a compressor that can fill to 2640 as opposed to 3500 . i have heard of cavers also overfilling tanks to 4k but your lp tanks are burst disked at 90% of 5/3 pressure so thats 3600. i would not attempt to push over 3400 on mine for a hot fill i would fill in water bath to 3200 instead. the previous comments by others are correct. except for a math error here or there. if you are looking at lp vs hp remember the more air you haul the more you can use. pro's is obvious the con's are 100 cf is 8 lbs of compensation you have to do as you consume the air. if you use a lp95 and overfill to 120 cf and plan to surface at 20cf. you may end up surfacing at 50 cf if you dont start your dive heavy enough to comp for the air used. now with all that weight you will still be comping for at the start of the dive you will be countering it with the bc and that has its problems with depth control. having a tank that if positive when empty can relieve some of this problem. im sure there are many other opinions concerning weight control. many of them universal and many are dependant or taylored to the diver or the environment. 100# and 300# divers do it differently but still manage to stay with in the few universal rules of ballance that really count.
you mentioned weight and the al 80. this should only be of concern when out of the water. in the water you are comped.

there is no perfect tank for everyone and what i could suggest is to take a tank in a pool aired up to surfacing volumn 200-300 lbs and get neutral. thats where you want to be at the end of the dive, or slight heavy with 3-500#. if you need a lot of weight (body type) use a steel tank in leu of a bunch of lead. out of the water 35# of tank and 10# lead is the same as 40# tank and 5 lead. your trim needs may make that decision for you. steel tank top heavy leadbelt makes bottom heavy.

if all this isnt enough are you doing both fresh and salt water diving then you have to ballast in fresh and then add weight to go to salt. going from 15 to 20# lead belt is a pain.. perhaps 5-10# in lead and a steel tank vs an al is a better solution.

good luck

 

[BigTuna]

i use faber lp95's they are 95 cub at the 10% or 2640. i routine overfill them to 3000-3300 depending on the fill method (wet or dry).

Does your shop do that for you or do you fill them yourself?

your lp tanks are burst disked at 90% of 5/3 pressure so thats 3600.

Excellent info!

100 cf is 8 lbs of compensation you have to do as you consume the air.

I've been accounting for the air in the tank by adding 5 lbs. You've made me realize that this may be OK for an Al80, but not for other air volumes. I have to start taking that into account. So the number I need to remember is air weighs 8 lbs per 100 cf.

if you use a lp95 and overfill to 120 cf and plan to surface at 20cf. you may end up surfacing at 50 cf if you dont start your dive heavy enough to comp for the air used. now with all that weight you will still be comping for at the start of the dive you will be countering it with the bc and that has its problems with depth control.

I see. The 120 cf of air is 25 cf more than 95 cf, so it weighs 2 lb more (1/4 of 8 lbs). So if I'm ballasted for only 95, I'll go positive when my remaining air reaches about 50 cf. If I had that extra 2 lb of ballast that I neglected to account for, then I could take the tank down to 20 cf, as planned, before going positive.

Thanks, KWS!