Please share tips for buying a used tank

[tbone1004]

so the A should be an up arrow, but that tank was likely born in 06/1995. I wouldn't buy it due to the age alone, but that's just me. It won't be the bad alloy, I just prefer to have AL80's with less than 10 years of use on them when I buy them, whether it is sitting or being used, they stress differently than steel.

Regarding the steel. You will have to determine what it actually is in order to determine your turn pressures. Read up on Rock Bottom Calculations and you will realize that the turn is based on cubic footage. I'm going to assume that the 90 is actually a 95 as that is a "normal" tank size, so with that it is 95cf at 2640+10%=2640psi. This is how all of the low pressure tanks are rated.
Based on that, your buddy will have 77.4cf/3000psi, and you will have 95cf/2640psi. Tank factors are 2.6cf/100psi for your buddy and 3.6cf/100psi for you. For every 250psi he burns, you will burn 100psi *nominal numbers* assuming you have the same sac rates. Without knowing the SAC rates of each diver, it is impossible to determine your turn pressures when calculating rock bottom. You can approximate.
If you were doing linear penetration for tech diving which is rule of thirds, then you have to calculate the dissimilar tank volumes. In this case, 77.4/3=25.8cf=>1000psi. Easy enough for your buddy. For you, you have to figure out at what pressure you will have used 25.8 cf, in the case of a LP95, that is ~700psi, so with a fill of 2600psi, you would have to turn at 1900psi, when your normal "third" would have been 1800psi.

 

[geoff w]

Google Luxfer aluminum tank and you will see plenty of information. There's a pretty good writeup on the auction web site about what to look out for when buying used scuba tanks at Buying Used Scuba Cylinders

So I was able to obtain a low pressure steel tank from my brother. I dropped it off today to get inspected and filled. He believes it is a 90cuft.

My new question is this

If I dive with a buddy who rents an al80 with 3,000psi and I have a 90cuft low pressure tank with 2400 psi, how do we determine the turn pressure? Will his gauge drop faster than mine since its roughly the same tank just less pressure? I'm sure I'm missing something here

If you and your buddy have mismatched tanks, or even the same size tanks with different fills, my general rule of thumb is whoever gets to a little over half tank first lets the other know and that's when we turn the dive. It also works if one buddy is an air hog and the other doesn't breathe. The same practice works with more than two divers in a team. Half a tank is half a tank, whether it's a big or little tank, high or low pressure, or different air consumption. If you figure the dive is at least half over and it's time to head back when the first diver gets to half a tank everyone should end up with their own air at the end and no drama. Of course, if you're swimming into current, expect a longer swim back to your exit point, or someone is having issues, you should factor in additional conservative air reserve for contingency and turn back before anyone gets down to half a tank. If someone is bad with math, make sure they know what their pressure gauge should read (in PSI or bar), not just a percentage of a their starting pressure. Half of a 3,000 PSI is meaningless if you only start with 2600 PSI!

 

[geoff w]

If you are looking at buying a tank out of visual and hydro, ask the seller if they will stand behind the tank if there is a problem. If they say "no", keep looking. There are always more tanks for sale out there and a cheap price doesn't make up for a tank that won't pass inspection and nobody will fill, unless you are planning to start making big wind chimes

 

[Diver0001]

So I was able to obtain a low pressure steel tank from my brother. I dropped it off today to get inspected and filled. He believes it is a 90cuft.

My new question is this

If I dive with a buddy who rents an al80 with 3,000psi and I have a 90cuft low pressure tank with 2400 psi, how do we determine the turn pressure? Will his gauge drop faster than mine since its roughly the same tank just less pressure? I'm sure I'm missing something here

Turn pressure is still 1/2 + 20% on a basic "out and back" type of dive. Depending on air consumption, one or the other of you will reach the turn pressure first. The first buddy to reach his/her turn pressure turns the dive.

R..

 

[Diver0001]

I never intended to buy a tank this soon. Also I was going to wait to buy a HP steel. However, I came across an AL80 for $60.00. I have used an 80 on every dive and jist rented one for 15.00.

I'm pretty sure I'm going to go look and purchase this tank this weekend. Can you share insight on what to look for. Tank needs hydro and vip. Poster says it also comes with diving weight but gives no information.

Luxfer Sherwood 06a95 80cf

View attachment 366470

If it's out of hydro then you need to go into this with open eyes. If it fails hydro and/or the maintenance costs are high because it needs extra work then it might not be money well spent. Don't look just at the asking price.... look at the "potential" price. In the worst case you get screwed for your entire investment.

The best policy is to buy tanks 2nd hand that have recent hydro's. They're out there.

That said, I've bought tanks before that were out of hydro but I paid next to nothing for them. If I'm taking a gamble then then seller should share the risk.

R..

 

[keeth1123]

So how much weight would you recommend to start with for a steel tank. I used 12lbs fresh water with an al80.

 

[Littlerayray]

In cold fresh water with my 117 hp steel Faber in 60 degree water with my 7mm farmer john I have 16lbs of weight so chances are you could drop easily 2lbs if not 4lbs total but everyone's different someone who is more lean with little or no body fat will require less weight then someone who is more or less obese

 

[geoff w]

So how much weight would you recommend to start with for a steel tank. I used 12lbs fresh water with an al80.

It depends on what steel tank you get. A low pressure steel 72 is only a couple of pounds more negative buoyant than an Al80. An old Worthington high pressure 80 is about 6 pounds more negative buoyant than an Al80. If you know what kind of steel tank you can usually Google the buoyancy characteristics. They're all different. An empty Al80 is about 3 lbs positively buoyant. I need about 6 lbs less lead when I go from an Al80 to steel HP 80 in salt water