If your answer is yes to both of these questions, then the decision is already made for you. Plus, the reasons for that decision make all the sense in the world. Let me explain.

DIR dictates that your tanks be steel LP 104?s. Actually, 104?s are most common, but anything between 95?s and 104?s is acceptable. Anything larger than 104?s and you get into diminishing marginal returns. Setting a standard like 104?s diminishes the need for dissimilar tank gas matching, which could introduce human error. Also, if your buddy doesn?t have 125?s like you do, then you?re killing your knees for nothing and tiring yourself from more drag on the dive. If you need more gas than this, then start staging with AL 80?s. You?ll see shortly that the DIR standard actually coincides with longer bottom/penetration times than many other options from which you could choose. Having a standard like this also follows the ?KISS? principle (Keep It Simple, Stupid). But the dive industry has to make a little money trying to chisel out market niches, so that?s why you have all the confusing choices. Do it DIR and forget everything else.

What about overfilling? Oohh . . . there is a ton of bickering over this. Simple rule of thumb that some technical dive shops have been doing for MANY MANY years and have had NO problems. The rule of thumb is that you should only overfill LP steel cylinders. Yes, we regularly overfill our AL 80?s to 3500 too, but that?s not really a big deal. In overfilling, I?m talking about 1100 to 1200 lb overfills. I wouldn?t do that on an AL 80.

Here?s an example, You have double aluminum 80?s and I have LP steel 104?s. Assuming you overfilled yours to 3500 psi, you?ve got 186 cf of gas. If I overfill my tanks, to the same pressure, I?ve got 273 cf of gas. Now which set of doubles is going to get you longer bottom times and penetrations? From a safety standpoint, would you rather enter with 186 cf or 273 cf of gas?

Some opponents of overfilling are afraid the tanks are going to blow up. Some tech shops are overfilling LP steels. Do they for some reason desire to die more than you or I? Well, that?s why God invented burst discs. The disc would go before the tank exploded. However, many tech divers do double their burst discs or insert shims because the discs create an additional fail point during a dive. So the diver does have a responsibility to take care of the tanks if he is going to do this. I?ve never heard of an exploding LP steel tank killing a cave diver, but I have heard of many deaths in caves as a result of running out of gas. So, I think it?s safe to say that the risk of explosions are statistically insignificant compared to running out of gas. I?d rather have the extra volume. Some say to just get larger tanks. Well, the largest I can get is 125s. That?s only 21 cf more per tank, plus you have the extra drag and weight. I can overfill 104?s and get 32 more cf per tank than a 125 and save the weight and drag. Seems like a no-brainer to me.

Some argue that it?s a DOT regulation thing & that overfilling is illegal. Well, first of all, there isn?t a single DOT engineer going to die for you in a cave. You?ve got to do that for yourself. So the incentive is higher for me to go with the larger volume than blindly follow some arbitrary standard by someone who doesn?t dive and can?t die for me. Yes, I said arbitrary for two reasons. One, the European?s have been filling their tanks to these pressures all along. It?s been legal there. As I've already said, so too have some tech fill stations. DOT just decided not to take ANY risks and rated LP?s conservatively. Secondly, there is a new series of tanks out on the market now and guess what!! Their ratings are the equivalent of the pressures and volumes we?ve been overfilling to all along. And except for maybe VERY minor points, the tanks are exactly the same as the old LP's. What happened? Bluntly put, I?d bet the DOT was finally proven wrong by the tech community and instead of the DOT loosing face, they agreed to rate higher a new tank series if one were manufactured. And there you have it ? tanks that are rated to like 3500 or so PSI and have the same volume as an LP 104 overfilled to the same pressure. So really, the issue of LP versus HP and overfilling will soon be moot.

There are three big reasons HP's haven't been very popular in the cave diving community. One, in order to get your HP100 up to 100 cf, you have to get a 3500 PSI fill, which is what the tank is rated to. If you go to a dive site that has no haskel and can?t get a fill up to that level, then you loose the full value of your tank. For example, your shop can only fill your double HP100?s to 2640. That means you only have 151 cf of gas versus the 200 cf you should have with a full fill. Then I come up with LP104?s and get my 2640 fill. Well, that gives me 208 cf of gas, which is what they?d be rated to together. The second reason is if you can get just as much volume from a full fill 104 at 2640 that you can an HP100 at 3500, then why would you subject your first stage, regs, burst disks, and o-rings to that higher pressure? It goes to reason, the higher the pressure, the quicker your parts are going to wear out. So I fail to see the benefit that HP tanks offer. The third reason is that HPs require the use of DIN. While DIN is DIR, sometimes people haven?t switched to DIN yet. So LP valves allow you to use yoke inserts until you make the switch. Of course, you shouldn't overfill the tanks if you are still diving yoke though.

The aluminum versus steel debate has been going on forever too. I think the only reason aluminum tanks ever got popular is because they were lighter, cheaper, and more readily available than steel. While each of these points is true, steel is still better. If I were doing nothing but rec open water diving again, I?d dive steels. As already mentioned, for a little extra initial investment, you can overfill LP steels and get A LOT more volume in a single tank than you can an aluminum. If your goal is to extend bottom time, then the case is closed. Plus, since steel tanks weigh more, you can use less weight on the dive. Any time you can drop weight from your belt, your trim will be better and drag will be reduced. And aluminum tanks get very buoyant after the middle of a dive. I?ve learned this just by diving stages. The butt of that Al 80 will ride real high in the water toward the end of the dive.

Bottom line, save yourself a lot of hassle and wasted money and go DIR. It will cost a little more in the beginning, but will save your butt in the end (no pun intended).

Exercise a little discipline while saving up for your doubles and don?t buy that set of used HPs for an ?unbeatable? price.