Diving doubles.

[Packhorse]

I am just about to recieve my manifold for my doubles. Im using 2 11.6 litre (100cf) Mares with a OMS manifold.
Now I have always been told to keep 50 bar reserve. I instictivly converted this to litres. I trained with 10L tanks so 50bar X 10L tanks = 500Liters of air.
When I dive with my 15litre tank (123cf) I keep a 35 bar reserve 35x15 =525 liters of air.
Now 50 bar= 725 psi & 35 bar = about 500psi so the metric way is very conservitive compared to imperial.
Now at 21 bar or 304psi will give me 500 liters of air when diving doubles. I feel this is plenty of reserve air volume but my question is is 21 bar too low pressure? Will this cause any problems like condensation forming in the tank or hassles with the dive shop when I get them filled.
What do other double divers keep as a reserve?
Also since the dive shop only has to connect 1 fill whip should I expect to only pay for 1 tank fill ?

 

[Mark Vlahos]

Please forgive me, but since I live in the USA I don't speak metric very well. I will relate my opinion in PSI.

First lets get out of the way the really good idea that deeper dives have you starting up with a larger reserve in your cylinder than when you choose to end a shallow dive. Lets just keep it to some magical and hypothetical number.

First, lets say that on a dive with a single Aluminum 80 I choose to climb back on the boat with 700 PSI in my cylinder. That translates into about 18 cubic feet.

Second, lets consider what the 18 cubic feet would be with double low pressure 95's.... Hmmm, that comes to a bit less than 300 PSI. Logic would indicate that a reserve of 300 PSI would be fine, Well, not really. You need to consider that your pressure guage might be inaccurate and actually be as much as 200 PSI off at the bottom end of the scale. You just might find that if you acutally need the air you might not have any where close to the 18 cubic feet you expected. With doubles I really don't like getting them below 500 PSI. I don't like it in a single either, but it is more tempting in doubles.

Sure your pressure guage might never be this inaccurate, and you might never need to breathe this reserve gas because you never experience any problems, and your buddy never does either. Well, in my book if I need the reserve gas I want to make darn sure it is actually there and not just numbers on a dial that has no breathable air behind it.

The message above makes no mention of how much gas it might take to get to the surface for both you and your buddy in an emergency at the beginning of the ascent or decompression obligations. We have all seen plenty of divers who try to extend their bottom times by balancing on the razors edge with regards to gas reserves. It is rarely a problem to reach the surface with just a bit too much gas in your cylinders.

Mark Vlahos

 

[Packhorse]

Thanks. What you say makes alot of sence. But the reason for me asking the question was not really based on gas planning. For deep dives I always calculate a turning pressure and stick with it. I supose could simplify the question by asking what is the minimum pressure that you should have in a tank to avoid problems with the tank.

 

[wunat]

Also since the dive shop only has to connect 1 fill whip should I expect to only pay for 1 tank fill ?

The shop might then decided to shut the isolator valve and fill the one tank you paid for.

 

[BarryNL]

I am just about to recieve my manifold for my doubles. Im using 2 11.6 litre (100cf) Mares with a OMS manifold.
Now I have always been told to keep 50 bar reserve. I instictivly converted this to litres. I trained with 10L tanks so 50bar X 10L tanks = 500Liters of air.
When I dive with my 15litre tank (123cf) I keep a 35 bar reserve 35x15 =525 liters of air.
Now 50 bar= 725 psi & 35 bar = about 500psi so the metric way is very conservitive compared to imperial.
Now at 21 bar or 304psi will give me 500 liters of air when diving doubles. I feel this is plenty of reserve air volume but my question is is 21 bar too low pressure? Will this cause any problems like condensation forming in the tank or hassles with the dive shop when I get them filled.
What do other double divers keep as a reserve?
Also since the dive shop only has to connect 1 fill whip should I expect to only pay for 1 tank fill ?

If you're diving doubles you're ready to do proper gas planning. This means working out your expected gas use and carrying 50% extra with you for reserve (a.k.a. "rule of thirds" when you're doing an in-and-out type dive). Doubles type dives tend do be deeper with more risk of unplanned decompression etc. so you're more likely to need more gas to get out of trouble, not less. And don't forget, you might need to get your buddy out on that gas too...

 

[Packhorse]

Thanks but maybe you missed my above post.

 

[BarryNL]

Thanks. What you say makes alot of sence. But the reason for me asking the question was not really based on gas planning. For deep dives I always calculate a turning pressure and stick with it. I supose could simplify the question by asking what is the minimum pressure that you should have in a tank to avoid problems with the tank.

Hmm, I guess in theory you need the pressure in the tank to be at least the intermediate pressure plus the ambient pressure. So, if your regulator intermediate pressure is 10 bar and you're diving at 20m (3 bar) you need at least 13 bar in the tank for the regulator to deliver air (for a balanced 1st stage, at least). In reality, this means the system should keep happily delivering air way below what you should ever have in your tank if you're paying attention to your SPG.

 

[Packhorse]

The reason I ask has more to do with getting as much usage out of the tanks as posible so I dont need to take another tank out as well.
Say I was planning a deep dive and expect to return with 80 bar (1856 Litres or 65cf)
I could latter do a shallower dive and use the doubles again.

 

[Packhorse]

Hmm, I guess in theory you need the pressure in the tank to be at least the intermediate pressure plus the ambient pressure. So, if your regulator intermediate pressure is 10 bar and you're diving at 20m (3 bar) you need at least 13 bar in the tank for the regulator to deliver air (for a balanced 1st stage, at least). In reality, this means the system should keep happily delivering air way below what you should ever have in your tank if you're paying attention to your SPG.

OK that covers air delivery concerns, although I feel 13bar or 300 litres @ 20 m is cutting things a bit fine. But as I said Im more interested in minimum pressure to eliminate problems with the tank. Eg some dive shops may require a viz before filling empty tanks & I think there are condensation concerns with tanks with low pressure.

 

[BarryNL]

OK that covers air delivery concerns, although I feel 13bar or 300 litres @ 20 m is cutting things a bit fine. But as I said Im more interested in minimum pressure to eliminate problems with the tank. Eg some dive shops may require a viz before filling empty tanks & I think there are condensation concerns with tanks with low pressure.

Hmm, again - shouldn't be any problems. The tank temperature in the water is always going to be very close to the water temperature - you can't get air out fast enough to cause a significant drop in ocean temperature . So you won't get condensation while diving. If you empty the tank out of the water you will get condensation but generally only on the outside unless the filling air wasn't as dry and it should be. I've emptied tanks for inspection before and the outside gets covered in condensation but the inside stays dry.

During filling the thing gets hotter, so you won't get condensation then either. I've never had a dive shop complain about filling a new empty tank when I've just bought it there - can't see why they'd complain any other time.