Stupid Question Time - 'Low on Air'

[surlytart]

I did my first warm water dives last week. Most were either in the 60-70 foot range or were multi-level dives, and most were 45-50 minute dives. I thought it odd that the outfit suggested beginning ascent at 500 PSI for most of the dives rather than surfacing with 500 PSI. One other diver on the trip wasn't too careful about his pressure and couldn't make a safety stop for one of those dives.

Does this seem a little foolhardy to anyone else or is it common practice in warm water?

 

[ShakaZulu]

Calculate how much air you need if your buddies first stage fail right before you guys ascend, and see if you have enough air for the two of your to surface, and do your safety stops?

 

[H2Andy]

the standard usually is "be back on the boat with 500 psi"

it is odd that they would ask you to ascend AT 500 psi

it's also very poor gas planning on their part

i like to use the "rock bottom" concept: the minimum amount of
gas i need to get myself and my buddy to the surface safely,
including safety stop.

for aluminum 80 tanks, a rule-of-thumb rock bottom is
700 psi down to 60 feet and 1,000 psi below 60 feet.

your usable gas is the starting pressure minus the rock bottom.

for example, if you start with 2800 psi and plan a dive to
80 feet, your rock bottom is 1,000 psi so your usable
gas is 1800 psi.

you have 1800 psi, so half of that is for the way out, half
for the way in. thus, you will use 900 psi on the way out.

that means that you turn your dive at 1900 and head back
to the boat, reaching the boat at 1,000 psi.

you now can spend some of that gas in a slow ascent
and get on the boat with 500 psi.

much safer dive (cause you planned it) and you still end
up with 500 psi back on the boat.

more importantly, at any point in the dive, should your buddy
lose 100% air, you both have enough air to make it to the surface
safely on your tank.

 

[fire_diver]

Calculate how much air you need if your buddies first stage fail right before you guys ascend, and see if you have enough air for the two of your to surface, and do your safety stops?

This is, of course the correct answer. However, from what I witnessed in Coz (and has been backed up by others) is that many, if not most, down there dont even know how to calculate their RMV. Probably dont know what RMV is. Which would tend to make it a tad difficult to calculate their rock bottom pressures.

Probably why the dive ops just tell people to surface when they hit (insert random low pressure reading here).

FD

 

[surlytart]

""Calculate how much air you need if your buddies first stage fail right before you guys ascend, and see if you have enough air for the two of your to surface, and do your safety stops?""

We'd have been screwed with my buddy's consumption rate. I remember telling him to surface on a couple of dives as I wasn't comfortable with the op's parameters.

There seem to be a lot of areas that I'd plan much more conservatively than others, if only because I just have tables to work from.

Thanks.

 

[nolatom]

500 is awful thin, even in perfect conditions. Picture your remaining air as belonging to your buddy, and vice-versa. If you have "too much" air remaining, you can use it at your safety stop, you'd be surprised how much interesting stuff there is to see.

 

[Charlie99]

Calculate how much air you need if your buddies first stage fail right before you guys ascend, and see if you have enough air for the two of your to surface, and do your safety stops?

Safety stops are OPTIONAL. I don't include them in my rock bottom calculations. Obviously, one can include them if desired, but IF there is a catastrophic failure right at rock bottom, AND we both breathe right at maximum rate, AND we spend the maximum calculated time at depth before starting to ascend then I won't have enough gas for a safety stop. OTOH, if we are so freaked out that we are both still breathing at 1cfm at the safety stop, then it's best to head on up. YMMV.

Surlytart -- it's common in high viz warm water resort areas for the ascent pressures to be a bit low. More common than 500psi is an ascent pressure of 700 psi from the 40 to 60' range.

There is no need to be back on the boat with 500psi. You need about 200psi to keep the reg working at 15' and to make sure no water gets into the tank. Of course, this assumes that you have looked at the zero point on the SPG and know where the zero really is. If you get yourself in the awkward position of going very low on pressure or cutting a safety stop short, my advice is to go ahead and do the safety stop until you are right around 200psi. (This assumes that surface conditions are reasonable. In rough conditions, you'll want to keep an additional reserve so you can use your reg as you grab the ladder, remove fins if necessary, etc.)

 

[H2Andy]

Safety stops are OPTIONAL. I don't include them in my rock bottom calculations.

you should. if anything, this will fatten up your air reserve a little.

There is no need to be back on the boat with 500psi.

below 500 psi, most gauges are not as acurate and shouldn't be relied on
for an accurate reading. thus, any dive planning below 500 psi should
be avoided.

 

[Imagesetter]

An instructor once mentioned a rough calculation as a minimum for divers. (not in a instructor setting) 100 psi for every 10 ft of depth, minimum 700 psi and additional if a swim is required before surfacing.

 

[Mr Carcharodon]

I did my first warm water dives last week. Most were either in the 60-70 foot range or were multi-level dives, and most were 45-50 minute dives. I thought it odd that the outfit suggested beginning ascent at 500 PSI for most of the dives rather than surfacing with 500 PSI. One other diver on the trip wasn't too careful about his pressure and couldn't make a safety stop for one of those dives.

Does this seem a little foolhardy to anyone else or is it common practice in warm water?

Using rock bottom, and making the assumptions: (1) a 70 foot starting depth; (2) a no deco dive; (3 and 4) a RMV of 0.6 cu ft/min for both you and your buddy; and (5) a 80 cu. ft. tank I get a turning pressure of 474 psig. That should be enough to you and you buddy to the surface with zero reserve.

But that seems like a lot of assumptions to me. Unless the divers are well trained it is likely that their RMV will go up signifcantly in a OOG situation. If you allow one of the divers to go to RMV of 1.2 cu ft./min. (to allow for stress due to OOG) then rock bottom goes to 690 psi. So the original suggestion of starting up at 500 psi is marginal for two divers in an emergency.