Rule of Thirds & Shallow Rec diving

[00wabbit]

Another one for Rock Bottom. Rock Bottom seems mathy at first but it's not too hard and there are some good phone apps they will calculate it quickly and show you the RB for various depths. Scubacalc (it's called Min Ascent Pressure)

 

[MXGratefulDiver]

i never let my tank get lower than 500# for fear of getting sea water in my tank...

... anything higher than 14.7 psi will get that job done ...

... Bob (Grateful Diver)

---------- Post added November 25th, 2013 at 12:43 PM ----------

Another one for Rock Bottom. Rock Bottom seems mathy at first but it's not too hard and there are some good phone apps they will calculate it quickly and show you the RB for various depths. Scubacalc (it's called Min Ascent Pressure)

The math isn't particularly difficult ... especially if you use "scuba math" by rounding things off.

But Rock Bottom calculations are intended to be a worst-case ascent scenario where two stressed-out divers are making a direct ascent to the surface while breathing off a single tank. It's a useful tool for planning when it's appropriate to leave the deepest part of your dive, and if you're boat diving and making a relatively square profile it's a good measure of minimum gas reserves. On the other hand, if you're shore diving, or diving a profile where you'll be coming upslope before making your ascent, it's often more reserves than you need, unless you're good enough at math to recalculate in your head as your depth changes.

Turn pressure is calculated based on one diver (you) breathing off your tank, and assumes that you're not in an emergency or breathing heavily due to a stress situation ... in other words, normal diving. For some profiles it's a more useful way to calculate when you should turn the dive, or begin ascending to a shallower depth.

If Rule of Thirds is a hammer, Rock Bottom is a crescent wrench ... more versatile and very useful to be sure ... but still only one tool that isn't always the optimal one to use for every situation ...

... Bob (Grateful Diver)

 

[freewillie]

The rule of thirds is for diving in overhead environments. If you are not in an overhead environment then the rule doesn't necessarily apply. It will keep you safe for shallow recreational diving but you will be missing out on getting appropriately safe longer bottom times.

In general the goal would be to get back to the surface with 500 psi or just over. That needs to include possible safety stop for at least 3 minutes. So if you decide on a certain gas level as your turn around time you need to be back at the finishing destination with over 500 to actually finish the safety stop AT 500 psi. The 500 psi then becomes the safety margin for recreational diving analogous to the 1/3 rule for overhead diving.

The extra time depends on conditions as pointed out earlier. I use a general rule of thumb turn around pressure is about 1400-1600 psi depending on type of dive and visibility. I use the shorter for shallow dives with good visibility, the higher for deeper dives with less visibility. I also "push" the lower limits for shore dives since for me much of the return dive tends to be over sand at shallow depths. If we get close to the 500 psi or at least get under 1000 psi we can always end the dive at that time and just have a longer surface swim back to the beach. I personally like a bigger margin for boat dives in case something unexpected happens or get confused on the navigation/bearings. Gives me a little extra time to get reoriented without having to compromise the safety stop.

 

[phoenix31tt]

... anything higher than 14.7 psi will get that job done ...
But Rock Bottom calculations are intended to be a worst-case ascent scenario where two stressed-out divers are making a direct ascent to the surface while breathing off a single tank. It's a useful tool for planning when it's appropriate to leave the deepest part of your dive, and if you're boat diving and making a relatively square profile it's a good measure of minimum gas reserves. On the other hand, if you're shore diving, or diving a profile where you'll be coming upslope before making your ascent, it's often more reserves than you need, unless you're good enough at math to recalculate in your head as your depth changes.

you don't have to recalculate... or at least i don't... i pre calculated a range of depths, they are easy to memorize for your most common depths and easier to put on a slate...

in that way you know your air limit for the depth you are currently at, my table looks like:

Depth	Rock Bottom PSI
<40	500​
<60	800​
<80	1100​
<100	1400​
<110	1700​
<130	2300​

 

[00wabbit]

you don't have to recalculate... or at least i don't... i pre calculated a range of depths, they are easy to memorize for your most common depths and easier to put on a slate...

in that way you know your air limit for the depth you are currently at, my table looks like:

Depth	Rock Bottom PSI
<40	500​
<60	800​
<80	1100​
<100	1400​
<110	1700​
<130	2300​

This is what I do. To keep on the tool analogy it makes it more like a torque wrench I guess. If you hit rock bottom for one depth just move up a bit and you have a little more time.
Of course this needs to be recalculated based on the tanks you are diving.

 

[iluvtheocean]

The rule of thirds is for diving in overhead environments. If you are not in an overhead environment then the rule doesn't necessarily apply. It will keep you safe for shallow recreational diving but you will be missing out on getting appropriately safe longer bottom times.

In general the goal would be to get back to the surface with 500 psi or just over. That needs to include possible safety stop for at least 3 minutes. So if you decide on a certain gas level as your turn around time you need to be back at the finishing destination with over 500 to actually finish the safety stop AT 500 psi. The 500 psi then becomes the safety margin for recreational diving analogous to the 1/3 rule for overhead diving.

The extra time depends on conditions as pointed out earlier. I use a general rule of thumb turn around pressure is about 1400-1600 psi depending on type of dive and visibility. I use the shorter for shallow dives with good visibility, the higher for deeper dives with less visibility. I also "push" the lower limits for shore dives since for me much of the return dive tends to be over sand at shallow depths. If we get close to the 500 psi or at least get under 1000 psi we can always end the dive at that time and just have a longer surface swim back to the beach. I personally like a bigger margin for boat dives in case something unexpected happens or get confused on the navigation/bearings. Gives me a little extra time to get reoriented without having to compromise the safety stop.

shore dives do tend to be a different animal -- worse case scenario there is a surface swim on your back or on snorkel and much shallower depths. i've come out with more than 1 shore dive with only 200 or so left.


if my buddy is diving a rental al80 and the boat say be back with 500 psi. that's roughly a 20+ cu ft difference between me and him @500 since i dive hp130's. not necessarily a bad thing since my time would then be limited by my buddy and/or ndl. and gives us both a slightly better safety margin. Better safe than sorry.

 

[MXGratefulDiver]

This is what I do. To keep on the tool analogy it makes it more like a torque wrench I guess. If you hit rock bottom for one depth just move up a bit and you have a little more time.
Of course this needs to be recalculated based on the tanks you are diving.

... and the consumption rate of the person you're diving with.

What you guys are doing is great for ballparking ... and quite a bit more useful than what most people do. On the other hand, remember what Rock Bottom is ... it's a calculation for a worst-case scenario where either you or your dive buddy ran out of air and the two of you are ascending while breathing from one tank. To calculate it accurately you need to take into account the consumption rate of both divers ...

... Bob (Grateful Diver)

---------- Post added November 25th, 2013 at 02:06 PM ----------

if my buddy is diving a rental al80 and the boat say be back with 500 psi. that's roughly a 20+ cu ft difference between me and him @500 since i dive hp130's. not necessarily a bad thing since my time would then be limited by my buddy and/or ndl. and gives us both a slightly better safety margin. Better safe than sorry.

... actually it's roughly six cubic feet difference ... which is still almost 50% more air on the HP130.

At 500 psi an AL80 holds roughly 13 cubic feet of air ... an HP130 holds roughly 19 cubic feet.

... Bob (Grateful Diver)

 

[boulderjohn]

By an amazing coincidence, Natalie Gibb's latest about.com scuba article is on the application of the rule of thirds:
The Rule of Thirds for Scuba Diving Gas Management - Diving Planning

 

[phoenix31tt]

... and the consumption rate of the person you're diving with.

What you guys are doing is great for ballparking ... and quite a bit more useful than what most people do. On the other hand, remember what Rock Bottom is ... it's a calculation for a worst-case scenario where either you or your dive buddy ran out of air and the two of you are ascending while breathing from one tank. To calculate it accurately you need to take into account the consumption rate of both divers ...

... Bob (Grateful Diver)

oh yes, i should've mentioned that those figures are calculations with my wifes rates as well... so it does take into account my buddy (which doesnt change )...

and we always dive with al80s

but yes i can see how it may not be as useful if you're the type that basically insta buddies or is constantly switching tank sizes..

 

[Rich Keller]

Hey guys,
I'd love to hear you opine about this rule in regards to "vacation divers." If you're under 50 feet do you really want to be getting back on the boat with 1000psi left in your tank?
I understand in tech diving it makes sense as it is an overhead environment, but this seems overly cautious when the surface is a cesa away. I'm not advocating running it empty, but when you dive tropical beginner level dives what psi do you want to return with?

I usually want to get back on the boat with at least 500psi. As others have already stated there are a lot of variables that might cause you to pad that a bit too. Also take into consideration that some of these people do not wear snorkels, because none of the cool kids do, so they may need more air at the surface to swim back to the boat.