Little River 11/4: Viz 40', PI 1.55

[Rick Palm]

Dive: Up the main line to the big dorf marker, turned Left for the Serpentine Tunnel, and then Left again and through the Florida Room to the 1100 foot marker, and back by way of the Merry Go Round for a total dive time of 46 minutes. No deco.

Conditions: Flow was PI 1.55*, with particulate matter in suspension reducing visibility to ~ 40 feet. There is much fine silt that has settled over the rocky bottom parts of the proximal system, with silt settling out in the bottom of the Serpentine Tunnel. With carelessness, someone could really muck things up there. A fun dive, but a long shot from the old days of crystal clear cobalt blue water, and the "Little River Pull and Glide" system of locomotion.

*Palm (Flow Rate) Index: 1.55 [The PI is computed by dividing the dive time entering the cave to the point where the dive is turned, by the dive time to exit. An index of 1 means no flow. A PI of less than 1 is a siphon, and a PI of greater than 1, is a spring. For this dive, it took me 28 minutes to swim to the 1100 foot marker, and 18 minutes to swim back: 28 divided by 18, is 1.55. The greater the PI, the higher the flow. Obviously, the PI is only applicable to dives where the ingress is the same as the egress. (Note: The Serpentine Tunnel is approximately the same length and character as the Merry Go Round, so the PI is still applicable)].

[aainslie]

Dude, math is your friend.

That's a flow rate of 11 fpm, and you were swimming at 50 fpm. Not a bad swim speed.

Spreadsheet attached. Look at the "speed calcs" section if the first page makes your head hurt.

Nice to see you're keeping track of times in and out. This is really useful info.

Edit... there's a tendency to swim quicker in than out. So the adjusted flow is probably around 15-20 fpm, making your swim speeds ~55-60 fpm in, ~40-45 out.

[RN]

I like that. According to your equation, I had a PI of 1.53 the other day in one of the river caves I was in. I'll have to check out Andrew's spreadsheet when I get home. I don't think it will help me figure out flow, though, since I don't know my exact penetration distance. But it will still be useful in the popular, marked caves.

[aainslie]

If you know your general swim speed you can still back out the flow rate:

PI = (swim+flow)/(swim-flow)
PI*swim-PI*flow=swim+flow
swim(PI-1)=flow(PI+1)
flow=swim*(PI-1)/(PI+1)

So for example if your PI is 1.53 and your swim speed is 60 fpm then the flow would be:

60*.53/2.53

Or about 12 to 13 fpm.

Of course you'd need to put you real speed in. And as i said higher, this tends to give too low a reading since we tend to swim in faster than out. So once more the real flow rate would probably be nearer 15-20 fpm.

[Rick Palm]

Cool! I also liked Andrew's spreadsheet, especially page 2. I like the PI because it's quick and easy for the mainstream caves. I liked Andrew's comment about keeping track of ingress and egress times; they help for future dive gas management, too. Divers should note their times, depths, END, and gas used, swim speed, etc., if for nothing else, to reinforce basic principles. For example, how many divers can compute their ENDs? I use Dalton's Law circle of equations; it works great and reinforces those important concepts of partial pressures, fractions of gas, and absolute pressures.

[aainslie]

OK, now that I snuck in the thought that a bit of math is useful, have a careful look at the first tab.

That was designed to determine turn pressures in a sump. But if you enter negative flow, you'll find that keeping double your required gas to exit can be done while breathing substantially more than 1/3 your gas. Even with extra safety margins in there.

So... if you use a bit of math, and believe that the aim of the exercise is to always have double the gas needed to exit, and if you know the flow rate (a lot of ifs, but all manageable) THEN you can go below thirds safely. Rick, with your speeds and a nice Murphy's allowance, you can safely drop to 2200 psi on a 3600 fill before turning. Even 2100 is reasonably conservative.

I now do this routinely. And believe me, I've had my fair share of scenarios where gas became unavailable.

I have some other tricks. But I'll save those for another time. As Rich, Chris and Elisha will tell you, my tanks tend to leave the water... emptier than most. Yet I guarantee you I'm in no risk of dying from a lack of gas.

Seriously, math is your friend in the caves.

A BIG caveat - DON'T DO THIS UNLESS YOU UNDERSTAND THE MATH WELL ENOUGH TO REPLICATE THAT SPREADSHEET WITHOUT USING MINE!!! And don't do it unless you've computed your swim speed and the flow rate in the cave you're about to do this in. You should NEVER rely on someone else's calculations if using the rule of doubles rather than the rule of thirds. Also don't pull this stunt on an unsuspecting buddy - and good luck writing the dissimilar doubles spreadsheet that I didn't bother to write since I only do this stuff when diving solo.

To the inevitable naysayers, can I just remind you of what I said last time I threw this idea out there - if you put zero flow in and add a reasonable Murphy's, you'll find that you are NOT safe turning on thirds in Peacock, or in most Mexican caves. the problem with thirds is, your level of risk varies on every dive. I personally find that alarming.


(OK, my umbrella's up, I'm ready for the responses... )

[Daedalus]

Witch! Witch! Burn the Heretic!

[marinediver]

Andrews tanks coming back low?? I thought steel 108's were supposed to float a little bit in the fill tub!!!

[EGIB]

I have some other tricks. But I'll save those for another time. As Rich, Chris and Elisha will tell you, my tanks tend to leave the water... emptier than most. Yet I guarantee you I'm in no risk of dying from a lack of gas.

I don't worry when they are low, I worry when they are full

[RN]

If you know your general swim speed you can still back out the flow rate:

PI = (swim+flow)/(swim-flow)
PI*swim-PI*flow=swim+flow
swim(PI-1)=flow(PI+1)
flow=swim*(PI-1)/(PI+1)

So for example if your PI is 1.53 and your swim speed is 60 fpm then the flow would be:

60*.53/2.53

Or about 12 to 13 fpm.

Of course you'd need to put you real speed in. And as i said higher, this tends to give too low a reading since we tend to swim in faster than out. So once more the real flow rate would probably be nearer 15-20 fpm.

I finally got a chance to open the spreadsheet. I'll have to sit down with it this weekend. Interesting stuff. I'm not sure I'm ready to start using it to plan my dives yet as I tend to go more conservative when I'm solo even though I could ease up a bit (I ended my last dive at your turn pressure...) I'd guess the flow rate on the last system I was in was in the 15 fpm range. I was probably moving about 35-40 fpm the first/last 1/3s due to the small passage, then increased it to 50-60 fpm during the middle portion of the dive. Got a spreadsheet for that?