Doing a more thorough Pre-Dive Check

[boulderjohn]

You forgot "Check computer is on and O2 percentage set".

About 17 years ago, I read a story on ScubaBoard about someone who got bent because he switched tanks from his original intent to dive nitrox and dived air instead. He forgot to change his computer, so he was diving air but going by a computer that thought he was on nitrox.

When I read it, I owned one of those computers that automatically switched oxygen settings after 24 hours (one of the really brilliant moves in computer design), and I had had that happen to me, where I had the dive all set to go, and the computer changed the mix on its own just before I got in the water. Fortunately, it went from nitrox to air (less serious), and I saw it as soon as I checked it during the dive.

I also realized that when using a wrist computer, that computer is the last thing many people put on, and it is easy to forget.

So I wrote to PADI, gave all that information, and said that the instruments you are using for the dive should be included in the predive safety check. I got an immediate enthusiastic reply. They said they loved the idea and were presently working on a new mnemonic. It would take a while to get out, but it would be out soon.

Still waiting. Should be any day now.

So I put an I for instruments into the BWRAF on my own. My mnemonic was Bruce Willis Ruins Another Independent Film. Sadly, that mnemonic is dsrespectful to his current medical condition.

 

[Wibble]

For me, the get into your kit and jump in checks must be done more than once: if you get distracted/whatever and forget to turn on your gas, then this must be caught on a second check.

What I do when diving open circuit; singles or twins is three checks.
0) Before putting your kit on the boat!

• Plan! Depth, gasses, special equipment, weights, etc.
• Fills: pressure, gas analysis (on neck of cylinder so you can see it on the boat)
• Kit: is it ready and working!

1) Before getting donning your kit on the boat:

• Gas: Turn on the valve(s). Breathe from BOTH regs. Check the SPG whilst doing that. Check manifold is open (if relevant). Puff some gas into the BCD/wing.
• Drysuit: Turn on drysuit inflate; route hose so you can reach it
• Harness: Check it's all ready (crotch strap in place), not fouled up. Check double enders. Check lights are threaded, attached and work.
• Me: pockets (SMBs, mask, notes, spare spool, knife), hood, mask, fins, gloves,
• Computer(s): set to right gas? (You've labelled the gas on the cylinder neck, so can check it matches)

2) Getting into your kit

• Untie your cylinder(s) and hold it whilst you turn around, don the kit and buckle up (checking nothing's under your belt)
• Drysuit: inflate hose connected. Dump valve open
• Gas: necklace in place; longhose -- check it's not caught, then hook under battery and round the neck and clipped off. Breathe from BOTH regs. Check the SPG whilst doing that. Puff some gas into the BCD/wing.
• Me: hood on, mask spit, mask on (backwards), fins on, gloves on, Computer(s) on wrist(s), weights (if relevant)

3) Pre jump (will do this at least twice; once whilst seated, once at the gate before jumping)

• Drysuit inflate puff; dump open (proves it's connected and turned on; axillary buoyancy)
• BCD inflate puff (or your normal ready pressure) (proves you've got buoyancy and won't sink)
• Breathe from BOTH regs. Check the SPG whilst doing that. (proves you can breathe)

 

[boulderjohn]

As for making sure the computer is on, with most of today's computers, it is less important, but it could be.

Years ago I dived with a student on a series of dives, and on some of them we noticed that our computers were giving decidedly different depths for our dives. On other dives, they were identical. They were both Shearwater computers.

After a discussion with Shearwater, they figured out the problem and corrected it in their firmware. They had just recently added altitude adjustments to their computers (based, BTW, on discussions with another member of our group). When we turned on our computers to make sure all settings were correct, it read the altitude. If, however, it took us too long to submerge on our dives, the computers would shut off. If they turned on again by submersion, they would assume you were at sea level and would act accordingly.

BTW, the only thing that affects is your knowledge of depths and where you hold your deco stops. The computer's actual decisions for decompression depend upon pressure, not the depth reading.

 

[Fastmarc]

3) Pre jump (will do this at least twice; once whilst seated, once at the gate before jumping)

• Drysuit inflate puff; dump open (proves it's connected and turned on; axillary buoyancy)
• BCD inflate puff (or your normal ready pressure) (proves you've got buoyancy and won't sink)
• Breathe from BOTH regs. Check the SPG whilst doing that. (proves you can breathe)

You didn't include putting on the mask correctly from the backwards position previously mentioned. Unless you enter the water that way?

 

[inquis]

BTW, the only thing that [knowledge of surface pressure] affects is your knowledge of depths and where you hold your deco stops.

And time at each stop, of course.

 

[lowwall]

...If, however, it took us too long to submerge on our dives, the computers would shut off. If they turned on again by submersion, they would assume you were at sea level and would act accordingly.

BTW, the only thing that affects is your knowledge of depths and where you hold your deco stops. The computer's actual decisions for decompression depend upon pressure, not the depth reading.

The wording of this is unclear. The second paragraph looks like you are saying that the altitude at which you are diving doesn't matter, which of course is untrue. The paragraph is applicable if you are discussing depths in salt versus freshwater.

 

[Wibble]

You didn't include putting on the mask correctly from the backwards position previously mentioned. Unless you enter the water that way?

Yes.

But you are floating and you can breathe.

Genuine question…. Do you really want to clutter up the checklist with obvious things?

Agreed, people have died because they’d forgotten their mask. But not people who have buoyancy and breathing gas.

In any case, you’re unlikely to do it again.

 

[boulderjohn]

The wording of this is unclear. The second paragraph looks like you are saying that the altitude at which you are diving doesn't matter, which of course is untrue. The paragraph is applicable if you are discussing depths in salt versus freshwater.

Yes, I would assume that if you are diving at altitude, you did set your computer for the appropriate kind of water. There aren't many high altitude oceans.

The computer calculates decompression needs based on pressure. It uses that pressure to notify you of your depth, based on an assumption of how much of that total pressure is air pressure.

 

[Fastmarc]

Sure you can; you learn how and practice it in a solo class. After you enter and are sorted out, near the surface, just lay back in the water and pause exhaling; look for bubbles around you. If you see some coming from behind your head you can even see if they are a gentle fizz or a solid stream.

Yes.

But you are floating and you can breathe.

Ah, ok. Cool.

Genuine question…. Do you really want to clutter up the checklist with obvious things?

True.

 

[boulderjohn]

Genuine question…. Do you really want to clutter up the checklist with obvious things?

This is an important point.

When I wrote my course on advanced dive planning, I had a whole section on these kinds of checks, and I based a lot of it on research I did years before working with assessment guides. There is a significant diminished return on a tool or process that is too long or too complicated--the whole thing goes unused.

As an example, in my earliest days of tech training, we used a process for changing gases on deco that was extremely long and complicated. It took forever, but it made damn sure we would not switch to the wrong gas. Once I got through that training and we were dong the dives for real, I immediately saw that no one, including the instructor who insisted we do it perfectly every time--did anything like it. It was too long and time consuming.

A very thorough and safe process that you don't use is no better than no process at all. As the old rock group Faces put it, "A nod is as good as a wink to a blind horse."