Instruments Should Confirm What You Already Know: Gas Math Underwater

Your gauge reads 1,800 psi.

Quick. How many cubic feet is that? How long until your turn pressure? What will the needle say in ten minutes?

If you have to stare at the gauge to answer, the gauge is running your dive. You're not.

What does "instruments confirm" mean?

It means the number on your wrist or your hip is never news.

Before you look, you already know roughly what it will say. You know your start pressure. You know your depth and how long you've been there. You know your breathing rate. So you know your gas.

Then you look. And one of two things happens.

It agrees. Good. You're on plan, and you've confirmed your picture of the dive is accurate.

It doesn't. Even better, in a way. The gap between what you expected and what you see is information. Something changed. Your job is to find out what, before it finds you.

That's the whole idea. The instrument checks your thinking. It doesn't replace it.

Why do UTD divers work this way?

Because UTD's second principle is the thinking team. No team member relies on another person or piece of equipment to make the sole decisions. No "trust me" dives.

An instrument you don't check against your own math is a "trust me." It's also why UTD-trained teams run their computers in gauge mode: the decompression plan lives in the team's heads, and the computer confirms the depth and time it runs on.

What should you track in your head?

Six numbers. All the time.

1. Time. How long you've been down, and how long the plan allows.
2. Depth and average depth. Where you are now, and the average your gas and decompression plan run on.
3. Gas, as volume. Psi is a pressure. Gas is cubic feet.
4. Gas rate. How fast you're using it at this depth.
5. Turn pressure and rock bottom. Worked out before the dive. See Rock Bottom Calculator.
6. Your team's gas. You're carrying gas for them too.

How do you do the math underwater?

You don't need a calculator. You need two habits and one number.

The number is your tank factor: the cubic feet in every 100 psi. UTD rounds an AL80 to 2.5. Double AL80s are 5. See Tank Factors.

Habit one: pressure to volume. Drop the last two zeros and multiply by your tank factor.

• 1,800 psi in an AL80: 18 × 2.5 = 45 cubic feet.
• 1,800 psi in double AL80s: 18 × 5 = 90 cubic feet.

Habit two: how fast it's going. Breathing rate × ATA ÷ tank factor × 100.

• At 66 feet you're at 3 ATA. Swimming at UTD's planning rate of 0.75 cubic feet a minute, that's 2.25 cubic feet a minute.
• In an AL80: 2.25 ÷ 2.5 × 100 = 90 psi a minute.
• In double AL80s: 45 psi a minute.

Now put it together. You started with 3,000 psi in an AL80 and you've spent 20 minutes at an average of 66 feet. Before you look, you know: 3,000 − (20 × 90) = 1,200 psi.

Look at the gauge. Does it say 1,200?

What if the gauge disagrees?

Say it reads 900. You're 300 psi light. That's not a rounding error. Something changed.

• Look for a leak. Have your teammate check your valve and first stage. Those are the places you can't see.
• Check your breathing. Are you working harder than planned? Cold? Stressed? Swimming against current?
• Tell your team. Your gas plan is their gas plan.
• Adjust. New numbers mean a new turn point. Make the call early, while it's an easy call.

Here's the reality check.

The comfortable belief: my computer tracks all this for me.

The truth: your computer knows your depth and your time. Some know your tank pressure. None of them know your teammate's gas, the current at the exit or how hard you're breathing because you're cold. You do. Or you should.

How do you build the habit?

On land first. Then in the water.

• Work the numbers before every dive. Start pressure, planned depth, expected gas at the turn. The gas planning worksheet is built for exactly this.
• Call it, then check it. Before every glance at your gauge, say the number in your head. Then look.
• One number per minute. Not everything at once. Time, then depth, then gas, then team.
• Make it a team habit. Share pressures on a schedule. Notice when someone's number surprises you.

It feels slow at first. Within a few dives it's automatic. Within a season you'll wonder how you ever dived any other way.

What's your gas right now?

Next dive, try it. Before every look at your gauge, call the number first. How close were you? What surprised you?

The divers who get good at this aren't math people. They're people who decided to stop being surprised. If you want to build that into your diving, it's at the core of Essentials of Tech.