Planning a dive with DiveLogic

A dive computer (chapter 22) tracks the dive you are making. A dive planner works out, before you get wet, the dive you intend to make: where you go, what you breathe, when you stop on the way up, and whether your gas and your oxygen exposure stay inside their limits.

Every earlier chapter feeds into a plan. The gas laws set the partial pressures you breathe. The body chapters set the limits on oxygen, narcosis and gas density. The decompression models turn depth and time into stops. This chapter shows how DiveLogic puts these together, and what it checks on the way.

What a planner needs from you

You describe the dive. DiveLogic plans it. You give the depth and time of each part of the dive (each level), the gases you carry with their cylinder sizes and fill pressures, the decompression model and its settings, and how fast you breathe. DiveLogic returns a schedule: every depth, time and gas from the surface back to the surface. With it come the checks on each gas, the oxygen exposure, the gas each cylinder uses, and a list of warnings.

The example in this chapter is 45 m for 25 minutes on trimix 21/35 (21% oxygen, 35% helium, the rest nitrogen). The diver carries it in twin 12 L cylinders filled to 232 bar. This is the bottom gas, the gas breathed at depth, worn on the back as back gas. For the ascent there is a deco gas, EAN50 (nitrox with 50% oxygen), in one 11 L stage cylinder at 200 bar, used from 21 m. The breathing rate, or SAC rate (surface air consumption, given here as litres per minute at the surface), is 15 L/min on the bottom and 12 L/min on the stops. It is the same dive as in chapter 25, and every number below was planned with DiveLogic.

The screens in this chapter are DiveLogic’s own planner, showing this dive. A plan moves through three steps: Plan, where you enter the dive, Review, where DiveLogic answers, and Save. The dive has one level: 45 m for 25 minutes. Read more about creating a plan in the Handbook.

Choosing the model and its conservatism

A decompression model tracks how much inert gas (nitrogen and helium) your body takes up, and sets a ceiling: the shallowest depth you can safely go to at that moment.

DiveLogic offers Bühlmann ZHL-16C (chapter 13), with 16 tissue compartments, and Thalmann E-L (chapter 14), the US Navy model, with a choice of parameter sets. This dive uses Bühlmann.

On Bühlmann you set the gradient factors (chapter 16): how much of the model’s limit you allow. GF low (30%) sets the first stop. GF high (70%) sets the limit at the surface. Lower numbers give a longer, more cautious ascent. Think of it like driving below the speed limit: the model’s limit is the speed limit, and the gradient factors are how far under it you stay. Thalmann takes no gradient factors; its parameter set decides how cautious it is. Read more about the decompression settings in the Handbook.

On the Review step the plan appears as a profile: depth against time, with the ceiling drawn under it. The line never goes above the ceiling. Each flat step on the way up is a stop.

Checking every gas where it is breathed

DiveLogic checks each gas at the depths you breathe it. The gas card for each mix shows its figures at the depth it is used: the deepest point of the dive for the bottom gas, and the switch depth for the deco gas. At 45 m the card for the 21/35 shows:

• PO2, the partial pressure of oxygen, also written ppO2 (chapter 6): 1.16 bar, below the working PO2 of 1.4 bar. PO2 is often given in ATA (atmospheres absolute), which is about the same as bar. The card also gives the mix’s MOD (maximum operating depth, the deepest you can breathe it at that limit): 56 m.
• Gas density, the mass of 1 L of the gas at depth (chapter 8): about 5.0 g/L, just below the 5.2 g/L advisory. Denser gas raises the work of breathing and can lead to hypercapnia (CO2 build-up).
• EAD, the equivalent air depth: the depth on air with the same nitrogen pressure, 21 m here.

The plan adds the END, the equivalent narcotic depth: the depth on air that would cause the same inert gas narcosis, counting nitrogen as the narcotic gas. On the bottom it is 20.6 m, below the 30 m the gas check warns at.

At the switch to EAN50 at 21 m, the PO2 is 1.55 bar, below the contingency PO2 of 1.6 bar, the limit for a deco gas, and its MOD at that limit is 22 m. This plan raises no warnings. Read more about gas cards in the Handbook.

The schedule

The schedule is the list of stops: depths you hold for a set time on the way up. DiveLogic places them every 3 m, at or below the ceiling. The first stop is at 24 m for 1 minute; the last, at 3 m, lasts 14 minutes.

You ascend at 9 m/min to the first stop, 3 m/min between stops and 1 m/min from the last stop to the surface. You switch to EAN50 at 21 m. The time to surface (TTS), from leaving the bottom, is 42 minutes, and you surface at a runtime (time since leaving the surface) of 69:35.

Chapter 25 covers how to carry out this schedule in the water. Read more about the schedule table in the Handbook.

Oxygen exposure through the dive

DiveLogic tracks 2 oxygen totals minute by minute, from the PO2 you breathe. The CNS %, or CNS clock, is the share used of the NOAA single-exposure limit for CNS oxygen toxicity, oxygen’s effect on the brain (chapter 6). OTU (oxygen tolerance units) measure the dose for pulmonary oxygen toxicity, the effect on the lungs and whole body (chapter 7).

Both rise faster after the switch to EAN50, because the PO2 is higher. This dive ends at 23% CNS and 62 OTU, shown in the strip across the top of the Review step. DiveLogic warns above 80% CNS and 200 OTU, and reports an error above 100% CNS and 300 OTU. Read more about oxygen exposure in the Handbook.

Gas used against gas carried

DiveLogic works out how much of each gas the dive uses, from your breathing rate and the pressure at each depth. The 21/35 uses 2,424 L of the 5,568 L carried and ends at 131 bar. The EAN50 uses 757 L of 2,200 L and also ends at 131 bar.

Each cylinder keeps a reserve: gas you plan not to use. By default DiveLogic uses the rule of thirds, so one third of the fill, 77 bar on the 21/35, is held back. The turn pressure is the lowest pressure you can leave the bottom with and still reach the next gas with the reserve intact: 87 bar here. This plan leaves the bottom with 141 bar. Chapter 24 covers turn pressure in detail. Read more about the gas view in the Handbook.

What DiveLogic checks on each gas

While you build the plan, the gas card for each mix shows its MOD, PO2, EAD and density at the depth it is used, and flags a mix that passes a limit there, including an EAD deeper than 30 m. Once the dive is planned, DiveLogic checks the gases along the whole schedule:

• PO2 at each level against the working limit, 1.4 bar by default, and against 1.6 bar, which is never allowed.
• PO2 at each gas switch against 1.6 bar. The switch depth you set for a decompression gas is the deepest point it is breathed, so a switch set too deep raises an error.
• Gas density at every point against 5.2 g/L (a warning) and 6.2 g/L (an error).
• Counterdiffusion at each gas switch (chapter 17): large jumps in the nitrogen or helium content, a switch at or above the ceiling, or a large change in narcotic depth. By default these checks apply to dives that reach 80 m. You can set them to apply to every dive in the ICD safety settings.
• Whether any gas can be breathed at the deepest point. If none can, the plan cannot move on to the review step.

One check is not made on an open circuit (OC) plan: hypoxia, a gas with too little oxygen to keep you conscious. The minimum PO2 is about 0.16 bar; below it a gas is not safe to breathe. A mix with less than 16% oxygen is below that at the surface, and DiveLogic does not warn when a plan breathes it there. The Mix limits tool gives the shallowest depth a mix can be breathed at. A hypoxic bottom mix needs a travel gas for the descent, and you have to add it to the plan yourself.

Warnings and errors

A finding from these checks is either a warning or an error.

• A warning means the plan is outside a usual limit, but the schedule is complete and consistent. You decide whether the finding matters on this dive. Examples are PO2 above 1.4 bar on the bottom, density above 5.2 g/L, and CNS above 80%.
• An error means the plan crosses a hard limit or could not be completed as set up. Examples are PO2 above 1.6 bar, density above 6.2 g/L, a counterdiffusion finding, and a cylinder that runs out.

DiveLogic still draws a schedule when there is an error, so that you can see what is wrong. A schedule with an error on it is not a plan to dive. Change the input behind the error: the gas, the depth, the time or the switch depth. Read the full list of messages in the Handbook.

Changing one thing

Change one setting at a time and see what DiveLogic reports. The planner below replans the dive each time: the strip, the profile (with the chapter’s plan dotted underneath) and the Dive Warnings list, printed as DiveLogic writes them. Some changes to try:

• Air as the bottom gas at 45 m gives an error: the density is 7.1 g/L.
• An EAN50 switch at 24 m gives an error: the PO2 at the switch is 1.71 bar.
• 60 m on 21/35 gives a PO2 of 1.48 bar (a warning) and a density of 6.4 g/L (an error). 18/45 has less oxygen and more helium, so at 60 m both errors go. Its density, 5.6 g/L, is still above the advisory, and the longer ascent uses part of the EAN50 reserve.
• A 40 minute bottom time uses part of the 21/35 reserve, which gives a warning.
• Every dive in the counterdiffusion setting shows the 3 findings DiveLogic makes on the switch from 21/35 to EAN50 at 21 m, which are not checked on a 45 m dive by default.

Contingencies

A contingency is a change to the dive that you did not plan but must be ready for. DiveLogic plans the common ones beside the main plan, each as a complete plan with its own schedule, gas figures and warnings. You carry them so that you have an answer in the water without working one out.

• Overrun: you stay longer or go deeper than planned. A new plan comes with two overrun rows, +5 min / +3 m and +5 min / +6 m, and you can set up to 6.
• Lost gas: on OC, DiveLogic plans the ascent without each decompression gas, as if that cylinder were lost or could not be used, and without all of them.
• Bailout: on a closed-circuit rebreather (CCR) (chapter 19), DiveLogic plans the ascent on OC from the rebreather dive, on all of the bailout gases you carry and on each one alone. This works on Bühlmann and Thalmann plans.

You set the overrun rows on the contingency card. Read more about contingencies in the Handbook, and about CCR bailout.

On this dive, a +5 min / +3 m overrun surfaces at 89:44 instead of 69:35, and the 21/35 is now a little too dense at 48 m (5.3 g/L). Losing the EAN50 means the whole ascent is on 21/35: the runtime rises to 125:35, and the 21/35 ends 23 bar inside its reserve. Both together would need 213 L more 21/35 than the diver carries. The lost-gas case is why a diver carries enough back gas to finish without the decompression gas, or a second decompression cylinder.

Taking the plan with you

DiveLogic plans without a connection. Plans are kept on the device, so you can build, open and change them on a boat or at a remote site, and they reach your account when the connection returns.

When the plan is final, Finalize it. The report holds the whole plan as one document: the profile, the schedule, the gas plan, the settings and any warnings, with scenarios for a rebreather dive. You can export it as a PDF or print it, and copy the schedule and contingencies onto a slate or wet notes. Read more about saving and sharing a plan in the Handbook.