A dive log for technical and decompression dives: what to record

Training comes first

Technical and decompression diving are taught as courses, with their own rules for planning, gas management and limits. Nothing in this guide replaces your training, your agency's standards or your instructor. It is only about keeping a good record of dives you were trained to do.

Why the log matters more

On a single-cylinder dive, a start pressure and an end pressure say most of it. On a decompression dive, the numbers are spread over cylinders. If you log only the back gas, your air consumption looks too high or too low, and you cannot see afterwards how your gas plan held up. A complete entry is also what a course director reads when you ask for the next level.

What to record for each cylinder

• The mix. Oxygen percentage, and helium percentage for trimix. Write the figure you analysed, not the one on the label.
• The size and the pressures. Cylinder size, and start and end pressure.
• What it was used for. Back gas, sidemount, stage, deco, bailout, diluent or oxygen.
• Twin sets. A twin set counts as one entry, marked as a pair.

Loggerfish takes all of that. A dive starts with one cylinder, and the form has an add-cylinder link for the rest, each with its use, oxygen and helium percentage and a twin-set option. Dive files that carry several cylinders import with them.

Mark the type of dive

Tick the types that apply: Deco, Technical, Doubles, Wreck, Dry suit, Night, Solo or Scooter. They cost one tap each, and they let you filter later: how many deco dives this year, which wreck dives were in doubles. Courses and specialities often ask for exactly these counts.

Gas switches and the profile

When a dive comes from a dive computer file, Loggerfish can show the depth profile and mark each gas switch on it. Loggerfish reads gas changes from Subsurface logs. Where the file has them, they come across, so you see when you moved to the deco gas without writing it down.

If you log by hand, put the switch depths and times in the notes. A line such as "50 % at 21 m, oxygen at 6 m" is enough to reconstruct the dive.

Air consumption across several cylinders

SAC is easy with one cylinder and less so with five. Loggerfish adds up the gas used from every cylinder you logged and divides it by the dive's time and its average depth. That has a limit worth knowing: every cylinder is charged at the dive's average depth, so a deco bottle breathed in the shallows counts for a little more than it should. Treat the figure as a trend, not a measurement. Our SAC guide has the sum for the simple case.

For planning, use your own SAC from harder dives, add the reserve your training requires, and check it against the plan from your agency's method.

The planner, and its limits

Loggerfish has a decompression planner for trained, certified decompression divers. It uses Bühlmann ZH-L16C with gradient factors, takes a travel gas and up to four deco gases, and checks each cylinder against the rule of thirds. It has been checked against published plans from other planners, but not reviewed by a technical diving instructor, and the app says so. It shows no plan until you confirm that you are trained for decompression diving, every time you open the app.

It is a planning aid. It is not a dive computer. Check any plan against a second planner, and follow your own computer underwater.

A short routine

1. Analyse your gas and write the figures down before the dive.
2. After the dive, enter every cylinder with its use and pressures.
3. Tick the dive types, and note the switch depths if no file carries them.
4. Export a copy of the log from time to time. Every export is free.

These calculations are planning aids, not a dive computer. Your training and your computer decide the dive.