Dive Computers Explained: What the Specs Actually Mean

Shopping for a dive computer is an exercise in decoding acronyms. Bühlmann ZHL-16C. VPM-B. Gradient factors. Air integration. Wireless transmitter. RGBM.

Most buyers ignore the specs and buy the one that looks good or comes recommended by their instructor. That’s fine — for recreational diving on easy profiles, almost any modern computer will keep you safe.

But if you want to understand what your computer is actually doing, this is the explainer you should have gotten in your open water course and didn’t.

The Algorithm: The Brain of the Computer

Every dive computer runs a decompression algorithm — a mathematical model that estimates how much inert gas (usually nitrogen, sometimes helium) has dissolved into your tissues, and how safely you can ascend without that gas forming bubbles.

The two dominant algorithm families you’ll see are:

Bühlmann ZHL-16C The industry standard. Developed by Dr. Albert Bühlmann at the University of Zurich over decades of research. Used in Shearwater, Suunto, and most serious dive computers. It models 16 theoretical tissue compartments, each with a different half-time for gas uptake and elimination.

It’s not perfect — no algorithm is — but it’s well-understood, widely tested, and the basis for most modern recreational and technical dive planning.

RGBM (Reduced Gradient Bubble Model) Used in Mares and Suunto (in some modes). Attempts to account for bubble formation, not just dissolved gas. More conservative in some scenarios. Less community consensus on exactly how conservative.

VPM-B (Varying Permeability Model) More common in technical diving planning software (like V-Planner or MultiDeco) than in wrist computers. Also bubble-based. Tends to produce shorter deco times than Bühlmann with low gradient factors.

For most divers: Bühlmann is what you want. It’s transparent, well-documented, and widely supported.

Gradient Factors: Bühlmann With a Dial

Here’s where it gets interesting. Bühlmann’s original model is actually quite aggressive — it pushes ascent ceilings close to the theoretical limits of what tissues can tolerate.

Gradient factors (GFs) are a way to add conservatism — or dial it back — as a percentage of Bühlmann’s calculated limit.

GF Low (GF Lo): Controls how early your first deco stop begins. A lower number = deeper first stop = more conservative. GF High (GF Hi): Controls the ceiling at the surface — how close to Bühlmann’s limit you’re allowed to ascend to.

Common settings:

GF 100/100 — Pure Bühlmann. No conservatism added. Not recommended.
GF 45/85 — Popular recreational/tech setting. Moderately conservative.
GF 30/85 — More conservative, favored by tech divers for deep/repetitive dives.
GF 20/80 — Conservative. Used for deep trimix, multi-day decompression diving.

Your computer may call these “conservatism levels” or let you set them directly. If it lets you set them directly, you have a serious computer worth learning.

Tissue Loading

Some computers display tissue loading as a graphic — a bar chart showing which of your theoretical compartments are carrying the most gas load. After a deep dive, your fast tissues (short half-time, like brain and spinal cord) are loaded hard. After a shallow hour-long dive, your slow tissues (long half-time, like joints) carry more.

This matters for repetitive dives and multi-day diving. Your surface interval affects how much off-gassing happens before your next dive. Your computer tracks this — don’t ignore it.

Air Integration

Wireless air integration means a transmitter on your first stage sends your tank pressure to your computer. Useful — one less gauge to scan, and the computer can calculate your remaining bottom time based on actual SAC rate.

Wired integration is rare in modern computers. Wireless transmitters from Shearwater, Suunto, and others are standard. If you dive multiple tanks or stage bottles, you’ll want a computer that supports multiple transmitters.

Which Computer?

Recreational / entry-level: Shearwater TERIC or Suunto D5. Both solid, both upgradeable as your diving progresses.

Intermediate / aspiring tech: Shearwater Perdix 2 or Peregrine TX. The Perdix is the workhorse standard. Robust, clear display, excellent GF control.

Technical: Shearwater Petrel 3 or Teric in tech mode. Multi-gas, supports trimix, full GF control, multiple transmitters.

The reason Shearwater dominates serious diving: transparent algorithm implementation, full GF control, active firmware development, and a display you can read at depth in low visibility. Once you dive one, most other computers feel like toys.

Abyssi syncs directly with Shearwater computers — full dive profile, gas mixes, deco data, all in your log automatically.