Dive Computer Algorithms — PADI DM / IDC Study Notes
Decompression Theory — Topics
How Dive Computers Work
A dive computer essentially writes a custom dive table for the exact profile you are diving — in real time. Because it tracks your actual depth at every moment rather than rounding to fixed depth increments, it eliminates the conservative rounding built into printed tables. The result is typically more bottom time than tables allow for the same level of safety.
The Five Main Algorithm Groups
There are five main groups of decompression algorithms used in recreational dive computers. Your computer's instruction manual should specify which it uses.
| Algorithm type | M-values | Gas washout model | Key characteristic |
|---|---|---|---|
| 1. Spencer limits, EE washout | Same as RDP | Exponential (each compartment at its own half time) | Can permit dives beyond what is safe — particularly short deep repetitive dives with short surface intervals |
| 2. Spencer limits, 60-minute washout | Same as RDP | 60-minute fixed | Closely matches what the RDP allows — the most directly comparable to the RDP table |
| 3. Bühlmann limits, EE washout | Further reduced (more conservative) | Exponential | Lower M-values offset the EE washout — produces results similar to RDP despite the different washout model |
| 4. RGBM and similar bubble models | Variable | Variable | Accounts for micro-bubble accumulation; penalises yo-yo profiles and fast ascents on repetitive dives |
| 5. Other / proprietary | Variable | Variable | Ongoing research area — many modern computers blend elements of multiple models |
Modern computer features
Most current recreational computers support: altitude settings and conservatism adjustment, nitrox diving, water temperature compensation, integrated air supply (tracks breathing rate), and Doppler-informed bubble tracking. Some support gas switching, extended range, trimix, and closed-circuit rebreathers.
PADI Guidelines for Diving with Computers
- Each diver must have their own computer — computers cannot be shared
- Each diver must use the same computer throughout a series of dives
- A computer has the same theoretical basis as a table — one is not inherently safer than the other
- All standard guidelines apply: deepest dives first, follow conservative profiles
- End the dive based on the most conservative computer in a buddy team
- Do not lend your computer to another diver if either of you has been diving
- Do not borrow a computer from another diver if either of you has been diving
- Do not change the battery between dives or underwater
- If the computer uses mixed gas, ensure it is set to the correct gas before diving
- Do not use a computer displaying any error or malfunction
- If the computer fails mid-dive: ascend slowly to 5 metres, make the longest safety stop your air supply allows, then stay out of the water for 12–24 hours before diving again with a working table or computer
- If diving at altitude, ensure the computer has an altitude mode
The computer does not get bent — you do
A dive computer is a tool, not a substitute for judgement. Know your computer's algorithm, understand its limitations, and recognise when its output does not make sense. Dive conservatively within its limits, not to them.
Decompression Theory — Topics