What does my dive computer actually measure?

Short version: pressure and time. That's it. Everything else on that screen — your no-stop time, your ceiling, the little tissue bar graph — is not a measurement. It's a calculation, run on a model of a body that isn't yours. Nothing on your wrist measures your tissues. Nothing can.

That's not a criticism. It's the most useful thing you can understand about the device you trust with your dive, so let's open it up.

What the sensors read

Pressure. A small pressure sensor behind a port in the case feels the water pressing on it. The computer turns that into depth by assuming a water density — which is why there's a salt / fresh setting. Seawater is about 2.5% denser than fresh, so the wrong setting shifts your displayed depth by about that much. The depth number is an inference; the pressure is the measurement.¹

Time. A clock. Every calculation is depth-and-time stepped forward every second or so.

Temperature. Mostly used to correct the pressure sensor and for your log. It doesn't change the decompression math.

Tank pressure — only if you have a wireless pressure transmitter on your first stage, and that's a second instrument talking to the first. (It's what makes the SAC number possible — next section.)

Your ascent rate isn't measured either: it's just how fast the depth number is changing. Everything past this point is arithmetic.

Same sensors, one more formula: your gas consumption (SAC)

Some computers also show your SAC — surface air consumption, the industry's name for it even when the cylinder holds nitrox: how fast you'd be breathing your cylinder's gas down if you were at the surface. It looks like a new measurement. It isn't. It's another calculation, from the depth sensor and the clock plus two more inputs: tank pressure, which needs a wireless pressure transmitter on your first stage, and your tank's size, which nothing measures — it's a number you typed into the settings. No transmitter, no SAC. Wrong tank size entered, wrong answer, delivered with complete confidence.

The idea is simple. At depth you breathe denser gas, so every breath takes more out of the tank: at 20 m (66 ft), three times what the same breath takes at the surface. Divide that depth effect back out, and what's left is your breathing, not the dive.²

SAC = (tank pressure used ÷ minutes) ÷ ambient pressure

Ambient pressure here is in bar or ATA — the two differ by about 1%, so either works (1 bar = 0.987 ATA = 14.5 psi). The answer comes out in bar per minute (or psi per minute) — which only means something for one particular tank. Multiply by the tank's size and you get liters per minute, a number that travels with you from tank to tank. Divers call that one RMV, respiratory minute volume, though many computers label it SAC anyway; your manual says which you're looking at.

RMV = SAC × tank size

Worked example — 30 minutes averaging 15 m (50 ft)

• Known: a 12 L tank; 100 bar (1,450 psi) used in 30 min; average depth 15 m (50 ft), so ambient pressure ≈ 2.5 bar (2.5 ATA).
• Unknown: SAC and RMV.
• Why: take out the depth effect, then turn tank pressure into gas volume.
• Equation: SAC = (tank pressure used ÷ minutes) ÷ ambient pressure; RMV = SAC × tank size
• Substitute: SAC = (100 ÷ 30) ÷ 2.5 = 1.33 bar/min (19 psi/min); RMV = 1.33 × 12 = 16 L/min (about 0.56 ft³/min).

Your computer does the same arithmetic continuously, using its own depth readings instead of one average — so it's smoothed, and it wobbles. The tank cools as you descend and its pressure reading dips with the temperature, not your breathing; give it a few minutes before you believe it.

Here's why it's worth watching anyway: of everything on that screen, SAC is the one number that is mostly about you. Work harder, get cold, get anxious, and it climbs. Those are the same things the decompression model can't see (why, in the tissues FAQ). If your SAC jumped on a dive, your body was telling you something your tissue graph never will.

(How much gas to carry for a given dive is gas planning, and that belongs to your training and your instructor, not this page.)

What it calculates: one equation, sixteen times

Most computers sold today run a version of the model Albert Bühlmann developed in Zurich, called ZH-L16.^3,4 The core is an equation that's older than scuba — J. S. Haldane wrote it down in 1908 for the Royal Navy.⁵ Each “tissue” closes half of the gap between the nitrogen it holds and the nitrogen you're breathing every half-time:

P_t = P₀ + (P_i − P₀) · (1 − 2^−t / T)

That's the whole idea the Tissue Loading tool animates. Here it is with numbers.

Worked example — 20 minutes at 18 m (60 ft) on air

Starting fresh from the surface. (Rounded: seawater adds about 1 bar per 10 m, and the small water-vapor correction the real model makes in your lungs is left out. Pressures are in bar, with ATA in brackets; 1 bar = 0.987 ATA = 14.5 psi.)

• Known: air is 79% nitrogen. Surface nitrogen P₀ = 0.80 bar (0.79 ATA). Ambient pressure at 18 m ≈ 2.82 bar (2.79 ATA). Time t = 20 min.
• Unknown: nitrogen in each tissue at the end of the 20 minutes.
• Why: the tissue chases the nitrogen you're breathing, closing half the gap every half-time.
• Equation: P_i = 2.82 × 0.79 = 2.23 bar (2.20 ATA); then P_t = 0.80 + (2.23 − 0.80) · (1 − 2^−20/T)
• Substitute, for five of the sixteen compartments:

Same dive, same twenty minutes, and the fast compartment is nearly full while the slow one has barely noticed you got wet. That spread is the reason for sixteen. (While you're descending or ascending, the depth is changing every second, so computers use a version of the same equation for a steadily changing pressure — the Schreiner equation. Same idea.)

The sixteen compartments

Bühlmann's model tracks sixteen compartments with nitrogen half-times running from about 4 minutes to 635 minutes — roughly 10½ hours:³

Three things worth knowing about them.

They aren't organs. There's no “compartment 7” in your body. The half-times were chosen to span fast tissues to slow ones and then tuned until the model's predictions matched what happened to real divers. They're a mathematical device. The test is whether the predictions hold — and they hold well enough to dive by.

Sixteen is a choice, not a law. Haldane used five compartments, with the slowest at 75 minutes — fine for the navy's dives of 1908, too short for long or repetitive diving.⁵ Adding slower compartments is how later models learned to handle multi-dive days. Other models use different numbers, and some computers don't run Bühlmann at all — there are bubble-model families (RGBM and relatives) and the DSAT model behind the PADI tables. Your manual says which yours runs. Two buddies on two different brands will get two different answers on the same dive, and both computers are “right.”

One compartment is always in charge. On a short, deep dive it's a fast one. On a long, shallow dive — or day four of a liveaboard — it's a slow one. The computer watches all sixteen and lets whichever is closest to its limit set your no-stop time. That's why the number can suddenly drop on a repetitive dive: a slow compartment you'd forgotten about is still carrying yesterday.

Fast and slow because you are. The compartments run from fast to slow because your body does: blood and brain take up and give up nitrogen in minutes; fat, joints and bone take hours.⁶ Which real tissues are fast, which are slow, and why — and why fat is slow even though it's full of blood vessels — is its own question, answered in What are fast and slow tissues?

The second equation — the limit

Loading is only half the model. The other half asks: how much excess gas can each compartment carry back to the surface without trouble? In Bühlmann's model each compartment gets a tolerated ambient pressure:³

P_amb,tol = (P_t − a) · b

where a and b are two coefficients per compartment, fitted to experimental dives. Fast compartments are allowed a big excess; slow ones are allowed very little. When the most restricted compartment's tolerated pressure is above the surface, you have a ceiling — a decompression stop. When none is, you're inside no-stop limits, and the minutes until one would cross is your no-stop time.

I'm deliberately not printing the a and b values here. Those numbers are what turn this from understanding into dive planning, and this page is for understanding.

Gradient factors — the conservatism setting on many computers, shown as two numbers like 40/85 — scale that tolerated excess down. GF 85 means “allow only 85% of what the original model allows.” Erik Baker introduced the idea in the late 1990s; it's now how most Bühlmann computers let you dial in more margin.^7,8

What it never asks

As of this writing, no dive computer I know of asks your age, height, weight, body fat, or sex. (One family does watch your heart rate and skin temperature during the dive — that's how hard you're working, not who you are.) Yet each of those things is suspected of shifting your risk — some with decent evidence, some pointing the opposite way from what divers have believed for decades, none understood well enough to put into an equation.⁹ So the computer leaves them out. It doesn't measure a single bubble, either.

Which means the model isn't built on you. It isn't even built on the “average” diver. It's built on whoever was in the test dives — and for much of decompression history, that meant young, fit, male, military volunteers in a chamber. The model learned what their bodies could tolerate, and then a margin was added. Following it to the last second of no-stop time is a bet on being that diver, not a guarantee. (More on that in What are my odds of getting bent?)

So ask yourself honestly: are you that diver? Are you their age? Their fitness? Did you sleep well, drink enough water, haul your own gear down a dock this morning? The computer doesn't know. You do. That's what your conservatism setting, your safety stop, and your rest days are for.

So read the screen for what it is: two honest measurements and a very good guess. And when you want the medicine behind it, DAN is the place.

For education only. This page explains the physics and the reasoning behind your dive computer. It is not for dive or gas planning. Plan every dive with your training, your dive computer and your gauges, and follow your instructor or dive professional.

References

1. Azzopardi E, Sayer MDJ. A review of the technical specifications of 47 models of diving decompression computer. Underwater Technology. 2010;29(2):63–72. ↩
2. Ange M. Estimating your air consumption. Alert Diver. February 2010. Divers Alert Network. ↩
3. Bühlmann AA. Decompression–Decompression Sickness. Berlin: Springer; 1984. ↩abc
4. Mitchell SJ, Doolette DJ. Recreational technical diving part 1: an introduction to technical diving methods and activities. Diving and Hyperbaric Medicine. 2013;43(2):86–93. ↩
5. Boycott AE, Damant GCC, Haldane JS. The prevention of compressed-air illness. Journal of Hygiene. 1908;8(3):342–443. ↩ab
6. Divers Alert Network. Introduction to decompression sickness. Chapter 1 in Decompression Sickness, DAN Dive Medical Reference Books. 2020. ↩
7. Baker EC. Understanding M-values. Immersed. 1998;3(3):23–27. ↩
8. Doolette DJ, Mitchell SJ. Recreational technical diving part 2: decompression from deep technical dives. Diving and Hyperbaric Medicine. 2013;43(2):96–104. ↩
9. Marroni A, Kot J, Pieri M, Pelliccia R, Balestra C. Identification of DCS risk factors in recreational diving: a multifactorial model based on the DAN DSL Database 2024. International Maritime Health. 2026;77(1):1–12. doi:10.5603/imh.108038 ↩