Tools
SAC/SRMV
Measure your personal breathing rate from a real dive so you can plan gas accurately for the next one.
마지막 업데이트 2026년 9월 15일
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What it is
This calculator measures how fast you actually breathe underwater, worked out from a dive you already did. You enter one or more measurements from the dive, each with the cylinder you were on, a depth, a time, and the pressure your gauge read at the start and end. It gives you your breathing rate as two numbers: SRMV (surface respiratory minute volume, also called RMV) and SAC (surface air consumption).
Each measurement carries its own cylinder, so a dive where you breathed a twinset and then a stage goes in as two measurements and both count. There is no separate cylinder setup to remember.
Your breathing rate is the single most important input to any gas plan, and it is personal. A relaxed diver and a nervous one on the same dive can use very different amounts of gas. Measure yours once from a calm dive, and you can plan gas honestly from then on. In the app this screen is titled SAC/SRMV, and the output field is labelled SRMV.
Why measure it
Every gas plan starts with “how fast do I breathe?” Guess too low and you plan a dive you cannot finish on the gas you carry. Guess too high and you cut dives short or haul cylinders you never needed. A measured rate from a real dive replaces the guess.
Once you have it, type the SRMV straight into the Gas Usage planner to size cylinders and find where your gas runs low on the next dive.
How to use it
Open the calculator
From the Plan tab, open SAC/SRMV.
Add a measurement
The cylinder is the first field of the Add Measurement form. Enter the cylinder you were breathing during this part of the dive (metric: Cylinder volume, the water volume in litres; imperial: Capacity in ft³ plus its rated working pressure, so an AL80 is 80 at 3000 psi), or tap Quick Fill, the button under the field, to pull one of your saved gas presets straight in.
Enter the total volume you breathed from, not the size of one cylinder. A twinset of two 12 L cylinders is 24 L. Twin AL80s are 2 × 80 ft³. The pressure drop on your gauge only means something against the size it came out of.
Then enter the average Depth of that part of the dive, the Time it took, and the Start Pressure and End Pressure your gauge showed. The form prefills sensible defaults (the first measurement starts at 180 bar (2611 psi) with an end 10 bar (145 psi) lower), and Save Measurement stays greyed out until the cylinder volume, depth and time are above zero and the start pressure is above the end. Tap Save Measurement when the numbers are valid.
Add a stage or a second cylinder
Breathed more than one cylinder on the dive? Add the time on each one as its own measurement, with its own cylinder in the cylinder volume field. A 25 minute leg on a 2 × 12 L twinset and a 15 minute leg on an 11.1 L stage are two measurements, and the tool adds both drops together over the time you were down.
Two measurements must never cover the same minutes. If they overlap, those minutes are counted twice on the clock while the gas is only counted once, so your rate comes out too low and the next gas plan you build on it will be sized too small. If you do not know exactly when you switched, split the dive at your best estimate: the totals still add up.
Changing the cylinder resets the start and end pressures to a full cylinder (200 and 190 bar, or 2901 and 2756 psi), because a different cylinder means a different gauge. Type in what the new gauge actually showed. A reading you typed yourself is left alone, field by field, so you can go back and correct the cylinder without losing it. Only the fields still holding what the app filled in start over, which keeps the previous cylinder’s numbers from ending up on the new one.
Add more segments if you want
For a multi-level dive, add a segment per level. The calculator weights them by how much gas each used, so a long deep segment counts more than a brief shallow one. Each new segment defaults its start pressure to the previous segment’s end, so a whole dive chains together cleanly, and the cylinder stays where you left it. Every saved segment appears under Measurements as its own card showing that segment’s depth, time, cylinder size, pressure drop, and its own breathing rate, while the Summary combines them into the weighted average.
Fix a mistake
Each saved measurement card has a delete control. Tap it to remove a segment you mis-entered, then add it again with the right numbers.
Read your rate
The Summary shows your weighted-average SRMV and SAC. Note the SRMV down and reuse it for planning.
If your measurements used more than one cylinder size, the summary shows the single SRMV and then a SAC per cylinder list, because a SAC only means something against one cylinder. The SRMV is still the number to carry forward.
Inputs and outputs
Inputs follow your unit settings: depth in metres or feet, pressure in bar or psi, volume in litres or cubic feet.
| Field | Meaning | Notes |
|---|---|---|
| Cylinder volume | The total size you breathed from in this measurement | Metric: water volume; imperial: capacity at working pressure (a hint shows the derived water volume). Enter a twinset as both cylinders together (2 × 12 L is 24 L). Quick Fill pulls a saved gas preset, doubling it when the preset is a twinset |
| Depth | Average depth of the segment | Use your computer’s average depth if you have it |
| Time | Minutes the segment lasted | Minimum 0.5 minutes |
| Start Pressure | Gauge reading at the start of the segment | |
| End Pressure | Gauge reading at the end | Must be lower than the start |
For each segment the tool works out the gas used (the pressure drop times that segment’s own cylinder volume), then divides by the load to get a segment rate. The load is time multiplied by the ambient pressure in ATA (1 + depth/10), which normalises the gas you breathed at depth back to the surface. That is why a deeper segment of the same length carries more weight. The tool then combines all segments into a weighted average.
| Output | Meaning |
|---|---|
| SRMV | Surface respiratory minute volume (also called RMV), your true breathing volume per minute at the surface, in litres or cubic feet per minute |
| SAC | Surface air consumption, the pressure your specific cylinder loses per minute at the surface, in bar or psi per minute. Listed once per cylinder when your measurements used more than one |
SRMV and SAC explained
Both numbers describe the same lungs, but in different units, and the difference matters when you plan.
SRMV is a volume of gas per minute, normalised to the surface. Because it is a true volume, it is yours regardless of cylinder. Carry it to any cylinder and it still tells the truth. This is the number to keep.
SAC is a pressure drop per minute for one particular cylinder. It is convenient because it reads in the same units as your gauge, but it is tied to that cylinder’s size. Put the same diver on a bigger cylinder and the SAC falls even though the breathing has not changed. Treat SAC as a cylinder-specific shorthand, not a portable figure.
In short: SRMV travels with you, SAC travels with the cylinder.
That is why a dive measured on two cylinders has one SRMV and two SACs. The summary shows the SRMV on its own and lists a SAC against each cylinder size, so you can read “1.08 bar/min on the 24 L twinset, 2.34 bar/min on the 11.1 L stage” and see that both describe the same breathing.
Behaviour and limits
- Garbage in, garbage out. An end pressure that must be lower than the start, an honest depth, and an accurate time are what make the result trustworthy. The app rejects an end pressure that is not below the start, and a cylinder volume of zero.
- Measurements must not overlap in time. The tool cannot tell that two measurements cover the same minutes. If they do, those minutes are counted twice and the gas only once, so the rate reads low. Low is the dangerous direction, because it sizes your next gas plan too small. Check that each measurement covers a different part of the dive.
- Measurements are cleared when you close the app. Write the SRMV down before you leave the screen.
- One dive is a snapshot. Measure a few dives across conditions you actually face, and plan from the highest sensible rate, not the lowest.
- The figure is your at-rest, in-control rate for that dive. Cold, current, and exertion push it higher, so add a margin when you plan demanding dives.
Related
- Gas Usage takes the SRMV you measure here and turns it into a gas plan for the next dive.
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