RMV vs. SAC: What’s the Difference and Why It Matters
Scuba divers use a lot of tools and numbers to make sure their dives are safe and predictable—but few are as important as knowing how much gas you actually use. That’s where SAC and RMV come in.
While many divers learn to calculate SAC first—especially during basic training—it has significant limitations. RMV is far more useful for planning dives, sharing gas calculations, and managing risk underwater.
What Is SAC (Surface Air Consumption)?
SAC measures how fast you consume gas, in PSI per minute at the surface. It’s calculated based on the pressure you used during a dive and normalized to 1 ATA.
SAC Formula:
Surface Air Consumption (SAC) and Respiratory Minute Volume (RMV)
\[ SAC = \frac{(\frac{\text{Starting PSI} - \text{Ending PSI}}{\text{Dive Time in Minutes}})}{\text{Average Depth in ATA}} \]
Use Case: SAC is mainly useful for logging and comparing breathing rates between dives—as long as the tank doesn’t change.
Why Experienced Divers Don’t Use SAC
- Tank-specific: PSI only makes sense if you know the tank size.
- Not volume-based: It doesn’t tell you how much gas you actually used.
- Not suitable for planning: You can’t plan a dive or reserve based on SAC alone.
This is why SAC is rarely used beyond entry-level training or basic dive logging.
What Is RMV (Respiratory Minute Volume)?
RMV tells you how much volume of gas (in cubic feet) you consume per minute at the surface.
RMV Formula:
\[ RMV = \text{SAC} \times (\frac{\text{Tank Size in cuft}}{\text{Tank Size in PSI}}) \]
Because RMV is in cubic feet per minute, it lets you plan dives across any tank or configuration. It’s tank-independent, making it ideal for gas planning.
Why RMV Is Better for Planning
If you want to answer questions like:
- How much gas do I need for a 40-minute dive at 60 feet?
- How much reserve gas should I bring?
- How long can I stay at depth with this tank?
...you need RMV. It forms the basis of all reliable gas planning methods, including those used by recreational and technical divers.
Final Thoughts
SAC is helpful for tracking your performance in your logbook. But when it comes to real-world safety and planning, RMV is the number that matters.