SAC vs RMV consumption rate in Scuba Diving

When planning a dive, the key element is the assessment of gas consumption, which allows for efficient management of air or breathing mixture supplies. In this context, two calculation methods are highlighted: SAC (Surface Air Consumption) and RMV (Respiratory Minute Volume). These two indicators differ in calculation methods and applications, which is important in the context of proper dive planning.

1. SAC (Surface Air Consumption rate):

   SAC is a measure of gas consumption expressed in the amount of BAR per minute consumed. This index is specific to the cylinder size it is determined on. For example, a SAC rate of 0.83 BAR/min means consuming 0.83 units of pressure per minute of diving. When calculating SAC, it is very important to consider the size of the dive cylinder, as it directly influences the result.

2. RMV (Respiratory Minute Volume):

   RMV is the amount of air consumption rate, measured in liters per minute on a surface . It is a size-independent measure, meaning that RMV can be used with any cylinder, as it refers to volume rather than pressure. It's crucial to note that RMV is an action-independent measure, which stands out for its utility in calculating gas usage without being affected by the external pressure changes typically associated with diving.

Using these pieces of information helps in calculating your gas consumption in a precise manner, enabling better planning and safety for your dives. Understanding your RMV is not just about knowing how much air you used, but also about optimizing your dives for efficiency and safety.

Comparison of both methods:

- Application:

SAC is useful for determining gas consumption with a specific cylinder, but its results are limited to the specific size of the cylinder. RMV, thanks to its universality, can be used with various types of cylinders, making this indicator more flexible and useful in diverse diving situations.

- Precision:

RMV provides more universal results that can be applied to different equipment configurations, which is particularly important in technical and exploration diving.

- Simplicity:

Calculating SAC is relatively simple, but its effectiveness is limited to specific equipment. RMV requires additional data about the cylinder, but offers greater precision and adaptability.

In summary, RMV offers divers the ability to plan gas more precisely in various conditions and with different types of cylinders, which is crucial in advanced diving, where different equipment and environmental parameters have a decisive impact on safety and diving efficiency.

What Affects Our breathing rate While Diving

As we wrote above, to calculate RMV, it will require us to gather several pieces of data during previous dives. For our calculations to be more reliable, it's best to calculate your RMV during each dive and then deduce an average. As we know, each dive can be somewhat different due to variations in gas consumption.

Factors that influence gas consumption underwater include:

• Our physical condition/state
• Diving experience
• Streamlining of diving gear
• The diver’s position during swimming
• Buoyancy control
• Excess tasks to be performed

As you can see, there are many factors that influence our respiratory gas consumption during diving. Therefore, we should frequently measure your breathing rate to utilize this information in planning gas needed for recreational, decompression, as well as overhead environment dives.

What Data Do We Need to Calculate RMV rate?

To accurately determine your RMV, it’s essential to gather specific data from your previous dives. Here’s what you’ll need:

1. Start Pressure: The pressure in your cylinder at the start of the dive.
2. End Pressure: The pressure remaining in your cylinder at the end of the dive.
3. Cylinder Size: The volume of your scuba tank.
4. Dive Time: The total duration of your dive.
5. Average Depth: The average depth in Ata or Bar.

As you can see, these are basic diving data that most computers can record, so we can enter the dive log and display all the data. If we don't have a transmitter that shows the starting and ending pressures, we must remember them. However, it is necessary to record this accurately because any incorrect values will give us a wrong result.

How to convert a Average Depth in meters into ambient pressure?

The average depth must be expressed in units of atmospheres or bars, which represent ambient pressure. Every dive computer or bottom timer will record this for us, but we must convert the depth to ambient pressure ourselves.

Water pressure

Water has weight, the typical density of seawater is around 1025 kg/m³, freshwater is around 1000kg/m³ and its weight exerts pressure on objects underneath it. As we dive deeper, the amount of water above us increases, resulting in greater pressure. For every additional 10 meters of depth, the pressure in the water increases by 1 bar, which is equivalent to atmospheric pressure. Therefore, as we dive to greater depths, we experience increased water pressure. Additionally, the pressure a diver experiences at a specific depth is the sum of all pressures acting on them from the water and the water's surface.

How to calculate RMV?

To calculate the Respiratory Minute Volume (RMV) during a dive or multiple dives, you'll need to gather the following data:

1. Gas Used: This can be calculated by subtracting the final pressure in the tank from the starting pressure and then multiplying this by the cylinder volume. For example, if the starting pressure is 200 bar and the final pressure is 100 bar in a tank with a volume of 24 liters, the gas used would be (200 bar - 100 bar) x 24 liters = 2400 liters.
2. Dive Duration: This is the total duration of the dive in minutes. For example, it could be 60 minutes for a particular dive.
3. Diving Depth (average or constant): This can be given as either an average depth or a constant depth. For instance, if the diving depth is 10 meters, which corresponds to 2 bars of pressure.
4. If the gas used during a dive was 2400 liters and the dive duration was 60 minutes at a constant depth of 2 bars, the RMV calculation would be as follows: RMV = (2400 liters / 60 min) ÷ 2 bar = 20 liters/min

Dive planning

When we know how much gas we are consuming on the surface, we can plan our dive as shown in the example below:

Let's consider planning a dive at a depth of 20 meters. We have a 15-liter cylinder filled to 200 bar with a 40 bar reserve. Assuming our previously calculated Respiratory Minute Volume (RMV) is 25 l/min, we can calculate the dive time as follows:

1. Calculate our gas usage at 20m depth in bars: Gas consumption at depth = 3 bars x 25 l/min = 75 l/min
2. Determine the total gas amount in the cylinder: Total gas volume = cylinder volume x (cylinder pressure - reserve pressure) Total gas volume = 15 liters x (200 bars - 40 bars) = 2400 liters
3. Calculate the potential dive duration based on the gas amount: Dive duration = Total gas volume / gas consumption at depth Dive duration = 2400 liters / 75 l/min"

This calculation helps in estimating the duration a dive can last based on the available gas supply, consumption rate, and planned depth.