Understanding Gas Laws: Filling Scuba Cylinders Safely

Filling scuba cylinders is a critical aspect of scuba diving, and a good understanding of the relevant gas laws is essential to ensure the safety of divers. The process of filling cylinders involves compressing air or gas mixtures into the cylinders using a high-pressure breathing air compressor or by decanting from high-pressure storage cylinders.

One of the key gas laws that comes into play when filling scuba cylinders is Boyle's Law, which states that the volume of a gas varies inversely with pressure at a constant temperature. This law helps divers understand the changes in gas volume and pressure as they descend underwater. Additionally, it explains why it is dangerous to hold one's breath during a dive, as the pressure inside the lungs can increase to fatal levels when ascending from deeper waters.

Another important law is Charles's Law, which states that the volume of a gas varies directly with temperature at constant pressure. This law is relevant when filling scuba cylinders as the temperature of the gas inside the cylinder increases during the filling process, and it can affect the amount of breathable air available to the diver.

Furthermore, Dalton's Law, which states that the total pressure exerted by a mixture of gases is the sum of the pressures exerted by each individual gas, is crucial when dealing with different breathing gas mixtures. It helps divers understand the partial pressures of gases in the mixture and their potential effects, such as nitrogen narcosis and oxygen toxicity.

In conclusion, a solid understanding of gas laws is vital when filling scuba cylinders to ensure the safety and well-being of divers during their underwater adventures. These laws help divers calculate and manage their gas supplies, plan their dives effectively, and avoid potentially life-threatening situations.

Filling scuba cylinders with compressed air

Pre-Filling Procedures:

• Inspection of Cylinders: Before filling, conduct a thorough external inspection of the scuba cylinders. Ensure they are suitable for filling by checking permanent and non-permanent markings, such as manufacturing standards, hydrostatic test dates, working pressure, serial number, owner's details, visual inspection dates, type of breathing gas, and cylinder colour coding.
• Permission to Fill: Obtain written permission from the cylinder owner to proceed with the filling process. This is a safety precaution and helps establish the cylinder's containment history.
• Residual Gas Check: Measure the gas pressure remaining in the cylinder. If the pressure is very low or the cylinder is empty, follow the appropriate procedures, including analysing the gas composition if qualified to do so.

Filling Process:

• Compressor Startup: Ensure the high-pressure breathing air compressor is functioning correctly. Check the compressor's performance, including direction of rotation, and make sure all components are in good condition.
• Cylinder Connection: Utilise safe handling procedures when connecting the cylinders to the filling equipment. Use appropriate fittings and adaptors that match the pressure rating and connector standard. Ensure airtight connections.
• Filling Procedure: Fill the cylinders at an appropriate rate to avoid excessive heating and control pressure loss due to cooling. Monitor the fill pressure and close the valves when the desired pressure is reached. Do not over-tighten the valves to prevent damage.
• Disconnection and Storage: After filling, check the valves and bleed the filler connection. Disconnect the cylinders, mark them as filled, and store them safely.

Post-Filling Procedures:

• Verification of Fill Pressure: Check the charge pressure of the cylinders after they have cooled to room temperature. Top up any cylinders that are under-pressured, ensuring the pressure is within 5% of the developed pressure for the given temperature.
• Recording Filling Data: Maintain a logbook or digital records of filling details, including owner information, cylinder specifications, inspection dates, gas type, and charging pressure.
• Safety Considerations: Regularly inspect and maintain the compressor, following the manufacturer's guidelines. Stay up to date with health and safety regulations, and ensure the filling station is well-ventilated and secure.

Filling cylinders with compressed breathing gas mixtures other than air

One of the most commonly used gas mixtures is nitrox, or Enriched Air Nitrox (EAN), which contains a higher percentage of oxygen (often 32-36%) and a lower percentage of nitrogen compared to atmospheric air. This reduction in nitrogen can decrease the risk of decompression sickness and allow for longer exposure to the same pressure or shorter decompression stop times.

Another gas mixture used in scuba diving is trimix, which is a combination of oxygen, helium, and nitrogen. Trimix is used to reduce the effects of nitrogen narcosis and increase dive time. By substituting helium for some of the nitrogen, the risk of oxygen toxicity is also reduced.

Heliox, a mixture of helium and oxygen, is used in technical and commercial diving. Heliox does not contain nitrogen, which dramatically increases the diving depth and time limits. However, helium has a higher thermal conductivity than nitrogen, so divers using heliox will lose heat faster.

When filling cylinders with compressed breathing gas mixtures other than air, it is crucial to follow safety protocols and consider the implications of gas laws:

• Boyle's law: States that the volume of a gas varies inversely with pressure at a constant temperature. This law is crucial in understanding the effects of pressure changes on gas volume in scuba tanks and divers' lungs. Divers must never hold their breath during ascent as it can cause severe and potentially fatal damage to the lungs.
• Charles's law: States that the volume of a gas varies directly with absolute temperature at a constant pressure. This law explains why the pressure in a scuba tank decreases when the temperature decreases. Dive shops often fill tanks in water to maintain a lower temperature and prevent overfilling.
• Dalton's law: States that the total pressure of a gas mixture is equal to the sum of the partial pressures of its component gases. This law is important for understanding the effects of different gas mixtures, such as nitrox, on the partial pressures of oxygen and nitrogen.
• Henry's law: States that the amount of gas dissolved in a liquid is directly proportional to the partial pressure of that gas. This law explains how increased pressure during a dive causes nitrogen to dissolve into a diver's bloodstream, potentially leading to decompression sickness during ascent if nitrogen bubbles form.

In conclusion, filling cylinders with compressed breathing gas mixtures other than air can provide several benefits to scuba divers, including increased safety and reduced risk of decompression sickness. However, it is essential to have a good understanding of gas laws and the potential dangers involved to ensure a safe diving experience.

Gas blending for scuba diving

Methods of Gas Blending

Gas blending methods include batch mixing by partial pressure or by mass fraction, and continuous blending processes. The completed blends are then analysed for composition to ensure the safety of the user.

Partial Pressure Blending

Partial pressure blending is done by decanting oxygen and helium into the diving cylinder and then topping up the mix with air from a diving air compressor. After each helium and oxygen transfer, the mix is allowed to cool, and its pressure is measured. Further gas is then decanted until the correct pressure is achieved. This process can take hours and is sometimes spread over several days at busy blending stations.

Continuous Blending

Continuous blending is the process of adding the component gases of the mixture together and then compressing it into a storage cylinder. The component gases are added to the intake of the compressor in a continuous flow at a pressure suited to the compressor design. This generally requires equipment to monitor and control the flow of the input gases, which are usually supplied from high-pressure storage cylinders.

Hazards of Gas Blending

There are several hazards associated with gas mixing and filling scuba cylinders:

• The use of oxygen and high-pressure gases presents a risk of fire and explosion.
• The composition of the mix must be safe for the depth and duration of the planned dive. An incorrect concentration of oxygen can lead to hypoxia or oxygen toxicity.
• The presence of large concentrations of gases that contain no oxygen, such as helium, can cause asphyxiation.
• Toxic contaminants can enter the cylinders from the diving air compressor.
• The high temperatures produced during gas blending can cause a compressor to overheat.

Training and Certification

Training and certification for scuba gas blending are provided by some diver training agencies and may be required by national legislation or standards. ISO 13293 provides minimum training standards for gas blenders for recreational diving services at two levels: Nitrox gas blender and Trimix gas blender.

Filling of cylinders

Inspection:

Before filling, the cylinder must undergo a thorough visual inspection to check for any signs of damage, corrosion, or other issues. This includes identifying the cylinder's permanent and non-permanent markings, such as the manufacturing standard, hydrostatic test date, working pressure, serial number, owner's name, visual inspection date, type of breathing gas, and external condition. Any cylinders that do not meet the legal requirements or are due for periodic inspection or testing should be rejected and sent for further examination.

Bleed and Purge:

Ensure that the cylinder valve is closed, and slowly open the valve to release any remaining air or gas. This step ensures that the cylinder is empty before filling.

Secure the Cylinder:

Place the cylinder in a secure position, either vertically or horizontally, using appropriate equipment or fixtures. This ensures that the cylinder remains stable during the filling process and prevents it from falling over.

Connect to the Filling Station:

Attach the appropriate filling adapter to the cylinder valve, matching the type of valve and gas being filled (e.g. air or nitrox). The filling station may consist of a high-pressure compressor or a cascade system.

Fill the Cylinder:

Once the cylinder is securely connected, use the chosen equipment to fill the cylinder with the desired gas, following specific guidelines and safety protocols. Fill the cylinder at an appropriate rate to avoid excessive heating and control pressure loss due to cooling.

Pressure Testing:

After filling, the cylinder should undergo a pressure test to verify that it has been filled to the correct pressure level as specified by the manufacturer or relevant standards. Take into account the temperature of the cylinder, as the pressure must be within an acceptable range relative to the temperature (usually within 5% of the developed pressure for that temperature).

Final Inspection and Labelling:

Once the cylinder passes the pressure test, conduct a final visual inspection to ensure it is in proper condition. Properly label the cylinder with information such as the type of gas filled, maximum operating pressure, and the date of filling. Ensure that the cylinder is stored securely and that filling data is recorded.

Remember, filling scuba cylinders is a complex process that requires adherence to safety protocols and a thorough understanding of the equipment and gas laws. It is crucial to rely on certified professionals to ensure the integrity of the cylinder and the safety of divers.

Gas planning

Scuba gas planning is a fundamental skill for divers to master, as it ensures their safety and enhances their underwater experience. It involves calculating the amounts and mixtures of gases required for a planned dive, taking into account the dive profile, decompression needs, and gas consumption rates. The purpose is to ensure divers have enough breathable gas suitable for the current depth and are aware of any potential emergencies.

Choice of Breathing Gases

The choice of breathing gas depends on the intended use and depth of the dive. The gas mixture must provide a safe partial pressure of oxygen (PO2) while being breathable at all planned depths. Air is typically used for shallow recreational diving, but for deeper dives, gases like nitrox, trimix, or heliox may be chosen to reduce the risk of decompression sickness and nitrogen narcosis.

Choice of Scuba Configuration

Divers must choose between open circuit and rebreather systems. Open circuit exhausts all exhaled gas, while rebreathers retain and restore exhaled gas for reuse. For carrying the cylinders, divers can opt for back mount or side mount configurations. Back mount involves firmly attaching one or more cylinders to a harness, while side mount suspends the primary cylinders at the diver's sides.

Estimation of Gas Quantities

Divers need to estimate the gas required for the planned dive, including bottom gas, travel gas, and decompression gases. They must also consider gas quantities for foreseeable emergencies, as stress can increase breathing rates and decrease swimming speed, leading to higher gas consumption during an emergency exit or ascent.

Choice of Cylinders

The choice of cylinders depends on the required gas quantities and pressures. Each cylinder's volume and working pressure must be sufficient to contain the needed gas at or below its working pressure. Divers should also consider the buoyancy and trim consequences of different cylinder choices.

Calculation of Gas Pressures

Divers need to calculate the required pressures for each gas in each cylinder to ensure they have sufficient quantities. This involves specifying critical pressures for relevant gas mixtures at different stages of the planned dive profile, taking into account individual and team members' gas consumption rates.

Gas Monitoring and Switching

During the dive, divers must monitor the remaining gas pressure in their cylinders and be aware of their team members' gas status. They may need to switch gases at certain depths to optimise decompression or avoid oxygen toxicity. Gas switching is typically done by manually opening the cylinder valve and stowing the original regulator.

Emergency Gas Provision

In an emergency, divers can share breathing gas by using a single demand valve or providing a secondary gas source. The standard approach is "octopus donation," where the secondary "octopus" demand valve is offered to the diver in need. Alternatively, two divers can share a single demand valve through buddy breathing, taking turns to breathe.

Frequently asked questions