Water Temperature and Thermal Protection
Quarry water is notoriously cold, often significantly colder than tropical oceans, even in summer. Thermoclines—sudden drops in temperature at different depths—are common. A standard 80-cubic-foot aluminum tank used in warm water has a substantial negative buoyancy when empty (around -3 to -4 lbs). In cold water, you’ll be wearing a thicker wetsuit or even a drysuit, which adds significant buoyancy that you must offset with weight. A smaller portable scuba tank has less negative buoyancy when empty. This dramatically changes your weighting requirements. You might need less lead on your weight belt initially, but as you consume air from the smaller tank, its buoyancy will change less than a large tank, affecting your trim and buoyancy control throughout the dive. You must conduct a proper buoyancy check at the surface with a near-empty tank to understand your ending buoyancy characteristic. Failure to adjust your weighting for the specific tank and exposure suit combination is a primary cause of buoyancy-related incidents in quarries.
Air Supply Management and Dive Planning
This is the most critical mathematical consideration. A typical quarry dive can easily reach 60-100 feet (18-30 meters). At depth, your air consumption rate skyrockets due to pressure. Using a portable tank with a lower capacity, such as a 3.0-liter cylinder holding approximately 19 cubic feet of air, demands meticulous planning compared to a standard 11.1-liter (80 cu ft) tank. You must calculate your Rock Bottom Gas (also known as Minimum Gas). This is the amount of air required for two divers to safely ascend from the deepest point of the dive, including a safety stop, at a controlled rate while sharing air.
Let’s calculate a conservative Rock Bottom Gas for a 30-meter (100-foot) quarry dive for two divers with a combined breathing rate of 60 cubic feet per minute (cfm) at the surface:
- Step 1: Determine the pressure at depth. 30 meters is 4 ATA (Atmospheres Absolute).
- Step 2: Calculate the gas consumption rate at depth. 60 cfm (surface) x 4 ATA = 240 cfm.
- Step 3: Determine the time needed for ascent. A safe ascent rate is 30 feet/9 meters per minute. From 100 feet, that’s about 3.3 minutes to reach 15 feet. Add 3 minutes for a safety stop at 15 feet (1.5 ATA). Total ascent time: 6.3 minutes.
- Step 4: Calculate the gas volume needed. 240 cfm x 6.3 minutes = 1,512 cubic feet of gas.
This simple calculation shows that the entire capacity of a 19 cu ft tank is insufficient just for a safe emergency ascent from 100 feet for two divers. This illustrates why portable tanks are not suitable as a primary air source for deep quarry dives with a buddy. Their use case is strictly for very shallow, short-duration dives, or as an emergency backup (pony bottle). Your dive plan must be built around the tank’s limited capacity, not the other way around. The following table compares tank capacities and their implications for a 60-foot quarry dive.
| Tank Type | Capacity (Cubic Feet) | Estimated Bottom Time at 60ft (avg. diver) | Suitable Quarry Use Case |
|---|---|---|---|
| Standard Aluminum 80 | 80 cu ft | 25-35 minutes | Primary source for recreational dive |
| Steel 100 | 100 cu ft | 35-45 minutes | Extended primary source for deeper dives |
| Portable 3.0L Cylinder | ~19 cu ft | 5-8 minutes | Shallow snorkeling-to-diving, backup only |
Entanglement and Snag Hazards
Quarries are often littered with submerged objects left behind from their industrial past: old machinery, cables, wires, and fishing lines left by anglers. A large, standard scuba tank mounted on your back is relatively protected. A portable tank, however, is often worn differently—strapped to a BC vest or slung under the arm. This positioning increases its profile and the risk of it snagging on an unseen obstacle. Before entering the water, you should have a clear mental map of your dive route, avoiding known hazard areas. You must also practice good buoyancy control and finning technique (e.g., frog kick) to avoid stirring up silt, which can reduce visibility to zero in seconds and disorient you, increasing the chance of entanglement. Carrying a sharp cutting tool (line cutter or shears) is not just a recommendation for quarry diving; it is an essential piece of safety equipment.
Access, Terrain, and Entry/Exit Points
Getting to the water’s edge in a quarry can be an adventure in itself. Unlike a dive boat with a platform, quarry entries often involve traversing uneven, rocky, or muddy paths. Using a heavy, standard twin-set is cumbersome and physically demanding in this environment. The lightweight nature of a portable tank is a significant advantage here, reducing fatigue before the dive even begins. However, you must assess the exit point with equal care. Climbing a slippery rock face or ladder with even a small amount of gear is more challenging than it looks. Ensure your hands are free for stability by having all equipment securely fastened. Test the exit ladder before your dive if possible—some are rusty, unstable, or covered in algae. The ease of transport with a portable tank is a major benefit, but it should not lead to complacency regarding the physical challenges of the entry and exit.
Local Regulations and Environmental Factors
Never assume a quarry is a free-for-all. Many are privately owned and managed by dive shops or clubs with strict rules. These rules can directly impact your equipment choices. Some quarries may prohibit the use of certain types of equipment or diving activities they deem higher risk. It is your responsibility to contact the quarry management or check their website for specific regulations regarding tank testing requirements (visual inspection and hydrostatic test stamps must be current), mandatory guide requirements, and designated dive areas. Furthermore, quaries have unique ecosystems. Avoid disturbing sediment more than necessary, and never touch or harass wildlife. Be aware of potential water quality issues; while most operating dive quarries maintain good water quality, runoff can sometimes affect visibility and introduce contaminants. Using a regulator with a sealed diaphragm can be advantageous in less-than-ideal water conditions.
Who is a Portable Tank in a Quarry Actually For?
Given the limitations, the primary legitimate user of a portable tank in a quarry is an experienced diver who employs it as a redundant air source, known as a “pony bottle.” This bottle is completely independent, with its own first and second stage regulator. It is slung from a D-ring or carried in a special pouch and is intended for use only in a true emergency, such as a primary air supply failure. It provides a crucial few minutes of air to conduct a controlled, safe ascent. For this purpose, even a small 19 cu ft or 30 cu ft pony bottle can be a lifesaver. It is not intended for extending a recreational dive. For snorkelers who want to make occasional short dives to 10-15 feet to look at something, a portable tank can offer a taste of scuba, but this should only be done under direct supervision and with a full understanding of the risks involved, including potential shallow water blackout from hyperventilation prior to the dive.