HP vs. LP Scuba Tanks丨Pros, Cons, and Application Scenarios

HP (High Pressure) cylinders have a working pressure of 300 bar (4351 psi) and are mostly made of carbon fiber or steel;

LP (Low Pressure) cylinders have a working pressure of 200 bar (2900 psi) and are primarily made of aluminum.

HP cylinders offer large capacity (a 12L HP holds 3600L of gas, while a same-sized LP holds only 2400L) and are lightweight (a carbon fiber 12L HP is about 10kg, while an aluminum 12L LP is about 15kg), but they come with high costs (carbon fiber HP $800-1200) and complex maintenance;

LP cylinders have a low cost ($300-400) and are impact-resistant, but they have small capacity and gas is consumed quickly during deep dives.

HP is suitable for technical diving (depth > 40m) and long-duration operations (such as scientific research), and is the top choice recommended by PADI for deep divers;

LP is suitable for recreational diving (depth ≤ 30m) and as a snorkeling backup, accounting for 70% of the recreational market.

A 12L HP carbon fiber cylinder lasts 90-120 minutes (at 40m), while an LP aluminum AL80 lasts 60-90 minutes (at the same depth).

Pros

High Pressure (HP) cylinders have a standard working pressure of 3442 psi (237 bar).

Their manufacturing process allows them to store 20%-30% more gas within the same physical volume.

For example, an HP100 cylinder is shorter than a standard aluminum tank but provides 20 cubic feet more gas capacity and offers -7 to -9 lbs of negative buoyancy, effectively replacing waist weights.

Low Pressure (LP) cylinders are rated at 2400 psi (165 bar) and have thicker walls, providing extremely high durability.

In remote areas where only 3000 psi filling is available, HP cylinders will lose about 15% of their gas capacity, while LP cylinders can gain an additional reserve of over 20%. Furthermore, their greater self-weight (usually 30-40 lbs) provides excellent ballast for drysuit divers.

HP Cylinders

Chromium-Molybdenum Steel Material

HP cylinders are almost entirely manufactured from Chromium-Molybdenum Steel.

This alloy steel possesses extremely high tensile strength, allowing manufacturers to make the cylinder walls relatively thin without sacrificing the structural safety factor (usually 3:1 or higher).

Although various brands exist on the market, the manufacturing processes for HP cylinders are mainly divided into two types:

1. Deep Draw: Such as Faber cylinders, which are stamped and stretched from a single steel plate. The bottom is usually rounded and requires an additional plastic tank boot to stand upright. These cylinders are relatively light and have moderate buoyancy characteristics.

2. Tube Closing: Early Worthington cylinders (now rarer but highly sought after) used this process. The bottom is flatter, allowing them to stand upright without a boot. These cylinders are typically heavier and have stronger negative buoyancy.

Compared to Aluminum 6061-T6, Chromium-Molybdenum steel has better elasticity and can withstand thousands of high-pressure fill-and-discharge cycles without being as prone to Sustained Load Cracking (SLC) at the neck as aluminum alloys.

This is why, in the second-hand market, a 20-year-old HP steel tank is still valued higher than a brand-new aluminum tank as long as it passes its hydrostatic test.

Buoyancy Characteristics

Unlike aluminum tanks, which exhibit +2 to +4 lbs of positive buoyancy at the end of a dive (500 psi), HP steel tanks maintain negative buoyancy throughout the entire dive.

Buoyancy Change Data Table (Example in Seawater)

Cylinder Specification	Material	Full Weight (inc. gas)	Full Buoyancy	Empty Buoyancy (500 psi)	Buoyancy Swing
HP 80	Steel	34 lbs	-8 lbs	-1.5 lbs	6.5 lbs
HP 100	Steel	40 lbs	-8.5 lbs	-2.5 lbs	6.0 lbs
HP 120	Steel	45 lbs	-9.5 lbs	-3.0 lbs	6.5 lbs
AL 80	Aluminum	35 lbs	-1.6 lbs	+4.0 lbs	5.6 lbs

• Weight Removal Logic: When using an AL80, a diver must carry an extra 6-8 lbs of lead to counteract the cylinder's positive buoyancy at the end of the dive to ensure a safe stop at 3 meters. With an HP100, the cylinder itself provides -2.5 lbs of sinking force even when empty. A diver can remove 6-8 lbs (to counteract the aluminum tank's buoyancy) plus 2.5 lbs (the steel tank's inherent negative buoyancy) from their weight belt, totaling nearly 10 lbs of weight reduction from the waist.

• Trim Optimization: Shifting weight from the waist (lower center of gravity) to the back (upper center of gravity) helps the diver naturally maintain a horizontal position in the water, reducing kicking drag caused by sinking legs.

DIN Valves

HP cylinders are primarily equipped with DIN (Deutsches Institut für Normung) valves.

• O-ring Capture Design: In a Yoke (Int) system, the O-ring is located on the valve surface and relies on clamp pressure for sealing. When pressure exceeds 3000 psi, a sharp impact or a loose clamp can easily cause the O-ring to be "extruded" by high-pressure gas, leading to catastrophic gas leakage.

• DIN Thread Locking: The DIN interface screws the regulator's first stage into the interior of the cylinder valve, "capturing" the O-ring deep inside. A 300 Bar DIN interface typically has 7 threads (compared to 5 threads for 200 Bar). This mechanical connection can withstand extreme physical impact and internal pressure. For a 3442 psi working environment, DIN is the only choice that conforms to safety engineering logic.

Available Gas Capacity

Although HP cylinders are rated at 3442 psi, in practice, thermal expansion is a major factor affecting available gas capacity (Charles's Law).

1. Temperature Rise from Fast Filling: When a compressor rapidly fills a cylinder to 3442 psi, the cylinder temperature rises significantly (potentially reaching 40-50°C).

2. Pressure Drop After Cooling: When the cylinder cools to room temperature or water temperature (e.g., 20°C), the pressure drops significantly. A cylinder filled to 3500 psi while hot may drop to 3000-3100 psi after cooling.

3. Actual Loss: Although a diver carries an HP100 cylinder, if the fill station operates too quickly without performing a "cold fill" or "top-off," the actual gas taken underwater may only be around 90 cubic feet.

To obtain the full 3442 psi (100 cuft) capacity, dive shops need to use water bath filling (submerging the cylinder in cold water) or staged filling (filling to 2000 psi first, then to full pressure after cooling), which places higher demands on the fill station's procedures.

Advantages of Doubles Configuration

In technical diving, twinset systems composed of HP cylinders provide an excellent streamlined profile.

• Dual HP100 vs. Dual AL80: Dual HP100s provide 200 cubic feet of gas with a total width of about 15.5 inches, concentrating the underwater weight on the back. In contrast, dual AL80s provide only 154 cubic feet, and because the aluminum tanks are longer and become positively buoyant at the tail, they tend to kick up at the bottom, increasing drag and reducing gas reserves by 23%.

• Manifold Compatibility: The 3442 psi standard of HP cylinders perfectly matches the 300 Bar rating of modern isolation manifolds. The shorter cylinder length places the valve position relatively lower; for divers with limited arm flexibility, the valves on HP cylinders are usually easier to reach during valve drills than those on long aluminum tanks.

While HP steel tanks have clear advantages for backmount, their use in sidemount configurations requires high-level buoyancy control skills.

Since HP steel tanks remain negatively buoyant even when empty, divers cannot "clip forward" the cylinder mounting points at the end of the dive to let them float alongside the body as they would with aluminum tanks.

HP steel tanks will always hang below the sides of the body.

This requires divers to adjust the wing position of the sidemount BCD or use a sliding D-ring system to dynamically adjust cylinder attachment points, preventing the tanks from dragging on coral or creating drag resistance.

Therefore, HP steel tanks in sidemount are generally only suitable for cold-water technical diving environments that require large gas loads and the use of drysuits (providing extra buoyancy), rather than recreational sidemount diving in warm waters.

HP Cylinders

Negative Buoyancy Characteristics

Unlike aluminum tanks, which produce positive buoyancy at the end of a dive, HP steel tanks maintain negative buoyancy throughout the dive.

Buoyancy and Weight Calculation Table (Example in Seawater)

Cylinder Model	Material	Full Buoyancy	Empty Buoyancy (500 psi)	Weight Belt Adjustment
Aluminum 80	Aluminum Alloy	-1.6 lbs	+4.4 lbs	Need +6 lbs lead to counteract empty tank floating
HP 100	Chromoly Steel	-8.5 lbs	-2.5 lbs	Can remove 6 lbs (aluminum offset) + 2 lbs (steel weight) = -8 lbs

• Weight Reduction Effect: When using an AL80, a diver must carry extra lead to hold down the +4.4 lbs of positive buoyancy produced by the empty tank; otherwise, they may float uncontrollably toward the surface during the 3m safety stop. When using an HP100, the cylinder itself has a sinking force of -2.5 lbs when empty. Divers can remove approximately 6-8 lbs of lead from their waist weight belt.

• Trim Posture Optimization: Reducing waist lead and using the cylinder's own weight to move the center of gravity up the spine can effectively improve a diver's Trim (horizontal posture), preventing "heavy legs" and reducing profile drag while swimming.

3AA Manufacturing

HP cylinders typically comply with US DOT-3AA or Canadian TC-3AAM specifications.

Their material is 4130 Chromium-Molybdenum Steel, a high-strength low-alloy steel.

• Strength and Wall Thickness: Because the tensile strength of chromoly steel is much higher than that of 6061-T6 aluminum alloy, manufacturers can make the walls relatively thin while maintaining the 3442 psi rating. This means that although HP cylinders are steel, their weight on land is not as extreme as imagined (an empty HP100 weighs about 33-34 lbs, only 2-3 lbs heavier than an AL80).

• Manufacturing Process: Modern HP cylinders (such as Faber) mostly use the Deep Draw process, stamped and stretched from a single steel plate, which ensures high consistency in wall thickness.

• Lifespan Advantage: Aluminum alloy cylinders are prone to Sustained Load Cracking (SLC) at the neck under long-term high-pressure cycles. In contrast, the lifespan of steel tanks is almost infinite as long as they pass hydrostatic tests every 5 years and visual inspections annually. 20-30-year-old HP steel tanks are still in circulation and performing perfectly.

DIN Valves

• Physical Connection Mechanism:

  • Yoke (Int): Relies on an external clamp for tightening; the O-ring is located on the valve face. When pressure exceeds 3000 psi, an impact can displace the clamp, causing the O-ring to be blown out by high-pressure gas, leading to catastrophic leakage.

  • DIN (G 5/8): The regulator's first stage screws into the valve, "capturing" the O-ring deep within the threads. A 300 Bar standard DIN interface has 7 threads (compared to 5 for 200 Bar), capable of withstanding extreme mechanical stress and internal pressure.

• Safety Considerations: For a working pressure of 3442 psi, DIN is the only design that meets engineering safety redundancy. All HP cylinders come factory-fitted with DIN valves (some have screw-in inserts for Yoke conversion, but it is generally recommended to remove them for HP use).

Filling

Although HP cylinders are rated at 3442 psi, in practice, the relationship between temperature and pressure described by Charles's Law significantly impacts the fill volume.

• Hot Fill Loss: When a compressor fills quickly, the cylinder temperature can rise sharply to 45-50°C. The gauge might show 3500 psi then, but as it cools to room temperature (25°C) or water temperature (20°C), the pressure will drop to 3000-3100 psi.

• Actual Gas Volume Deduction: If a fill station does not perform "water bath filling" or "top-offs after cooling," an HP100 cylinder may only carry 90 cubic feet of gas underwater.

• Countermeasures: Divers with HP cylinders usually need to find dive shops that provide "cold fill" services or accept a second pressurization after the first fill has cooled to achieve full rated performance.

Streamlining Advantages

In technical diving twinset configurations, the advantages of HP cylinders are further magnified.

• Drag Profile: A twinset system of dual HP100s is about 15.5 inches wide and sits tight against the back. In contrast, a dual AL80 system used to get similar gas capacity is wider and, due to the positive buoyancy of the aluminum tank tails, causes an unstable "tail-up" posture.

• Manifold Matching: Modern isolation manifolds are usually rated for 300 Bar. The 237 Bar working pressure of HP cylinders is well within the optimal range. The shorter body makes the valve height moderate, making it easy for divers to reach the valve knobs during underwater valve drills, which is especially important for divers with limited flexibility wearing drysuits.

Modern HP steel tanks are typically hot-dipped galvanized or phosphated and then coated with a polymer layer. High-quality HP steel tanks (like those with hot-dip galvanization) will undergo a sacrificial oxidation reaction if the surface is scratched, forming a dense zinc oxide protective layer that prevents rust from penetrating the steel. This feature solves the early steel tank rusting problem, greatly increasing durability in seawater environments; maintenance simply requires a fresh-water rinse after diving.

Cons

HP (3442 psi) cylinders are limited by compressor output capacity.

If only filled to the common standard of 3000 psi, gas capacity will drop by approximately 13% (e.g., 100 cu ft drops to 87 cu ft), and the cooling effect after a hot fill will cause final pressure to drop further.

LP (2400/2640 psi) cylinders have weight and volume redundancy; for the same gas capacity, the empty tank weight is typically 1.5 to 3 kg more than an HP cylinder.

The utility of LP cylinders depends entirely on non-standard "overfilling" (Cave Fill).

If a dive shop strictly follows DOT/TC nameplate pressures, the capacity/weight ratio is even worse than cheap aluminum tanks, and specific hydrostatic tests are required every 5 years to maintain the "+" mark (10% extra capacity), making maintenance more cumbersome.

Disadvantages of HP Cylinders

Chronic Underfilling

The design advantages of HP cylinders are built upon a working pressure of 3442 psi (237 bar).

In reality, the logistical support network has not fully caught up with HP cylinder specifications.

• Compressor Cut-off Pressure Bottleneck
  Air compressors worldwide (especially on remote islands or older liveaboards) have safety valves (PRV) and auto-shutoff settings typically locked at 3000 psi (207 bar) or 3200 psi (220 bar).
  Operators often refuse or are unable to adjust this limit. When you bring an HP100 (100 cu ft @ 3442 psi) for a fill, the physical capacity conversion follows a linear decay:
  $$ \text{Actual Gas} = \text{Rated Gas} \times (\frac{\text{Actual Pressure}}{\text{Rated Pressure}}) $$
  If filled only to 3000 psi, the calculation is: $ 100 \times (3000 / 3442) \approx 87.1 $.
  The user pays for the cost and weight of a 100 cu ft tank but only gets 87 cu ft of gas. By comparison, a standard Aluminum 80 has 77.4 cu ft at 3000 psi; the gap narrows, but the maintenance and initial investment for HP steel are much higher.

• Thermodynamic Cooling Effect
  The Ideal Gas Law ($PV=nRT$) shows that pressure is proportional to temperature at a constant volume. Fast filling (Flash Fill) causes intense friction, raising the tank temperature to 45°C - 60°C.

  • False Full: At the moment filling ends, the gauge may show 3500 psi.

  • Cooling Drop: After sitting for an hour to reach ambient temperature (e.g., 25°C), the pressure drops significantly. Empirically, for every 10°F (~5.5°C) drop, pressure falls by about 100 psi. A tank hot-filled to 3500 psi often stabilizes at 3100 psi - 3200 psi.

  • Lack of Top-offs: To reach a true 3442 psi, dive shops need to perform a "top-off" after cooling. In commercial operations, few shops are willing to spend the extra time and steps.

Valve Standards

To safely handle pressures over 237 bar, industrial standards mandate or strongly recommend the DIN (G 5/8") interface.

This deep-thread design fully encloses the O-ring within the metal structure, preventing O-ring extrusion under high pressure.

However, the existing stock of regulators (especially rentals and entry-level gear) is still dominated by Yoke (Int/A-clamp).

• Physical Hazards of Adapters (Insert/Puck)
  Mounting a Yoke regulator on a DIN valve requires a brass adapter called an "Insert."

  • Double O-ring Risk: Native DIN requires only one O-ring. With an adapter, an O-ring is added to the "adapter-valve" interface. This O-ring behind the adapter is often ignored; if it perishes or extrudes underwater, it causes a catastrophic free flow.

  • Galvanic Corrosion: Adapters are often left in the valve for long periods. Without regular cleaning, seawater forms salt crystals and galvanic corrosion between the brass adapter and steel valve, "cold-welding" it inside and requiring forceful removal that often damages threads.

• Geometric Changes and Head Interference
  Yoke regulators are bulky, and with an adapter, the first stage extends further back.

  • Leverage Effect: The first stage protrudes 20mm - 25mm further. This increases the moment arm, subjecting the valve to greater torque impacts (e.g., if the tank tips over).

  • Back-of-Head Impact: When a diver looks up (e.g., checking the surface), the back of the head hits the protruding first stage. This limits neck range of motion, stiffens trim, and causes vibrations through the regulator that increase jaw fatigue.

  • Hose Routing Stress: The shifted position changes the curves of low and high-pressure hoses. For divers with short hoses, this causes over-stretching and limits head rotation.

Buoyancy Characteristics

• Excessive Negative Buoyancy
  While HP steel walls are thinner than LP, the density of steel (~7.8 g/cm³) means an empty HP100 still has negative buoyancy of -2.5 kg to -3.5 kg.

  • Tropical Diving Dilemma: In tropical waters with 3mm wetsuits or skins, body buoyancy is low. Carrying an HP tank might mean the diver sinks even without any lead weights. This "over-weighted" state makes it impossible to fine-tune buoyancy through breathing, forcing constant reliance on BCD inflation.

  • BCD Lift Deficiency Risk: If a BCD failure occurs (e.g., inflator hose disconnect, bladder tear), the heavy HP tank becomes a deadly anchor. For divers using small-lift travel wings (e.g., 25 lbs / 11 kg), large HP tanks (like HP120 or HP133) may pose a risk of being unable to establish positive buoyancy to swim back up.

Maintenance and Testing

Burst Disk Confusion

HP cylinders typically require burst disks rated at 5250 psi.

Standard aluminum or LP tanks use 4000 psi or 3360 psi disks.

Repair Risks

If an inexperienced technician replaces an HP disk with a standard aluminum one during an annual Visual Inspection, the disk may burst at the fill station during the next 3442 psi fill, causing loud panic and gas waste.

Steel tanks are prone to internal rust.

Removing rust involves "tumbling," using abrasives to grind the inner wall.

Since HP walls have smaller design margins (to save weight) and handle extreme pressure, their tolerance for wall thinning is much lower than LP tanks.

Frequent or deep tumbling may cause the wall thickness to fall below DOT/TC minimums, failing the hydrostatic test and leading to the premature scrapping of an expensive cylinder.

Disadvantages of LP Cylinders

Dependence on Gas Capacity Performance

The popularity of LP cylinders in technical diving is built on an open secret: Overfilling.

However, for average divers or in non-tech-friendly regions, this becomes a major barrier.

• Inefficiency at Nameplate Pressure
  The LP philosophy is "low pressure, high volume," but this "high volume" is underwhelming by modern standards.

  • Data Example: An LP85 tank is usually marked 2640 psi (including the "+" mark). At this compliant pressure, it holds exactly 85 cu ft.

  • Reality Gap: In remote areas without high-pressure capacity, or if a shop refuses to fill past 2400 psi (base rating) for liability reasons, this heavy steel tank holds only about 77 cu ft.

  • Comparison Blow: This is less than a fully filled, cheap AL80 (@ 3000 psi = 77.4 cu ft). The user carries a much heavier tank without any gas reserve advantage.

• Difficulty of Obtaining Cave Fills
  Technical divers often fill LP tanks to 3500 psi or higher, reaching a staggering 110+ cu ft.

  • Commercial Refusal: Most recreational dive centers bound by insurance strictly forbid filling past the Service Pressure on the nameplate.

  • Equipment Limits: Even if a shop is willing, many old compressors have PRVs set at 3200 psi, physically unable to provide the "overfill" pressure. Unless you have a private compressor or are a regular at a specific tech shop, most of the LP's potential is wasted.

Physical Burden

• Extreme Empty Weight
  To maintain strength at lower pressures and provide large volume, LP walls are thick.

  • Single Tank Weight: An empty LP85 or LP95 typically weighs 34-38 lbs (15.5-17.2 kg). In contrast, the larger capacity HP100 weighs only 32-33 lbs.

  • Doubles Systems: When assembled with a manifold and bands, the system weight easily exceeds 35-40 kg.

  • Shore Diving Nightmare: For shore dives requiring gear to be carried to the water, or boat dives with ladders, the extra weight of LP tanks accelerates fatigue and increases the risk of knee and spinal injuries.

• High Inertia Underwater
  Greater mass means greater inertia.

  • Posture Control: When turning or stopping quickly in water, heavy LP tanks create significant inertial moments. Divers may feel the tanks trying to "swing" past their bodies, making fine trim adjustments more taxing on core muscles.

  • Trim Issues: Some long LP tanks (like old LP95s or LP120s) have a center of gravity that makes divers prone to "tail-heavy" (feet sinking) issues or difficulty recovering from head-down positions.

The "+" Rating

LP nameplates often have a plus sign (e.g., 2400+), allowing a fill to 110% of rated pressure (2640 psi).

According to DOT and TC regulations, steel tanks must undergo a hydrostatic test every 5 years.

• • Test Principle: The tester pressurizes the tank to 5/3 times the rated pressure and measures expansion.

  • Conditions to Keep the Plus: Elastic expansion must be within a specific range, and permanent deformation must be near zero.

  • Human Factors: If the test facility's gear isn't precise or the tester doesn't know how to calculate REE values, they often play it safe—passing the test but refusing to stamp the "+".

• Permanent Capacity Loss
  Once a tank loses the "+" in a test, it can legally only be filled to the base pressure (e.g., 2400 psi).

  • Your tank instantly and permanently loses 10% of its capacity.

  • An LP85 without the plus legally becomes an LP77, but remains just as heavy and bulky. This causes huge fluctuations in the market value of used LP tanks; buyers must check for the plus sign next to the latest hydro stamp.

Drag

To compensate for lower gas density, LP tanks must be physically larger.

• Diameter Differences
  Most standard HP and aluminum tanks are 7.25 inches (184mm) in diameter.
  Many high-capacity LPs (LP85, LP95, LP120) are 8.0 inches (203mm).

  • Frontal Area: An increase of 0.75 inches significantly increases the frontal area of a doubles configuration.

  • Drag Formula: Fluid drag increases with the square of velocity. When swimming against a current, thicker LP tanks create more drag, forcing more exertion and increasing SAC (Surface Air Consumption) rate, partially negating the advantage of the larger volume.

• Backplate and Wing Compatibility
  8-inch tanks in a doubles setup require a wider manifold center distance.

  • BCD Squeeze: Many standard wings are designed for 7.25-inch tanks. Using 8-inch LP doubles may cause the inflated wing to get squeezed between the tanks and backplate, hindering inflation or making exhaust valves hard to operate (the "Taco effect").

Mismatch Risks

Burst Disk Ratings

Standard LP valves should have disks rated for 4000 psi or lower (about 3600-3750 psi depending on the rating).

If a diver intends to "overfill" to 3500 psi, factory disks are highly prone to bursting during filling or after thermal expansion.

Unauthorized Modification Hazards

To prevent bursts during overfills, many users swap LP disks for HP specs (5250 psi).

While this prevents bursts, it violates DOT regulations. In an accident investigation, such modifications void insurance.

Furthermore, during Visual Inspections, responsible technicians will fail a tank with the wrong disk rating, forcing a swap back to LP disks, putting "overfill" users in a cycle of yearly part swaps.

Application Scenarios

LP (Low Pressure) steel cylinders (rated at 2400 psi) are often used to offset the positive buoyancy of a drysuit due to their negative buoyancy (approx. -8 lbs when full) and thick-wall construction.

They also allow for extra capacity (overfill) at remote fill stations that only have 3000 psi output capability.

HP (High Pressure) steel cylinders (rated at 3442 psi) are about 15% smaller in volume for the same gas amount (e.g., 100 cu ft), offering lower drag.

They are better suited for wetsuit diving or smaller divers.

However, insufficient fill station pressure will result in a 13-15% loss of gas capacity.

Cold Water & Drysuit

Advantages of LP Steel

Low Pressure Steel cylinders are made of Chromoly Steel, using increased wall thickness to compensate for lower yield strength, making them physically heavier than same-volume High Pressure (HP) tanks.

Cylinder Model	Rated Pressure	Material	Empty Weight (no valve)	Empty Buoyancy (Saltwater)	Full Buoyancy (Saltwater)
Faber LP 85	2640 psi	Steel	31.0 lbs	-3.8 lbs	-8.5 lbs
Faber HP 100	3442 psi	Steel	33.0 lbs	-2.5 lbs	-10.0 lbs
Worthington LP 95	2400 psi	Steel	37.0 lbs	-5.5 lbs	-12.5 lbs
AL 80 (Ref)	3000 psi	Aluminum	31.6 lbs	+4.4 lbs	-1.5 lbs

Data shows that a Worthington LP 95 still provides -5.5 lbs of sinking force when gas is exhausted (empty), replacing a standard 5 lb lead block.

For a doubles system (Double LP95s), a diver has 11 lbs of permanent ballast on their back, significantly reducing reliance on a weight belt.

Swing Weight

Air weighs approximately 0.0807 lbs/cu ft.

• HP100 @ 3442 psi: Contains about 8 lbs of gas.

• LP95 @ 2640 psi: Contains about 7.6 lbs of gas.

When ending a dive and returning to 3-6 meters for a safety stop, the cylinder is nearly empty, and buoyancy reaches its maximum (strongest tendency toward positive buoyancy).

• Using HP Cylinders: If weight calculations are imprecise, the slight negative buoyancy (or neutrality) of an empty tank may fail to counteract the expansion of residual air in a drysuit, leading to an uncontrolled ascent.

• Using LP Cylinders: Their inherent high negative buoyancy provides an extra safety margin. Even if the tank is completely empty, an LP95 acts like a "back-mounted lead block," stabilizing the diver at safety stop depth—a physical property highly practical for stops in rough cold-water seas.

Operational Impact

LP 85 and LP 95 tanks are typically slimmer and longer than HP tanks of the same capacity.

• Diameter: LP is usually 7.0 inches (178mm), whereas HP100 and HP120 are usually 7.25 inches (184mm) or 8.0 inches.

• Length: A Faber LP 85 is approx. 26 inches (66cm) long, while an HP 100 is approx. 24 inches (61cm).

In Doubles configurations, the longer LP tanks allow the bottom bands to be mounted further down.

Based on leverage principles, this places the upper part of the tanks (along with the Manifold) lower on the diver's back.

• Head Clearance: The lower manifold position gives more space for the head to tilt back, which is invaluable in cold water where divers frequently look up at the surface or buddies, and where drysuit neck seals already limit movement.

• Valve Reach: Although the manifold is lower, because LP tanks are longer, divers can adjust backplate height to find a balance point where they don't hit their head but can still easily reach valve knobs with stiff dry gloves. Short, stubby HP tanks often force a compromise between "hitting your head" and "not reaching the valves."

V-Weight and P-Weight Systems

V-Weight

• HP Cylinders: Because the body is shorter and wider, the gap between doubles is relatively wide but short. Standard V-weight blocks may be unstable or cause too much weight concentration.

• LP Cylinders: The slim, long body forms a narrow, deep central channel when doubled. This allows for longer V-weight strips (up to 8-10 lbs), distributing weight evenly along the spine rather than concentrating it on a single lumbar point.

Center of Gravity (CoG) and Torque Adjustment

• HP Solution: Since the tanks are short, the center of gravity is concentrated in the upper-middle back. To keep the legs down, divers often need ankle weights, which increase leg inertia and reduce kicking efficiency.

• LP Solution: Longer LP tanks extend the weight toward the hips. This mass distribution naturally presses down the lower body, creating a Pitch Down Moment, maintaining a horizontal trim without needing ankle weights.

Cave Diving & Cave Fills

Structural Integrity

Why can LP tanks handle a 1000 psi overfill while HPs cannot?

• LP Steel: Made of Chromoly, designed with a focus on safety through wall thickness rather than extreme yield strength. This "heavy" structure gives it excellent fatigue cycle resistance. Even with long-term overfilling, the elastic deformation of thick-walled steel stays within safe thresholds.

• HP Steel: To save weight, HP tanks use higher-strength specialty steels with thinner walls. Their design pressure is already close to the material's elastic limit. Forcing a 3442 psi HP tank to 4000+ psi creates massive stress concentration at the Neck Threads, drastically shortening its life and potentially causing neck cracks.

Regional Differences

Choosing HP or LP often depends on whether your dive site can and will provide a "Cave Fill."

• Florida (High Springs/Cave Country): Fill stations here (like Cave Adventurers, Amigos) have industrial compressor banks with output pressures set to 4500 psi. When staff see LP doubles, the default is to fill to 3500+ psi. In this environment, the high-volume advantage of LP is fully unlocked.

• Yucatan, Mexico (Cenotes): While also a cave mecca, most shops use aluminum (AL80). The few shops renting doubles often have compressors set to auto-stop at 3000-3200 psi. If you bring heavy LP steel tanks to Mexico and only get 3000 psi, you'll be "carrying a dead-heavy piece of iron with no air." In this case, HP tanks or local aluminum ones are the rational choice.

Valves and Burst Disks

To accommodate Cave Fills, divers must watch component ratings.

Standard LP valves usually come with 4000 psi (5/3 of working pressure) burst disks.

• Risk: During a "hot fill" to 3600-3700 psi on a hot summer day, internal pressure may hit 4000 psi, causing the disk to rupture and immediate gas loss.

• Modification: Cave divers often replace LP disks with 5250 psi versions (standard for HP). While technically a violation of DOT regulations regarding safety devices, it is a standard operating procedure in the cave community to prevent catastrophic gas loss underwater or at the fill station.

Gas Planning Rules

Using LP tanks with a Cave Fill also simplifies gas planning calculations.

• LP @ 3600 psi: Start pressure is 3600.

  • 1/3 in: 1200 psi.

  • Turn Pressure: 2400 psi.

  • Calculations are intuitive; integer division reduces mental load and the risk of pressure-induced errors underwater.

• HP @ 3442 psi: Start pressure is 3442 (actual gauge might show 3400 or 3500).

  • If calculated strictly at 3400, 1/3 is about 1130 psi.

  • Turn Pressure: 2270 psi.

  • Non-integer readings require more brainpower for estimation on the gauge.

In the North American cave community (led by Florida), LP steel cylinders dominate due to an industry unwritten rule known as "Cave Fill"—filling LP steel tanks rated at 2400 psi (165 bar) or 2640 psi up to 3500-3600 psi (240-248 bar).

Through this physical overfill, a twin LP104 system can carry over 280 cu ft of gas, which is about 40% more reserve than a standard-rated twin HP100 system.

Physiological Fit

Torso Measurement

The "fit" of dive gear isn't just about BCD size; it involves the geometric mapping of Cylinder Length and Backplate Length relative to the diver's spine (C7 vertebra to sacrum).

Physical fit considers two rigid interference zones:

1. Cervical Interference: Whether the first stage and manifold block the neck from tilting back.

2. Gluteal Interference: Whether the tank bottom hinders hip movement or hits the back of the thighs.

Physical Meaning of Data

• HP100 (Faber/Worthington): Length is approx. 24.0 inches (610mm). This is a compact mass with a relatively high Center of Mass.

• LP85 (Faber): Length is approx. 26.0 inches (660mm).

• LP95/LP104: Length is usually 26.5 to 28 inches.

For a diver with a torso length of 50cm, wearing a 70cm tank set means 20cm must extend toward the head or hips. This extension determines hydrodynamic performance.

Short Torso

Avoiding the "Turtle Effect"

When a short-torso diver uses long LP tanks, they often have to mount the tanks high to avoid them hitting the hips.

• Consequence: The manifold sits too high, right against the back of the head.

• Operational Limit: The diver cannot look up (navigating in Trim requires a 45-60 degree neck tilt), forcing them to "turtle" their neck. This leads to extreme neck fatigue and limited field of vision.

Leg Space Release

The 61cm length of an HP100 allows the tank bottom to stop right at the junction of the lumbar spine and sacrum.

• Kinematic Advantage: During frog or back kicks, the hamstrings and glutes need to contract and lift. The short tank leaves enough mechanical space to ensure kicks don't strike the tank boots, maintaining propulsive efficiency.

Long Torso

For divers over 180cm (5'11"), HP tanks often seem "too short," causing severe center-of-gravity imbalance.

Solving "Floaty Feet"

A diver in horizontal Trim is a lever.

• HP Issue: Since the tanks are short, all the weight is concentrated near the lungs (center of buoyancy). The lower body lacks the Moment Arm to keep the air in drysuit legs down.

• Result: Divers feel like they're wearing a heavy backpack; the upper body sinks while feet float up uncontrollably. To stay level, they must strain abdominal muscles or add ankle weights (reducing streamlining).

The "Keel" Effect of LP Tanks

The longer body of an LP85 or LP95 extends approx. 3-4 kg of steel mass from the shoulder blades down to the upper hips.

• Torque Calculation: Moving mass down 10-15cm relative to the body's rotation center (usually near the navel) significantly increases the Pitch-down Torque.

• Posture Correction: This natural physical ballast acts like a ship's keel, automatically holding down the lower body and allowing easy horizontal trim without extra ankle weights.

Valve Operation

Reach Distance

• HP Tanks (Wide and Short): HP steel tanks (especially the 8.0-inch diameter Worthington series) increase the lateral width of doubles. For narrow-shouldered divers, hands must reach further "back and out" to touch knobs. If the tank is hung too low (to clear the head), the valves may sit below the shoulder blades, requiring extreme "anti-joint" arm movements.

LP Tanks (Narrow and Long): LP tanks are usually 7.0 inches (178mm) in diameter.

A doubled LP set is about 1-2 inches narrower than an HP set. This narrower profile keeps valves closer to the spine, reducing shoulder abduction.

Additionally, because the tanks are long, they can be mounted lower (without hitting legs), placing valves in the "golden zone" at the base of the neck—where hands naturally reach when reaching over the shoulder.

Center of Mass Distribution

In addition to fore-aft balance (Trim), the diameter of the cylinder directly affects side-to-side rolling stability.

Parameter	HP Double System (8.0" Dia)	LP Double System (7.0" Dia)	Physical Impact
Center of Mass Distance (CoM Offset)	Farther from the spine	Closer to the spine	The further the mass is from the axis of rotation, the greater the Rotational Inertia.
Fluid Cross-section	Wider	Narrower	A narrower cylinder body reduces the projected area against lateral water flow.

Roll Damping

• HP Doubles: Like a flat, wide board. When banking side-to-side in the water, it requires overcoming a larger water resistance torque, but it provides higher passive anti-roll stability when stationary.

• LP Doubles: Like a compact cylinder. The design close to the back makes the diver more agile when turning in confined spaces (such as shipwreck penetration), offering a stronger sense of "diver-gear unity," but requires finer body control during side-hovering.

HP (High Pressure) steel cylinders (such as HP100) are usually shorter (approx. 24 inches/61cm), concentrating the center of gravity in the shoulder blade area of the back.

This is suitable for divers under 170cm or those with short torsos, preventing the bottom of the cylinder from interfering with leg finning;

In contrast, LP (Low Pressure) steel cylinders (such as LP85/95) have a slender profile (approx. 26-28 inches/66-71cm), which effectively extends the weight down the spine.

This provides necessary lower-body ballast torque for divers over 180cm, solving the "head-heavy" lever balance issue.