If you’ve ever handled a portable scuba tank, you’ve likely noticed a plus (+) symbol stamped near its valve. This symbol is not a manufacturer’s logo or a simple decoration; it is a critical piece of information that indicates the cylinder has been tested and approved for filling to a pressure 10% higher than its official working pressure. Essentially, it grants a certified fill station permission to safely pressurize the tank beyond its nominal rating, providing you with a valuable reserve of air.
The concept of a “+” rating is rooted in the rigorous hydrostatic testing process that scuba cylinders must undergo periodically, typically every five years. During this test, the tank is filled with water and pressurized to a level significantly higher than its working pressure—often 5/3 or 3/2 times the working pressure. This “proof pressure” tests the tank’s structural integrity without the risk of a rapid expansion that would occur with compressed air. If the cylinder passes this test without sustaining permanent deformation (measured by a permanent expansion test), and if the tank’s specifications allow for it, the testing facility may stamp the “+” symbol next to the test date. This signifies that the cylinder’s walls demonstrated exceptional strength and elasticity, safely withstanding the extreme test pressure and earning the right to hold an extra 10% of gas during normal use.
Understanding Working Pressure and the + Rating
To fully grasp the significance of the plus symbol, you must first understand the standard pressures involved. Every scuba tank has a “working pressure” (WP) stamped on it, which is the maximum pressure it is designed to hold under normal diving conditions. Common working pressures are 200 bar, 232 bar (3000 PSI), or 207 bar. The “+” symbol authorizes an “overfill” to a “service pressure” that is 10% above this WP.
Let’s look at a common example with a standard aluminum 80-cubic-foot tank, which has a working pressure of 207 bar (3000 PSI).
| Tank Specification | Pressure (Bar) | Pressure (PSI) | Significance |
|---|---|---|---|
| Working Pressure (WP) | 207 bar | 3000 PSI | Standard maximum fill pressure. |
| + Rated Service Pressure | 227.7 bar | 3300 PSI | Authorized fill pressure with the + symbol. |
| Extra Air Gained | +20.7 bar | +300 PSI | Approximately 8 extra cubic feet of air. |
This extra 300 PSI translates directly to more bottom time. For a diver using air at a moderate depth and breathing rate, those 8 extra cubic feet could mean several additional minutes of dive time, which can be crucial for safety stops, dealing with unexpected currents, or simply extending a pleasant dive. It’s a significant bonus that comes at no extra cost, provided your tank has the “+”.
The Hydrostatic Test: Earning the Plus
The journey to earning the “+” is demanding. The hydrostatic test is a destructive test in spirit, but it’s performed in a way that ensures safety. The tank is placed inside a water-filled chamber, known as a bomb calorimeter. It is then pressurized with water to its proof pressure. Since water is nearly incompressible, it stores very little energy, so if the tank were to rupture, it would not explode; it would simply leak.
The key measurement is “permanent expansion.” As the tank is pressurized, it expands elastically, like a spring. When the pressure is released, it should return to its original size. The test measures any permanent growth. If the permanent expansion is less than 10% of the total expansion observed during pressurization, the tank passes. The “+” is only awarded if the cylinder not only passes but does so with a significant margin of safety, demonstrating it can handle the additional 10% service pressure with ease. It’s a mark of quality and resilience.
Important Limitations and Safety Considerations
While the “+” symbol is beneficial, it is governed by strict rules and is not universally applicable.
1. Validity Period: The “+” rating is only valid for the hydrostatic test cycle during which it was awarded. When the tank is due for its next hydrostatic test (in 5 years), the “+” symbol expires. It may be re-earned during the new test, but it is not automatic. The tank must pass the test’s permanent expansion criteria again to receive a new “+” stamp next to the new test date.
2. Visual Inspection (VIP): The “+” rating does not override the need for annual Visual Inspections. A tank can have a valid “+” but fail its VIP due to internal corrosion, external damage, or a damaged valve. A fill station is obligated to refuse a fill, even to a “+” rated tank, if it fails the visual inspection. Safety always comes first.
3. Not All Tanks Qualify: Some cylinders, particularly those made from certain steels or with specific manufacturing specifications, are not designed to be filled beyond their working pressure. They will never receive a “+” symbol. It is crucial to never exceed a tank’s stamped working pressure unless the “+” is present and current.
4. Fill Station Discretion: Ultimately, the dive shop or fill station has the final say. Even with a valid “+” symbol, the technician may choose, based on their own shop policy or a visual assessment of the tank’s age and condition, to fill it only to its working pressure. It is always best to communicate with the fill technician.
The Practical Impact on Your Dive
For the recreational diver, the “+” symbol means more than just a number on a gauge. It provides a tangible safety and performance buffer. That additional air can be the difference between a normal safety stop and a rushed, stressful ascent. It allows for more flexibility in dive planning and can reduce anxiety about air consumption. When selecting gear, a portable scuba tank that boasts a current “+” rating is a smarter choice, indicating a cylinder that has proven its robustness. However, this benefit is entirely dependent on the tank being in otherwise perfect condition, underscoring the non-negotiable importance of regular professional maintenance and inspections.
The presence of the plus symbol is a direct result of the scuba industry’s relentless focus on safety through standardized testing. It represents a cylinder that has been stress-tested beyond its normal limits and certified for enhanced performance. While it offers a clear advantage, it also serves as a reminder of the responsibilities we bear as divers to understand our equipment thoroughly and maintain it meticulously. The next time you gear up, take a moment to check for that little “+”; it’s a small stamp that carries a great deal of weight.