Knowing whether a scuba tank is empty means checking if the internal air pressure has dropped below the safe diving threshold—typically around 500 PSI (or 30–35 bar). The most reliable way to check is by using a pressure gauge, but physical cues like changes in buoyancy or the absence of a hissing sound when opening the valve can also offer clues. In the guide below, we explain how to identify an empty tank, why it matters, and what checks you should perform before and after every dive.

 

What Does “Empty” Mean for a Scuba Tank?

A Tank Isn’t Truly Empty at Zero PSI

In scuba diving, an “empty” tank doesn’t mean it contains no air. In fact, tanks should never be drained to zero PSI. Doing so can allow moisture and contaminants to enter, which can corrode the interior walls and compromise tank safety. Even if no air comes out when you crack the valve, there may still be residual pressure inside. If the tank can no longer supply breathable air through a regulator, it’s considered empty from a diver’s standpoint.

Understanding the Reserve Pressure Standard (500 PSI / 30–35 Bar)

Professional divers and training agencies agree that a tank is considered empty when it reaches 500 PSI. This is not a hard cutoff for pressure, but a safety reserve designed to provide enough time for a safe, controlled ascent and to deal with unexpected situations like currents or out-of-air buddies. It's essential to plan your dive to begin your ascent well before reaching this threshold.

Why Some Pressure Is Left in the Tank After a Dive

Leaving 300–500 PSI in a tank after diving isn’t wasteful—it’s intentional. This residual pressure protects the tank from internal oxidation by keeping moisture out. A tank that’s completely drained to 0 PSI becomes vulnerable to rust, even if it appears clean on the outside. Dive shops may refuse to refill fully depressurized tanks because it’s difficult to guarantee they remain structurally sound and safe to pressurize again.

 

Primary Ways to Check If a Scuba Tank Is Empty

Use a Submersible Pressure Gauge (SPG) or Tank Checker

The most accurate and recommended method is using an SPG connected to your first-stage regulator. Alternatively, a tank checker can be attached directly to the valve for a quick reading. A full scuba tank typically reads 3000 PSI (200 bar), depending on the tank type. If the needle is under 500 PSI, the tank is no longer suitable for diving and should be refilled.

Beginners should make it a habit to check the pressure before every dive. This not only ensures you have enough air for the dive plan but also alerts you to any slow leaks or issues in the valve or O-rings.

Related Reading: How Much Pressure to Fill Scuba Tanks

Open the Tank Valve Briefly to Listen for Air

If a gauge isn’t available, you can carefully open the valve for a second. If you hear a short hiss, the tank still holds pressure. However, this method doesn’t give an accurate reading of how much air is left. It’s useful in a pinch, but not a substitute for a proper gauge.

Be cautious: opening the valve too forcefully may cause internal damage, and if the tank is empty, this action may introduce moisture. Always close the valve immediately after.

Check the Tank’s Weight—Only Useful With a Reference

Compressed air has weight. A full aluminum 80 cu ft tank weighs roughly 6 pounds (2.7 kg) more than when empty. If you’re familiar with how the tank normally feels, you might notice a lighter-than-usual tank as an indicator that it’s close to empty.

However, this method is only helpful if you’ve handled full tanks frequently or have a full tank nearby for comparison. Otherwise, the difference is too subtle for most divers—especially beginners.

Test Buoyancy in Water as a Real-Time Indicator

Aluminum tanks become slightly positively buoyant as they empty, meaning they tend to float more. If, during your dive, you find it harder to stay down or you feel yourself rising unexpectedly during your safety stop, your tank may be nearly empty.

This buoyancy shift is a real-time signal, especially during the last moments of a dive. That said, buoyancy is not a precise measurement tool and should never be relied on for pre-dive air checks.

 

When Should You Check If a Tank Is Empty?

Before the Dive—Part of the BWRAF Safety Check

Checking tank pressure is a fundamental part of the pre-dive routine known as BWRAF (“Begin With Review And Friend”). Before entering the water, open the valve fully and check your SPG to ensure the tank is filled appropriately for your dive—typically around 3000 PSI for most recreational dives.

Failing to check before diving risks running out of air underwater—a situation that can escalate quickly, especially for beginners or those diving in currents.

After the Dive—To Prepare for the Next Use

Once the dive ends, always check the remaining air. If the reading is under 500 PSI, mark the tank for refilling. This also helps you evaluate your air consumption, especially if you're learning to track your Surface Air Consumption (SAC) rate.

Avoid storing a tank with less than 300 PSI, as internal condensation can occur. A quick post-dive check is a simple way to protect your gear and get ahead of your next dive.

Before Storage or Returning Rental Equipment

Whether you own or rent, it’s best practice to return or store tanks with residual pressure. Most dive centers require tanks to be brought back with pressure inside—ideally above 300 PSI.

A tank at 0 PSI may be rejected for refilling due to potential contamination. For personal storage, this step helps extend the life of your cylinder and reduces the risk of internal corrosion during long periods of inactivity.

 

What Happens If You Dive with an Empty or Nearly Empty Tank?

Risks of Regulator Failure Mid-Dive

When pressure drops too low, your regulator may fail to deliver air consistently—or stop entirely. You could experience difficulty breathing, weaker airflow, or complete loss of supply. If this occurs at depth, you may panic or ascend too quickly, both of which increase the risk of injury.

Checking your gauge regularly prevents this and gives you time to react calmly if your air gets low.

Potential for Lung Overexpansion on Emergency Ascent

Running out of air can force an emergency ascent. If a diver holds their breath while ascending rapidly, the expanding air in the lungs can cause serious damage—like lung overexpansion injuries or an arterial gas embolism, which can be life-threatening.

Proper pre-dive checks and conservative planning reduce the chances of ever needing such a risky ascent.

Why “Just a Little Air” Is Still Not Safe

Some divers think that a few hundred PSI is “enough for a quick dive.” This mindset is dangerous. Underwater conditions can change fast—strong currents, increased exertion, or helping a buddy can quickly drain those last breaths of air.

A tank should be fully pressurized before diving. It’s not worth gambling your safety for the sake of squeezing in one more dive on low air.

 

Common Misconceptions About Empty Tanks

A Tank Without a Hissing Sound Isn’t Always Empty

Opening the valve and hearing nothing doesn’t mean the tank is truly empty. It may still have low pressure—perhaps below 200 PSI—that’s just not enough to make a noticeable sound. Silence isn’t confirmation. Only a gauge can give you a reliable answer.

A Floating Tank Doesn’t Always Mean It’s Completely Out of Air

While buoyancy shifts can indicate a low tank, a floating tank isn’t always empty. Depending on the tank material and the diver’s weight configuration, positive buoyancy may occur even with some air remaining. Always double-check using your SPG.

Valve Covers Are Not a Reliable Indicator of Fill Status

Some divers assume a missing or placed valve cover indicates whether a tank is full or empty. In truth, valve caps only protect the threads. They don’t tell you anything about the tank’s air level.

 

Pro Tips for Managing Tank Air Supply Safely

Know Your Tank’s Capacity and Air Consumption Rate

Familiarize yourself with your tank size and how long it typically lasts you. An 80 cu ft tank at 3000 PSI may last around 45 minutes, but this varies widely depending on your depth, fitness level, and breathing rate.

Logging your air use after each dive can help you improve planning and avoid surprises underwater.

Use a Dive Computer or SPG Continuously

Don't just check your pressure at the beginning of a dive. Make it a habit to glance at your SPG or dive computer every few minutes, especially after changing depth or exerting yourself. Staying aware is the best way to avoid running low unexpectedly.

Always Follow the Rule of Thirds

The “rule of thirds” is a smart strategy: use one-third of your air to swim out, one-third to return, and save one-third for emergencies. This technique gives you flexibility and keeps you in control—especially when diving in unfamiliar areas or with newer dive buddies.

Related Reading: What Is The 120 Rule In Scuba Diving?

Conclusion

In summary, the safest way to tell if a scuba tank is empty is by checking the pressure with a gauge—anything below 500 PSI means it’s no longer safe to dive. While signs like weak buoyancy or no hissing sound can offer hints, only a gauge gives a clear answer. Always check before and after every dive to stay safe and protect your gear.