Quick answer: Hyperbaric chamber pressure uses ATA (atmospheres absolute), where 1.0 ATA is normal sea-level air. A 1.3 ATA chamber sits about 4.4 PSIG higher, roughly 10 feet of seawater, while 2.0 ATA equals 14.7 PSIG (33 FSW) and 3.0 ATA equals 29.4 PSIG (66 FSW). However, clinical HBOT usually runs between 2.0 and 3.0 ATA, because it needs close to 100% oxygen in a rigid hard-shell chamber.
Hyperbaric chamber pressure gets more attention than any other spec, and honestly, it causes more confusion too. But an ATA rating tells you only one thing: how hard the air inside the vessel pushes. It won’t tell you what the patient is breathing, though. It also won’t tell you how the chamber was built, or whether a session there counts as medical treatment at all.
We build hard-shell chambers at PCCI Hyperbaric Systems, so these numbers cross our engineers’ desks daily. First, this guide converts ATA into units people recognize: PSIG, feet of seawater and kPa. Then it walks through what each pressure level means for oxygen, construction and safety.
What Is Hyperbaric Chamber Pressure?
Hyperbaric Chamber Pressure is the total air or oxygen pressure inside a sealed chamber once it climbs past normal atmospheric pressure. After all, “hyperbaric” literally means above normal pressure. Down at sea level, for example, air presses on you at about 14.7 pounds per square inch. That’s the 1.0 ATA starting line.
However, when you pressurize a chamber, the air inside gets denser. So each breath packs in more molecules of every gas, oxygen included. That rise in hyperbaric chamber pressure is the basic physics behind hyperbaric oxygen therapy (HBOT). But pressure is only one input. The other is what’s in the air, so we’ll get to that in a minute.
What Does ATA Mean in a Hyperbaric Chamber?
ATA stand for “atmospheres absolute” which is the total unit of pressure measurement used inside a hyperbaric chamber. The word “absolute” matters, because it counts the atmosphere you’re already living under. One atmosphere of extra pressure on top of that gives you 2.0 ATA. Two extra gives you 3.0.
Gauges on a chamber panel, however, usually show something different: PSIG, or pounds per square inch gauge. A gauge reads zero at sea level, so it only shows pressure above the room around it. For that reason, 2.0 ATA appears as 14.7 PSIG on the dial rather than 29.4. People mix these two up constantly, and plenty of chamber listings do too.
The last unit you’ll run into is FSW, short for feet of seawater. It comes from diving. Per the US Navy Diving Manual, every 33 feet of seawater adds roughly one atmosphere.
Hyperbaric Chamber Pressure Levels: ATA, PSI, FSW and kPa
Each hyperbaric chamber pressure conversion below uses 33 FSW per atmosphere. It also uses the NIST value for one atmosphere: 14.696 psi, or 101.325 kPa.
Pressure | PSIG (above ambient) | kPa (absolute) | Depth (FSW) |
1.0 ATA | 0 | 101 | Sea level |
1.3 ATA | 4.4 | 132 | 9.9 |
1.5 ATA | 7.3 | 152 | 16.5 |
2.0 ATA | 14.7 | 203 | 33 |
3.0 ATA | 29.4 | 304 | 66 |
A quick tip: if a listing promises “19 to 29 PSI” of hyperbaric chamber pressure, ask whether that’s absolute or gauge pressure. Only then compare it with anything else. After all, the two differ by a whole atmosphere.
1.3 ATA: What This Pressure Level Means
Pressure in a 1.3 ATA hyperbaric chamber is 30% higher than at normal sea level. In other words, that’s about 4.4 PSIG, of extra pressure or about 19.11 PSIA (pounds per square inch absolute) of total pressure, matching the pressure felt roughly 10 feet underwater.
You’ll mostly find this number on soft-shell chambers: the fabric bags that a compressor inflates and a zipper closes. Their FDA 510(k) clearance covers just one use, acute mountain sickness, and they’re meant to run on plain ambient air. Still, sellers often market them as “mild hyperbaric oxygen therapy.”
But that label deserves a closer look. The Undersea and Hyperbaric Medical Society (UHMS) sets clinical standards in this field. Specifically, it defines HBOT as breathing close to 100% oxygen at 1.4 ATA or more. So below 1.4 ATA, UHMS says, the exposure doesn’t meet its definition. A 1.3 ATA bag on room air raises pressure, sure, but it isn’t clinical hyperbaric therapy.
There’s also an overlooked detail: ASME PVHO-1, the safety standard for pressure vessels that hold people, applies above 2 PSIG. So a 1.3 ATA chamber is well past that line, yet fabric chambers aren’t built as PVHO-1 vessels.
What about 1.5 ATA?
It shows up on a lot of spec sheets because it’s near the top end of what flexible chambers and some personal hard shells advertise. In hyperbaric chamber pressure PSI terms, 1.5 ATA is about 7.3 PSIG, or 16.5 FSW. Still, it doesn’t turn a session into clinical HBOT on its own. After all, the breathing gas and the setting count as much as the gauge.
2.0 ATA: The Starting Point for Clinical Hyperbaric Oxygen Therapy
The pressure in a 2.0 ATA hyperbaric chamber is twice sea level. That’s 14.7 PSIG, or the same as 33 feet of seawater.
So why is this the hyperbaric chamber pressure everyone talks about?
Mostly because UHMS describes accepted clinical HBOT as running between 2.0 and 3.0 ATA, with the exact pressure depending on the condition. A hyperbaric chamber at 2.0 ATA is therefore where most medical protocols start. It’s also where you need a medical-grade hyperbaric chamber. In practice, that means a rigid pressure vessel, medical-grade oxygen and trained staff watching the treatment.
Some sellers now advertise an “exercise-ready hard shell hyperbaric chamber – 2.0 ATA.” When one of those is on your list, skip the marketing. Instead, ask for the rated working pressure, the oxygen delivery method, the FDA 510(k) clearance and the ASME PVHO-1 paperwork. Keep in mind that exercising under pressure isn’t an FDA-cleared use for any chamber, so a physician should weigh in first.
3.0 ATA: What Changes at Higher Chamber Pressure?
A 3.0 ATA hyperbaric chamber operates at 29.4 PSIG (pounds per square inch gauge) or 66 FSW (feet of seawater) of pressure reaches three times normal atmospheric pressure. Still, clinicians don’t reach for 3.0 ATA by default. They pick it when a protocol calls for it.
Outside routine HBOT, though, hyperbaric chamber pressure can go far beyond 3.0. For instance, we rate our clinical multiplace installations for a 6 ATA working pressure. Similarly, on the industrial side, our tunneling and mining systems include TBM manlocks rated up to 10 bar (147 PSIG) for workers in compressed-air tunnels.
1.3 vs 2.0 vs 3.0 ATA: Side-by-Side Comparison
Attribute | 1.3 ATA | 2.0 ATA | 3.0 ATA |
Gauge pressure and depth | 4.4 PSIG, 9.9 FSW | 14.7 PSIG, 33 FSW | 29.4 PSIG, 66 FSW |
Typical chamber | Soft-shell or personal hard shell | Hard-shell monoplace or multiplace | Hard-shell monoplace or multiplace |
Typical breathing gas | Ambient air | Near-100% medical oxygen | Near-100% medical oxygen |
Meets UHMS HBOT definition | No, below 1.4 ATA | Yes, with medical oxygen | Yes, with medical oxygen |
FDA 510(k) context | Acute mountain sickness | Cleared medical uses | Cleared medical uses |
Now look at the bottom three rows, because each one depends on the gas a person breathes. That’s the point most comparisons miss.
Does Higher Hyperbaric Chamber Pressure Mean More Oxygen?
Only when the patient is breathing the right gas. Pressure raises the oxygen partial pressure, but oxygen concentration decides how big a slice of that pressure is oxygen in the first place.
However, the result depends on two key factors:
- The pressure inside the chamber
- The oxygen concentration being breathed.
The math is short. First, oxygen partial pressure equals the oxygen fraction times the absolute pressure (Dalton’s law). Henry’s law then explains why that number matters: as partial pressure rises, more oxygen dissolves straight into blood plasma. Plasma holds about 0.003 mL of oxygen per deciliter for every mmHg of arterial oxygen pressure. That sounds tiny until the partial pressure jumps by a factor of ten.
Same pressure, different oxygen exposure
So here’s what the lungs actually receive at each hyperbaric chamber pressure in five common setups:
Scenario | Gas breathed | Inspired oxygen partial pressure |
Sea level, 1.0 ATA | Ambient air (21%) | 160 mmHg |
Mild hyperbaric, 1.3 ATA | Ambient air (21%) | 207 mmHg |
2.0 ATA on air | Ambient air (21%) | 319 mmHg |
Clinical HBOT, 2.0 ATA | Near-100% oxygen | 1,520 mmHg |
Clinical HBOT, 3.0 ATA | Near-100% oxygen | 2,280 mmHg |
Compare rows three and four. Although they share the exact same hyperbaric chamber pressure, the air-breathing patient gets about a fifth of the oxygen. That’s why ATA by itself can’t describe a treatment.
You may also see summaries claiming 2.0 ATA delivers about 1,824 mmHg of oxygen. In fact, that number belongs to 2.4 ATA rather than 2.0. Real arterial values land lower than any of these, because carbon dioxide and water vapor take up room in the lungs.
Why the oxygen delivery method matters too
Oxygen reaches a patient in two main ways. First, an oxygen-filled monoplace floods the whole vessel with it. An air-pressurized chamber keeps the room on ordinary air while the patient breathes oxygen through a hood or mask. Our multiplace chambers and Althea 4200 monoplace both work the second way. As a result, the atmosphere inside stays close to normal air, which lowers fire risk.
Meanwhile, some soft-shell sellers bolt on an oxygen concentrator and a mask. UHMS points out that these chambers are designed for air only, though, and a concentrator doesn’t produce USP medical-grade oxygen.
Not sure which pressure fits your program?
Pressure rating, oxygen delivery and room layout have to work as one system, so the right answer depends on what you're treating. When you tell us about your application, our engineers will help you narrow it down.
How Chamber Pressure Affects Hard-Shell Chamber Design
From an engineering standpoint, hyperbaric chamber pressure drives almost everything else, because every square inch of the vessel carries that load. So raising the rating changes the whole design:
- Shell: Higher pressure needs thicker walls or a stronger shape, so most hard shells are cylinders.
- Doors: On our multiplace chambers, internal pressure seals the doors shut, so there are no latches to fail.
- Viewports: Acrylic windows also carry their own PVHO-1 rating for pressure and temperature.
- Seals and valves: Each penetration has to hold for the full treatment, not just the first few minutes.
- Pressure control: Controlled compression and decompression, plus relief valves, keep occupants safe when something changes.
- Monitoring: Operators track pressure, oxygen concentration and temperature until the run ends.
Also, rated hyperbaric chamber pressure and treatment pressure are not the same thing. ASME Section VIII, Division 1 generally requires a hydrostatic test at 1.3 times the vessel’s maximum allowable working pressure. So a properly built chamber always keeps a margin above the pressure it treats at.
Pressure, Safety Controls and Standards
Whatever its hyperbaric chamber pressure, any medical-grade hyperbaric chamber answers to several standards. First, ASME PVHO-1 covers the pressure vessel itself, and PVHO-2 covers inspection once it’s in service. Then NFPA 99, Chapter 14, sets the fire safety rules for hyperbaric facilities and treats any atmosphere above 23.5% oxygen as oxygen-enriched. Finally, the FDA classifies hyperbaric chambers as Class II medical devices. It also recognizes both PVHO-1 and NFPA 99 as consensus standards.
Hyperbaric chamber pressure also affects the body. For instance, middle-ear barotrauma is the most common HBOT side effect, according to a 2017 review by Heyboer and colleagues. That’s why slow compression and ear equalization matter at every level. In August 2025, the FDA also reminded providers to follow device instructions, keep equipment maintained, monitor patients and take fire prevention seriously.
Does Every HBOT Treatment Use the Same Pressure?
No. Instead, the clinical protocol sets the pressure, the oxygen concentration and how long the session runs. US Navy Treatment Table 6 for decompression sickness, for example, starts at 60 FSW, or about 2.8 ATA. That’s neither 2.0 nor 3.0. Wound care protocols also often land somewhere in between.
So higher ATA doesn’t equal better treatment. Rather, a physician chooses the pressure that fits the indication. Our military applications work in dive medicine also follows those protocol-driven profiles.
Pressure can go the other direction, too. Our hypobaric altitude chambers drop below 1.0 ATA so aircrews can learn to recognize hypoxia before they meet it in the air.
How to Read a Hyperbaric Chamber Pressure Specification
When you compare hyperbaric chamber pressure specs side by side, check all seven of these:
- ATA: total absolute pressure, so the atmosphere is included.
- PSIG: pressure above ambient. Make sure it isn’t PSIA.
- FSW: depth equivalent, handy when reading treatment tables.
- Maximum working pressure: what the vessel is rated to hold, rather than the pressure it treats at.
- Oxygen delivery: whole-chamber fill, hood or mask, as well as the oxygen concentration supplied.
- Chamber type: soft-shell, hard-shell monoplace or multiplace, since each handles pressure differently.
- Standards: FDA 510(k), ASME PVHO-1 and NFPA 99 documents, so you can actually verify claims.
When a seller can’t answer all seven, you’re getting a number without context. Labs also have their own checklist on top of this one, and our research chambers add programmable pressure profiles and dATA logging for that reason.
Where PCCI Hyperbaric Systems Fits
We design, engineer and build hard-shell hyperbaric chambers, and we don’t build anything else. That covers monoplace and multiplace medical systems as well as tunneling, hypobaric and research chambers. So on every project, we weigh hyperbaric chamber pressure alongside oxygen delivery, controls, fire safety and how people will use the chamber.
About Ramesh Dixit, PE
Our division head, Ramesh Dixit, PE, sits on the ASME PVHO, NFPA 99 and UHMS safety committees, and that work shapes how we design Hyperbaric Chambers and related products.
Frequently Asked Questions
How many PSI is 2.0 ATA?
The pressure in a 2.0 ATA hyperbaric chamber is 14.7 PSIG, which means 14.7 psi above the surrounding air. Counted in absolute terms it’s 29.4 PSIA, because ATA includes the atmosphere already around you. The same pressure also equals 33 feet of seawater, or roughly 203 kPa absolute.
Is 1.3 ATA considered hyperbaric oxygen therapy?
Not under the clinical definition. That’s because UHMS defines HBOT as breathing close to 100% oxygen at 1.4 ATA or higher. Meanwhile, most 1.3 ATA chambers are soft-shell units running on ambient air. They create a mild hyperbaric environment, but the FDA has cleared them only for acute mountain sickness.
What pressure is used in clinical hyperbaric oxygen therapy?
Generally, UHMS describes clinical hyperbaric oxygen therapy pressure as falling between 2.0 and 3.0 ATA, with near-100% medical oxygen. Still, the exact number depends on the condition and the protocol. The Navy’s main decompression sickness table, for example, starts near 2.8 ATA, while some wound protocols sit lower in that range.
Is a higher ATA always better?
No. More pressure raises oxygen partial pressure, but it also raises the risk of oxygen toxicity and ear barotrauma. Physicians match the pressure to the indication instead of chasing the highest number. A chamber’s oxygen delivery and construction matter just as much as its ATA rating when you compare options.
What is the difference between 1.3 ATA and 2.0 ATA?
At 1.3 ATA, pressure is 30% above sea level, or 4.4 PSIG, while 2.0 ATA doubles it to 14.7 PSIG. But the bigger difference is the gas. Because 2.0 ATA chambers usually deliver close to 100% medical oxygen, inspired oxygen pressure ends up about seven times higher.
