Hyperbaric Glossary: ATA, kPa, BIBS, PVHO and More

[ Gauge and fittings ] Reference imagery; final photography in production

How to use this glossary

This page collects the terms our guides use, in one place. Each entry gives a short definition that matches the full guide it comes from, and a link to that guide. The terms are grouped by topic: pressure units, chamber types, standards and regulators, oxygen and breathing, clinical and billing terms, and buying and ownership terms. A table near the end lists every term with its plain definition and the page that explains it.

The definitions describe what a term means and who uses it. They do not say what a chamber does for your health, and they do not rank brands. Where a word has two definitions from two bodies, we give both and say who uses which. Our best hyperbaric chambers page uses these same terms when it scores chambers.

Pressure units

ATA (atmospheres absolute). The total pressure on you, counting the atmosphere that is already there. Sea level is 1.0 ATA, so a chamber at 1.5 ATA adds half an atmosphere on top of normal air pressure. Our pressure ratings guide covers the three tiers: 1.3, 1.5 and 2.0 to 3.0 ATA.

Atmosphere. The US National Institute of Standards and Technology defines the standard atmosphere as 101.325 kPa, which is about 14.7 psi. A 2.0 ATA chamber is one full atmosphere above ambient.

kPa (kilopascal). A metric unit of pressure. Spec sheets that use it usually mean the pressure added above ambient: 30 kPa above ambient is about 1.3 ATA, and 50 kPa is about 1.5 ATA. The UHMS writes its 2.0 ATA floor as 202.65 kPa, which is the absolute figure, exactly twice 101.325.

psi, psia and psig. Pounds per square inch. Psia is absolute and psig is gauge, meaning above ambient. The two differ by exactly one atmosphere, so a 1.3 ATA chamber can be written as 19.1 psi absolute or 4.4 psi gauge, and both are correct. The 2.0 ATA vs 1.3 ATA head to head converts each figure exactly.

Gauge pressure and absolute pressure. Gauge pressure is measured relative to the atmosphere around you. Absolute pressure counts that atmosphere too. ATA and psia are absolute. Psig, kPa above ambient and feet of seawater are gauge figures.

Feet of seawater (fsw). A diving unit of depth used for pressure. NOAA puts one extra atmosphere at every 33 feet (10.06 meters) of seawater. So 1.3 ATA is about 9.9 feet of seawater and 2.0 ATA is 33 feet. Navy treatment tables give depth in fsw, not ATA.

Pressure differential. The difference between the pressure inside a vessel and the pressure outside it. It is the number that loads the shell. ASME PVHO-1 applies to vessels that hold a person at more than 2 psi of differential pressure, which the UHMS puts at about 1.14 ATA.

Test pressure and operating pressure. Operating pressure is what a chamber runs at in a session. Test pressure is what the shell was tested to. When a maker says a shell is tested to 150 kPa and limited to 50, that gap is the safety margin, and it is one of the numbers we score.

Chamber types

Hyperbaric chamber. Any chamber pressurized above sea level. Medical hyperbaric oxygen, under the UHMS definition, needs 2.0 ATA or more. Home chambers sit near 1.3 ATA for soft shells and 1.5 ATA for hard shells.

Monoplace. One person per session: a capsule you lie down in, sized like a small couch. NFPA 99 calls this Class B. Our monoplace vs multiplace guide compares capacity, price bands, power and space.

Multiplace. More than one person per session, seated together in a walk-in chamber. The hard-shell range we price seats 1 to 6. NFPA 99 calls this Class A. In hospital multiplace chambers the room holds air and each person breathes oxygen through a mask or hood.

Hard shell. A chamber with a rigid, pressure-tested hull, usually a cylinder, because a cylinder spreads load evenly. Hard shells run from 1.5 to 3.0 ATA. Our hard vs soft shell guide sets out the physics and the price gap.

Soft shell. A fabric chamber, closed by zippers and pressurized with room air by an electric compressor. Soft shells top out near 1.3 ATA. The FDA says the zippered bags it has cleared are for acute mountain sickness only.

Sitting and lying chambers. Most chambers are lying tubes. Seated chambers use less floor length and more height. Whether you sit or lie is a separate choice from seat count, and our sitting vs lying guide compares sizes, doors and access.

Mild hyperbaric. The UHMS calls exposures below 1.5 ATA mild, usually with under 95% oxygen by loose mask. Its position statement on fabric chambers uses a different number and describes them as operating below 1.4 ATA.

Medical hyperbaric oxygen (HBOT). The UHMS defines it as a medical procedure on a physician's prescription and oversight, with the whole body inside a hard-sided chamber built to ASME PVHO-1 and NFPA 99, at 2.0 ATA or more, breathing medical grade oxygen above 99%. Our mild vs medical guide compares the two line by line.

Decompression, recompression and decompression chamber. Decompression is coming back from higher pressure to surface pressure. Recompression is raising pressure again to treat decompression illness, and a recompression chamber delivers it. Decompression chamber is the everyday name for the same vessel. Our decompression vs hyperbaric guide sorts the names.

Hypobaric (altitude) chamber. A chamber that lowers pressure below 1.0 ATA to simulate altitude. It does the opposite of a hyperbaric chamber and is used in aviation training.

Isolation (float) tank. A shallow pool of water saturated with Epsom salt, kept near skin temperature in a dark, soundproof pod. It is a different machine from a hyperbaric chamber, as our isolation tank guide explains.

Standards and regulators

FDA cleared. The FDA reviewed a 510(k) premarket notification for that device and found it substantially equivalent to a device already legally on the US market, for a stated list of uses. A clearance covers a device and its listed indications, not every use a seller names.

FDA registered. The company that makes or distributes the device filed its annual establishment registration. The FDA states that registration does not denote approval, clearance or authorization. Our FDA cleared vs registered guide shows how to tell the two apart.

510(k) and K-number. A 510(k) is the premarket notification a maker files for a Class II device. The K-number identifies one clearance. You can type it into the FDA 510(k) database to see the device name, applicant, product code and decision date.

Class II and 21 CFR 868.5470. The FDA classifies hyperbaric chambers under 21 CFR 868.5470 as Class II devices, product code CBF. Mild and medical chambers share that classification. What differs is the clearance for each model.

Rx only (prescription device). Under 21 CFR 801.109 a prescription device carries "Rx only" and is to be sold only on a licensed practitioner's order. Every chamber clearance our prescription guide opened, seven of them, is marked Prescription Use.

ASME PVHO-1. The American Society of Mechanical Engineers' safety standard for pressure vessels for human occupancy. It covers design, fabrication, inspection, testing, marking and stamping of vessels run above 2 psi of differential pressure. It is an engineering standard, not an FDA clearance. Our ASME PVHO-1 guide covers who requires it.

ASME PVHO-2. The companion standard for operation and maintenance of PVHOs built to PVHO-1. It gives the criteria for judging whether an acrylic window is still serviceable. Our maintenance guide covers its inspection types.

NFPA 99. The National Fire Protection Association's Health Care Facilities Code. Its Chapter 14 covers fire, electrical and pressure hazards of hyperbaric chambers used at 0 to 100 psi gauge, so mild chambers are in scope. It becomes enforceable when a state or city adopts it through its fire code. Our NFPA 99 guide covers the 2023 vote on mild chambers.

Class A, B and C. The NFPA 99 chamber classes. Class A is human, multiple occupancy (multiplace). Class B is human, single occupancy (monoplace). Class C is for animals. A chamber built for one person, hard or soft, is Class B.

CE marking. The manufacturer's own declaration that a product meets the EU rules that apply to it. The European Commission says it does not mean an authority approved the product as safe. Medical devices fall under Regulation (EU) 2017/745. Our CE marking guide shows how to check a claim.

Notified body and NANDO. A notified body takes part in the conformity assessment for most medical devices above the lowest class. Its four-digit number sits next to the CE mark. NANDO is the Commission's database where you look that number up.

EUDAMED. The EU medical device database. Every whole hyperbaric chamber we found in it, 21 records on 9 October 2026, is registered as Class IIb.

UHMS. The Undersea and Hyperbaric Medical Society, a professional society, not a regulator. The terms mild and medical hyperbaric come from it. It also publishes the accreditation manual and credentialing guidelines our clinic guides cite.

Oxygen and breathing

BIBS (built-in breathing system). A set of masks or hoods plumbed into the chamber, so the person inside breathes a gas that differs from the chamber air. The US Navy lists it among the basic parts of a recompression chamber. Our BIBS and oxygen delivery guide covers each delivery method.

Overboard dump. A line that carries exhaled gas from the BIBS mask out of the chamber instead of into the chamber air. The Navy manual lists it as an optional part. Without one, exhaled oxygen adds to the oxygen around the occupant.

Oronasal mask, hood and head tent. Three ways to carry oxygen to the face in an air-filled chamber. An oronasal mask covers the nose and mouth. A hood is a clear cover over the head that seals at the neck. A head tent is the name one hospital uses for its own head enclosure.

Oxygen-filled vs air-filled chamber. A hospital monoplace is often filled with 100% oxygen, so the patient breathes the chamber gas. A multiplace holds air, with oxygen by mask, hood or head tent. A home soft chamber holds room air from its compressor.

Oxygen-enriched atmosphere and the 23.5% line. Air is 20.95% oxygen. A Des Moines fire department letter says NFPA 99 uses 23.5% to define an oxygen-enriched atmosphere, where items ignite more easily and burn hotter and faster. The UHMS accreditation manual, citing NFPA 99, calls for alarms in Class A chambers above 23.5%.

Oxygen concentrator. An electric machine that pulls in room air, strips out most of the nitrogen, and delivers oxygen at roughly 90% or more through tubing to a mask or cannula. In the US it is a prescription medical device. Our oxygen concentrator guide covers flow and purity.

Compressor. The chamber's own air pump. It pushes room air, about 21% oxygen, into the chamber to raise the pressure. It is a separate machine from a concentrator, and one cannot stand in for the other. The FDA says it has not cleared soft, zippered chambers for use with oxygen concentrators.

Medical grade oxygen. Oxygen above 99.0% purity that meets the United States Pharmacopeia standard. The UHMS calls it the only acceptable gas for medical hyperbaric oxygen.

Clinical and billing terms

Time at pressure and total session time. A hospital session runs about two hours, and the UHMS puts treatment at 90 to 120 minutes. That total includes compression, oxygen periods and decompression. Time at pressure is the part in between. Our session length guide breaks down each part.

Air break. A short period of breathing air between oxygen periods. Navy Treatment Table 9 holds 90 minutes of oxygen at 45 feet, split into three 30 minute periods with two 5 minute air breaks.

Treatment tables 6 and 9. US Navy schedules for chamber treatment. Table 6 starts at 60 feet of seawater, which Divers Alert Network equates with 2.8 ATA, and runs 4 hours 45 minutes, not counting descent. Depth in these tables is given in fsw.

NCD 20.29. The Medicare National Coverage Determination for hyperbaric oxygen. It limits payment to therapy in a chamber for 15 listed conditions and names 22 it does not cover. It does not name a pressure. Our insurance and CPT codes guide covers the detail.

CPT 99183 and HCPCS G0277. CPT 99183 is physician attendance and supervision of hyperbaric oxygen therapy, per session. HCPCS G0277 is hyperbaric oxygen under pressure, full body chamber, per 30 minute interval. The CMS manual says G0277 is for hospital outpatient departments only.

CHT and CHRN. Certified Hyperbaric Technologist and Certified Hyperbaric Registered Nurse. Both come from the National Board of Diving and Hyperbaric Medical Technology, a private board, not a state. The CHT is renewed every 2 years and the CHRN every 4. Our clinic staffing guide covers the 40-hour course and the 480 clinical hours.

Safety director. The person a hyperbaric facility designates in writing, as the UHMS accreditation manual asks, citing NFPA 99. The manual gives the safety director written authority to remove any potentially hazardous supply or equipment from the chamber.

Immediately available. The UHMS credentialing guidelines say the attending provider must be immediately available for every treatment, meaning in the same or a connected building and able to reach the chamber as soon as asked.

Buying and ownership terms

Installed cost. The list price plus what it takes to get a chamber running in your room. Hard-shell home capsules land 10 to 20% above list once installed. Our cost guide walks through the line items.

White glove delivery. A paid service that moves the chamber into the room. On a standard freight delivery the crate stops at the curb, and moving it inside is the buyer's job. Our delivery guide covers freight claims too.

Full and limited warranty. Under the Magnuson-Moss Warranty Act a written warranty must be titled full or limited. A full warranty meets five federal minimum standards. One that misses any of them is limited. Our warranty guide lists the five.

Service contract. A paid add-on plan. Under 16 CFR 700.11 an agreement is a service contract, not a written warranty, if it costs something on top of the purchase price or is made after the purchase.

Fair market value lease and $1 buyout lease. Under UCC 1-203, a lease you cannot cancel that lets you own the chamber for a nominal payment, such as $1, works like a sale on credit. A fair market value lease leaves the chamber with the lessor. Our financing and leasing guide compares them.

Section 179. A tax election a business can make when it buys a chamber and uses it more than 50% for business, under the rules in IRS Publication 946. Rev. Proc. 2025-32 sets the 2026 cap at $2,560,000. Ask a tax adviser before you rely on it.

Design life (acrylic windows). Acrylic windows have a 10-year design life that runs from the date of fabrication. A Perry Baromedical advisory puts it at 10 years or 10,000 cycles, whichever comes first. After that, PVHO-2 sets a stricter inspection regime.

Crazing. A defect that acrylic window inspection forms check for, along with cracks, discoloration and blisters. Perry Baromedical says cleaners with high concentrations of alcohol are known to start it.

Remarketer and remanufacturer. The FDA chart marks remarketers of used devices as not required to register. A remanufacturer must register. Our selling guide and used chamber checklist cover the paperwork on each side of a sale.

Break-even. The number of paid clinic sessions a home chamber has to replace to cost the same. For a capsule near $64,000 installed it sits at roughly 430 to 850 sessions. Our worth it guide shows the arithmetic.

Every term at a glance

The table repeats each term in one line and names the guide that explains it in full. Use it to find the right page fast.

Definitions drawn from the linked Wellness Vaults guides, which cite the regulator, standards body or maker for each fact. Read 22 October 2026.

Terms that get mixed up

A few pairs cause most of the confusion we see. Cleared and registered sound alike but mean different things: one is a review, the other is a filing. Cleared and approved differ too, because approval is the word the FDA uses for the PMA pathway for Class III devices.

Absolute and gauge pressure trip up spec sheet comparisons. A 1.3 ATA chamber is 4.4 psi gauge and 19.1 psi absolute. A seller who quotes one figure and a rival who quotes the other are describing the same pressure.

The concentrator and the compressor are two machines. The compressor raises the pressure with room air. The concentrator makes oxygen for a mask or cannula. The FDA has not cleared soft chambers for use with a concentrator, even though the two are sold bundled.

PVHO-1 and FDA clearance answer different questions. PVHO-1 is about how the vessel is built and tested. A clearance is about the device and the uses the FDA reviewed. Ask for both separately. CE marking is a third, separate system for the EU, and one does not imply the others.

Mild has two numbers. The UHMS indications page draws the line at 1.5 ATA. Its fabric chamber position statement uses 1.4 ATA. UnitedHealthcare defines hyperbaric oxygen therapy as at least 1.4 ATA with near 100% oxygen. Ask which definition a seller is using.

What this glossary does not cover

This page defines terms. It does not say whether hyperbaric oxygen helps any condition, and it does not rate brands. For what the FDA has cleared chambers to treat, read our benefits guide. For scored chambers, see the best hyperbaric chambers page.

Some terms change by place. Fire code adoption, prescription practice and lending rules differ by state and country. Where a definition depends on local adoption, such as NFPA 99, ask your fire marshal or regulator which edition applies.