Hard shell vs soft shell

Two products share the name "hyperbaric chamber" and they are not variants of one another. One is a fabric envelope, typically urethane-coated nylon over a frame, inflated with air by small electric compressors and holding a few pounds per square inch above ambient. The other is a rigid pressure vessel — steel with acrylic viewports, or an acrylic cylinder in a steel cradle — designed, stamped and inspected as a pressure vessel for human occupancy. Almost every downstream question in this guide has a different answer depending on which one you buy.

The line that separates them is a code threshold, not a marketing tier

ASME PVHO-1, the Safety Standard for Pressure Vessels for Human Occupancy, is one of the two consensus standards FDA recognises for this product code (FDA Product Classification, CBF; FDA recognition record for ASME PVHO-1-2023). The Undersea and Hyperbaric Medical Society summarises its scope precisely: "The ASME-PVHO-1 standard applies to any pressure vessel that encloses a human within the pressure boundary, with an external or internal pressure exceeding 2 psig above ambient pressure (1.14 atmospheres absolute)" (UHMS Position Statement, Low-Pressure Fabric Hyperbaric Chambers, created 22 January 2017, revised 10 July 2018).

That threshold is why 1.3 ATA fabric chambers exist as a product category at all. A common fabric chamber operating pressure is 4.4 psi — for example the Summit to Sea Grand Dive, published as "4.4 PSI (1.3 ATA)" (retailer specification sheet). That is above 2 psig, so it is not outside the PVHO conversation, but it sits in a very different engineering and regulatory world from a vessel rated to 30.0 psig. A clinical monoplace chamber such as the Sechrist 3600H is published at a maximum operating pressure of "30.0 pounds per square inch gauge (psig); 3.0 atmospheres absolute (ATA)," with two relief valves set at 36.00 psi (Sechrist monoplace H-series specifications, manufacturer brochure). The pressure difference between the two products is roughly sevenfold in gauge terms.

What each is cleared for, which is narrower than most buyers expect

Fabric and semi-rigid low-pressure chambers do hold FDA clearances, and those clearances are for one indication. Summit to Sea's Shallow Dive, Dive and Grand Dive — described in the clearance as soft, urethane-coated nylon bags inflated with atmospheric air by a small electric pump, operating at 3–4 psi — were cleared "to treat acute mountain sickness (AMS), as prescribed by or under the direction of a physician" (K072757, cleared 8 February 2008). The Revitalair 430+, a semi-rigid polyester-fabric chamber with aluminium rings at no greater than 1.5 ATA, was cleared "to treat acute mountain sickness under the prescription of a health professional" and "designed for use at physician offices and health institutions" (K220290, cleared 4 May 2023).

FDA has also addressed the oxygen question directly, and this is the single most commonly ignored sentence in the category: "The FDA has also cleared a large, zippered bag that is intended to treat altitude sickness only. These zippered chambers for treating altitude sickness provide pressure but do not attach to oxygen tanks. The FDA has not cleared these bags for use with oxygen tanks or oxygen concentrators. However, the FDA is aware of instances in which people used these bags to create homemade HBOT devices, which can pose the risk of fire and suffocation" (FDA consumer update, archived; content current 26 July 2021).

The evidence framing is different too

UHMS states its position plainly: "Hyperbaric oxygen therapy is defined as an intervention in which an individual breathes near 100% oxygen while wholly enclosed inside a hyperbaric chamber at a pressure equal to or greater than 1.4 ATA," that "ALL UHMS currently approved indications require that patients breathe near 100% oxygen while enclosed in a chamber pressurized to a minimum of 2 ATA," and that "Mild hyperbaric exposures with air deliver no more oxygen to the body than breathing oxygen by mask at sea-level pressure" (UHMS Position Statement). Published UHMS-affiliated authors define "'mild' hyperbaric oxygen therapy … as oxygen administration at pressures lower than 1.5 ATA" and write that "there is presently limited evidence for efficacy of 100% oxygen breathing at lower pressures" (Huang, Bennett and Moon, 2024).

We do not restate that as a claim about outcomes. We restate it as a claim about applicability: a 1.3 ATA fabric chamber pressurised with air is not reproducing the exposure conditions used in the indication-level literature, so studies conducted at 2.0–2.5 ATA on 100% oxygen do not describe it. What the published research actually used works through that in detail, study by study.

Published specifications that actually differ

Note what the table does not do. It does not fill the blanks. Sechrist's brochure genuinely does not publish an electrical specification, so the cell says so; asking the manufacturer or the installer is the correct next step, and guessing 240 V because it sounds right is how installations get wired wrong. Side-by-side specifications across many more models, in filterable form, are on our companion site comparehyperbaric.com.

The consequences you are actually choosing between

Fabric. Light enough that two people can move it, so freight is ordinary parcel or light LTL and there is no floor-loading question worth raising in a normal building. Runs on a household circuit in the published examples. Sets up and strikes down. Cleared for acute mountain sickness, prescription only, and not cleared for use with an oxygen concentrator. Cannot reach the pressures used in the indication-level literature. Seals, zips and the fabric envelope itself are the wearing parts.

Rigid. A pressure vessel with a stamped design, an inspection history, and acrylic viewports that carry a documented design life and service-life rules under ASME PVHO-2 (ASME PVHO-2-2019, publicly available preview pages). A published 2,220 lb dry weight means a rigging plan, a floor check and a delivery method, not a doorway and two friends. It can reach clinical pressures, which is also why its oxygen handling, ventilation and fire precautions are a different order of problem — see ventilation and safety.

Which is the better buy?

Honestly: it depends on a question that is not about the equipment. If you want a low-pressure air environment, at home, with the least installation friction, a fabric chamber does that and a rigid vessel is an expensive way to do it badly. If your interest is in reproducing the exposures used in the published hyperbaric literature, a fabric chamber cannot do it at any price, and buying one on that expectation is the most common expensive mistake in this market. There is no version of this where a fabric chamber becomes a clinical chamber by adding oxygen to it — that is the specific configuration FDA has described as creating a homemade device.

If the honest answer for your situation is "neither, yet," that is a legitimate outcome of reading this page. The next page deals with the setting question, which is where several buyers discover the real constraint.