Pressure Units in Hyperbaric Medicine
Hyperbaric medicine inherits its pressure vocabulary from diving physics, which creates a mix of five related units that frequently confuses readers of the clinical literature. Understanding each unit and the relationships between them is foundational to interpreting HBOT trial reports, device specifications, and treatment protocols accurately.
ATA — Atmospheres Absolute
The US clinical standard. ATA measures total pressure relative to a true vacuum. 1 ATA equals normal atmospheric pressure at sea level (14.7 psi, 760 mmHg). Most UHMS protocols are expressed in ATA — 2.0 ATA for neurological HBOT, 2.4 ATA for wound care, 2.8–3.0 ATA for emergency indications.
ATG — Atmospheres Gauge
Pressure above atmospheric. Related to ATA by: ATA = ATG + 1. So 2.0 ATA = 1.0 ATG. Less commonly used in clinical HBOT literature but appears in some engineering specifications and older military documents.
psi — Pounds per Square Inch
The US engineering unit. Can be absolute (relative to vacuum) or gauge (relative to atmospheric). 1 ATA = 14.7 psi absolute = 0 psi gauge. 2.0 ATA = 29.4 psi absolute = 14.7 psi gauge. Device specifications and chamber safety ratings typically use psi.
mmHg — Millimetres of Mercury
Medical physiology standard, familiar from blood pressure measurement. 1 ATA = 760 mmHg. Arterial oxygen tension (PaO₂) is typically reported in mmHg — normal breathing at sea level produces PaO₂ ≈ 100 mmHg; HBOT at 2.0 ATA with 100% O₂ exceeds 1,000 mmHg.
fsw — Feet of Seawater
The diving-medicine unit, inherited from US Navy practice. 1 ATA (sea level) corresponds to 0 fsw; each additional 33 fsw adds approximately 1 ATA. Treatment Table 6 at 2.8 ATA is equivalent to 60 fsw. Dive-training and decompression literature uses fsw predominantly.
Conversion Formulas
- ATA ↔ ATG: ATA = ATG + 1
- ATA ↔ psi absolute: psi = ATA × 14.6959
- ATA ↔ psi gauge: psi_g = (ATA − 1) × 14.6959
- ATA ↔ mmHg: mmHg = ATA × 760
- ATA ↔ fsw: fsw = (ATA − 1) × 33
Why This Matters Clinically
A trial reporting treatment at "2 atmospheres" could mean 2.0 ATA (effectively 1 ATA above sea level) or 2.0 ATG (effectively 3.0 ATA absolute). Device specifications in psi often omit whether the number is absolute or gauge. Dive-medicine literature expressed in fsw requires conversion to ATA for direct comparison with clinical HBOT protocols. Confusion here can lead to significant misinterpretation of trial pressure and dose.
When reading HBOT literature, always confirm the unit basis. When writing or specifying protocols, always include the unit (ATA, ATG, psi absolute, psi gauge, mmHg, fsw) explicitly. This converter helps you cross-check conversions quickly.
Research disclaimer: This tool is a general reference calculator. It is not a substitute for facility-specific protocols or physician-supervised calculation of individual treatment pressures. All calculations are approximations based on standard conversion factors.