Chamber Pressure in ATA and What the Console Shows in PSI
Hyperbaric work is written in atmospheres absolute. A treatment sheet says 2.4 ATA, a chamber's rating plate says 3 ATA, an air-break schedule is keyed to depth — yet the pressure gauge in front of the operator is very often a psi dial, and psi hardware from a US builder is quoted in pounds per square inch too. Moving between the two is a daily task, and the trap is that ATA and a gauge dial do not start counting from the same place.
What the Numbers on a Treatment Sheet Mean
ATA counts from vacuum
The dial counts from the room
Depth language kept on dry land
Pressure is only one part of the dose
Checking a Prescribed Depth Against the Gauge
Both boxes accept input and update each other, which suits the two directions this job actually runs in.
Put the ATA figure in the left box
Type 1.3, 2.0, 2.4 or 2.8 — whatever the schedule specifies. The psi value appears as you type, and a comma decimal such as 2,4 is read the same as 2.4.
Subtract one atmosphere to get the dial reading
The result is absolute pressure. Take 14.696 off it and you have the psig figure the console should show at depth: 2.0 ATA gives 29.39 psia and 14.70 psig.
Reverse it for an imported rating plate
Press ↔ to work psi → atm when a chamber, a regulator or a relief valve is stamped in psi. The hand calculation is a division by 14.696, or a multiplication by 0.068 045 964.
Copy the bare figure into the log
The copy button on each field puts the number alone on the clipboard, without the unit or the thousands spacing, so it drops straight into a treatment record or a spreadsheet. Ctrl + C in a field behaves identically.
Protocol Pressures in ATA, PSIA, PSIG and Feet of Seawater
The pressures that appear on chamber schedules, shown as absolute pressure, as the gauge reading at depth, and as the diving-derived depth equivalent using the clinical rounding of one atmosphere to 33 fsw or 10 msw.
| Chamber pressure | Absolute (psia) | Console reading (psig) | Depth equivalent (fsw) | Depth equivalent (msw) |
|---|---|---|---|---|
| 1.0 ATA — door open, surface | 14.70 psia | 0.00 psig | 0 fsw | 0 msw |
| 1.3 ATA — soft-shell chambers | 19.10 psia | 4.41 psig | 9.9 fsw | 3.0 msw |
| 1.5 ATA | 22.04 psia | 7.35 psig | 16.5 fsw | 5.0 msw |
| 1.75 ATA | 25.72 psia | 11.02 psig | 24.8 fsw | 7.5 msw |
| 2.0 ATA | 29.39 psia | 14.70 psig | 33 fsw | 10 msw |
| 2.4 ATA — common clinical depth | 35.27 psia | 20.57 psig | 46.2 fsw | 14 msw |
| 2.8 ATA | 41.15 psia | 26.45 psig | 59.4 fsw | 18 msw |
| 3.0 ATA | 44.09 psia | 29.39 psig | 66 fsw | 20 msw |
Read down the psig column and the pattern is easy to hold in your head: every extra atmosphere adds a shade under 14.7 psi to the dial and another 33 feet to the notional depth.
How the Converter Fits Chamber Work
Both scales move together
Typing in either box updates the other immediately, so a whole schedule — surface, treatment depth, air-break depth — can be stepped through in one sitting.
Two-way for imported hardware
Chambers, valves and fittings arrive stamped in psi from one supplier and in atmospheres from another; the ↔ button covers whichever way the paperwork runs.
Bar and kPa for the plant room
Searchable dropdowns on both sides list every pressure unit in the app, so a compressor or air-bank figure in bar can be brought into the same conversation.
Clean figures for the record
Results carry up to eight decimals, and the copy button strips the unit so nothing but digits lands in a log sheet or a maintenance note.
Questions Asked at the Chamber Console
Why does the gauge read about 20.6 psi when the schedule calls for 2.4 ATA?
Because the two scales have different zeros. ATA is absolute: 2.4 ATA is 35.27 psia, counted from a perfect vacuum. A conventional dial reads gauge pressure, which is zero in room air, so it shows only the 1.4 atmospheres that were added — 20.57 psig. The relationship is simply psia = psig + 14.696. An absolute-reading gauge, if the chamber has one, will show the 35.27 figure instead.
Where does the 2.4 ATA treatment depth come from?
It descends from established diving and hyperbaric tables that settled on depths around 45 feet of seawater, and 2.4 ATA is the pressure at that depth. Published clinical protocols cluster at 2.0, 2.4 and 2.8 ATA for that historical reason, with 1.3 to 1.5 ATA used by low-pressure soft-shell systems. Which pressure applies in any given case is a clinical decision, not a conversion one.
What does a chamber "depth" in feet of seawater mean when there is no water?
It is a pressure expressed as the depth that would produce it. Seawater adds roughly one atmosphere for every 33 feet, so 33 fsw means one atmosphere of added pressure and 66 fsw means two. Because it measures what was added, fsw is a gauge scale: 0 fsw is the surface, and 46.2 fsw corresponds to 2.4 ATA absolute. Metric services use msw on the same basis, with 10 msw standing in for one atmosphere.
Do monoplace and multiplace chambers reach the same pressures?
The pressures overlap, but they are produced differently. A monoplace chamber is usually filled with oxygen and the occupant breathes the chamber atmosphere directly. A multiplace chamber is compressed with air while occupants breathe oxygen through hoods or masks, which is why attendants can work inside. Both are typically rated to 3 ATA (44.09 psia, 29.39 psig) or a little above, and each carries its own certified working pressure on the rating plate.
Why do ears need clearing while the chamber is compressing but not once it holds?
Discomfort follows the rate of change, not the level. While pressure is rising, the air outside the eardrum gains on the air behind it until the Eustachian tube lets the middle ear catch up; once the chamber steadies at 2.4 ATA the two sides are equal and nothing is pushing. The largest proportional change also happens in the first few feet — going from 1.0 to 1.3 ATA is a 30 % jump, whereas 2.4 to 2.8 ATA is under 17 % — which is why compression is usually slowest at the start.
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