Decompression tables appendix

The decompression tables in Appendix A are computed for working chamber pressures from 0 to 14 psig and from 14 to 50 psig (inclusive) in 2‑pound increments, and for exposure times from one‑half hour up to "over 8 hours." Decompression Tables in 1926 Subpart S App A.

• This is stated in the appendix explanation and defines the grid of pressures and exposure times that the tables cover.
• If your exact pressure or exposure time doesn't match a table entry, follow the table guidance about using the next higher value rather than interpolating (see related Q&A).

The appendix requires decompressions to be conducted by two or more stages, with a maximum of four stages (the four‑stage limit applies to working chamber pressures of 40 psig or over). Decompression Tables in 1926 Subpart S App A.

• Use Table No. 2 to determine exactly how many stages are required for a given pressure and exposure time.

Stage 1 must reduce ambient pressure by an amount ranging from 10 to a maximum of 16 psig, and the pressure at the end of stage 1 must not be less than 4 psig. The reduction in Stage 1 must occur at a rate not greater than 5 pounds per minute. Decompression Tables in 1926 Subpart S App A.

• In practice this means Stage 1 is a comparatively fast reduction, but it cannot drop pressure below 4 psig and cannot exceed 5 psig per minute.

Stage 2 and any subsequent stages must reduce pressure at a slower rate and in no event at a rate greater than 1 pound per minute. Decompression Tables in 1926 Subpart S App A.

• The tables give the required reduction and the time for each stage; the rule limits the maximum rate to 1 psig/min for these later stages.

Use Decompression Table No. 1 to find the total decompression time (in minutes) for a given working chamber pressure and exposure time, and use Decompression Table No. 2 to get the detailed stage breakdown (number of stages, pressure reductions, terminal pressures, time per stage, and rate). Decompression Tables in 1926 Subpart S App A.

• Table No. 1 = total minutes for the entire decompression.
• Table No. 2 = per‑stage instructions (from/to pressures, minutes for that stage, and minutes per pound rate).

Decompression Table No. 2 provides: (a) the number of stages required; (b) the pressure reduction and terminal pressure for each required stage; (c) the time in minutes during which the pressure reduction is accomplished for each required stage; and (d) the pressure reduction rate in minutes per pound for each required stage. Decompression Tables in 1926 Subpart S App A.

• Use Table No. 2 when you need the step‑by‑step decomposition plan, not just the total time.

No — do not interpolate between table values. If the actual working chamber pressure or exposure time falls between values shown, you must use the next higher table value for pressure or exposure time. Decompression Tables in 1926 Subpart S App A.

• This "use the next higher value" rule ensures a more conservative decompression schedule.

It means that during any given stage the pressure must be reduced at a constant rate for that stage (the tables give that uniform rate as minutes per pound). Decompression Tables in 1926 Subpart S App A.

• Practically, you set a steady depressurization rate for the stage (not a variable or stepped rate) equal to the minutes/pound value shown for that stage in Table No. 2.

The total decompression time for a 20 psig working chamber and a 4‑hour exposure is 43 minutes, as shown in Decompression Table No. 1 and illustrated by Example No. 1 in the appendix. Decompression Tables in 1926 Subpart S App A.

• Example No. 1 in the appendix shows Stage 1 reduces 20 psig to 4 psig at 5 psig/min (3.2 minutes rounded per the table example to 3 minutes listed) and Stage 2 reduces 4 psig to 0 at a slow rate for the balance of the 43 minutes.

Example No. 2 in the appendix shows that for 24 psig and a 5‑hour exposure, the procedure includes reducing pressure in stages to 4 psig and then transferring men to a special decompression chamber while maintaining the 4‑psig pressure during transfer. The total decompression time in Table No. 1 for 24 psig/5 hours is 117 minutes. Decompression Tables in 1926 Subpart S App A.

• The example emphasizes maintaining the indicated pressure (4 psig) during transfer operations when instructed by the table.

The pressure at the end of Stage 1 must not be less than 4 psig. Decompression Tables in 1926 Subpart S App A.

• Stage 1 reductions are specified to leave the chamber at or above 4 psig; subsequent stages then reduce further at the slower rates defined in the tables.

To get the elapsed minutes for a stage, multiply the minutes‑per‑pound rate by the number of pounds of pressure reduced in that stage; the table also often lists the elapsed minutes directly. Decompression Tables in 1926 Subpart S App A.

• Example: if a stage reduces from 8 psig to 4 psig (4 pounds) at 1.00 minute per pound, the stage takes 4 × 1.00 = 4 minutes.
• The tables frequently display both the rate and the elapsed minutes so you can verify the math.

When the actual working chamber pressure falls between the tabulated 2‑pound increments, use the next higher working chamber pressure shown on the tables (do not interpolate). Decompression Tables in 1926 Subpart S App A.

• For example, if your working pressure is 21 psig, use the 22‑psig row in the tables to determine decompression times and stages.

Total decompression time is presented in Decompression Table No. 1 as minutes (an integer number of minutes) for each combination of working chamber pressure and exposure time. Decompression Tables in 1926 Subpart S App A.

• The tables show the total minutes; use Table No. 2 to break that total into stage times and verify the per‑stage rates.

The appendix examples show stage times presented as whole minutes consistent with the table entries; when computing stage times from rates you should follow the table values rather than attempt finer interpolation. Decompression Tables in 1926 Subpart S App A.

• The appendix warns not to interpolate and to use the next higher table values; the examples follow that approach when listing stage elapsed minutes.

The appendix includes entries for the exposure category "Over 8" hours; when the actual exposure exceeds 8 hours use the table row/column entries labeled "Over 8" and follow the listed total times and stage breakdowns. Decompression Tables in 1926 Subpart S App A.

• As with other values, if your exposure is between a numbered entry and the "Over 8" category you must use the next higher category per the "no interpolation" rule.

For 22 psig and a 3‑hour exposure, Decompression Table No. 1 lists a total decompression time of 38 minutes. Decompression Tables in 1926 Subpart S App A.

• If you need the stage breakdown for that cell, consult Decompression Table No. 2 for the exact number of stages, pressure reductions, and per‑stage times associated with 22 psig and a 3‑hour exposure.

No — the appendix states that the pressure reduction in each stage is accomplished at a uniform rate, so a stage must be performed at a constant (uniform) rate for its duration. Decompression Tables in 1926 Subpart S App A.

• For compliance, set and maintain the stage’s minutes‑per‑pound rate as given in Table No. 2 for the entire stage period.