Gas Switch Checker
One deco gas switch, three rules. Burton's rule of fifths on the fractions, the BSAC 0.5 bar nitrogen step at your switch depth, and Doolette's timing and oxygen checks. They disagree on the most common switch in the sport, and the verdict tells you which one to trust and what to change.
Inputs
O₂ He N₂
Current gas (before the switch)
—
Usually the bottom mix, e.g. Tx 18/45. N₂ is the remainder.
Switch gas (after the switch)
—
The deco gas you switch onto, e.g. EAN50 = 50 / 0, or 35/25.
Oxygen ceilings used here: 1.4 bar for any mix up to 80% O₂, 1.6 bar for pure oxygen. Inferred from the NOAA exposure data, not a NOAA rule.
Verdict
1 · Rule of fifths
Burton 2004. Fractions only, blind to depth. Field count, no trial.
He drops — pts, so N₂ may rise — pts. It rises — pts.
2 · 0.5 bar nitrogen step
BSAC; MultiDeco and V-Planner default. Partial pressure at your switch depth. Convention, no trial.
At — m, PN₂ goes — → — bar, a step of — bar.
3 · Timing and oxygen
Doolette & Mitchell. Switch shallow, breathe the highest safe O₂ after, decompress the ear first. Model plus one human trial at 30 m.
- —
- —
- Spend the previous stop, or at least 3 minutes, on the current gas at this depth before switching. The inner ear's half-time is about 8.8 min, so 3 minutes sheds about a fifth of its excess.
Burton's dissolved-gas load, before and after
Pressure × lipid solubility (He 0.015, N₂ 0.067) at — m: — → —. This is the quantity the rule of fifths protects. Bubbles form on tension, not quantity, which is the rule's known weakness.
Sources: Burton, scubaengineer.com 2004; BSAC recommended gas mixtures; Doolette & Mitchell, J Appl Physiol 2003 and Diving Hyperb Med 2013, 2015; Doolette & Gerth, NEDU TR 12-04. The full walk-through is in Gas Switching and Isobaric Counterdiffusion.
Common questions
What is isobaric counterdiffusion (ICD)?
ICD is what happens when you change the inert gas you breathe without changing depth, classically switching from helium-rich trimix to nitrogen-rich deco gas at a stop. Helium leaves the tissues while nitrogen enters. In one tissue, the inner ear, the total can briefly rise even though ambient pressure never moved. The characteristic injury is inner-ear DCS: vertigo and vomiting a few minutes after the switch, at a constant depth.
Why three rules?
Because they measure different things. Burton's rule of fifths reads the cylinder fractions and is blind to depth. The BSAC 0.5 bar rule reads the nitrogen step in bar at the switch depth. Doolette's rule reads the state of the inner ear: decompress it before the switch, switch shallow, breathe the highest safe oxygen after. They disagree on the most common switch in the sport, Tx 18/45 to EAN50, and this tool shows you all three so you can see why.
Which rule has evidence behind it?
Only the timing rule. It comes from a model of the inner ear and from a US Navy trial that switched heliox to air at 30 m after an hour at depth and found low risk. The rule of fifths rests on Burton's count of his own dives. The 0.5 bar figure is a BSAC convention and a planner default with no trial behind it. Mitchell and Doolette's 2015 position is that typical switches would rarely, if ever, cause inner-ear bubbles on their own; an under-decompressed ear and a PFO do most of the damage.
Is ICD caused by helium leaving tissues faster than nitrogen enters?
Helium is the faster gas: it diffuses about 2.65 times faster than nitrogen. In the inner ear that speed is exactly the problem, because helium in the fluid chambers diffuses back into the blood-supplied part faster than nitrogen leaves it. At the same time, nitrogen is about 4.5 times more soluble, so arriving blood delivers more nitrogen than departing blood removes helium. Both routes add gas to the same small tissue at once.
What do I change if the switch fails the 0.5 bar check?
Put a helium-bearing deco gas in between, such as 35/25 at 30 m, so no single step is over half a bar. Or keep helium in the shallow gas, such as 50/25 instead of EAN50. The tool computes the minimum helium for your oxygen choice and the two-step alternative.
What are the oxygen ceilings in this tool?
PO₂ 1.4 bar for any mix up to 80% oxygen and 1.6 bar for pure oxygen only. They are inferred from the NOAA exposure data, not a NOAA rule, and they put EAN50 at 18 m and 35/25 at 30 m.
Three heuristics, not a guarantee. None of these rules has been shown to prevent inner-ear DCS. The best evidence says the profile before the switch and a right-to-left shunt do most of the damage, and the switch adds a small transient. A switch that passes here is not thereby safe. Use it inside proper decompression training and cross-check your planning software.