Gas switch calculator

Calculate the partial pressure differences between two gases for different ambient pressures

From Gas

O2:
HE
N2: 40

To Gas

O2:
HE
N2: 50

Depth
[m]		ppO2
[bar]		ppN2
[bar]		ppHe
[bar]		Gas
O2/He		Density
[kg/m³]		IANTD 5% rule
6from
to
diff		1.30
0.80
0.50		0.14
0.80
0.66		0.16
0.00
0.16		81/10
50/0
 		2.06
2.14
 		10% He drop allow max
2% N2 increase
41% N2 increase
10from
to
diff		1.30
1.00
0.30		0.33
1.00
0.67		0.37
0.00
0.37		65/19
50/0
 		2.34
2.68
 		19% He drop allow max
3.8% N2 increase
34% N2 increase
20from
to
diff		1.30
1.50
0.20		0.80
1.50
0.70		0.90
0.00
0.90		43/30
50/0
 		3.02
4.02
 		30% He drop allow max
6% N2 increase
23% N2 increase
30from
to
diff		1.30
2.00
0.70		1.27
2.00
0.73		1.43
0.00
1.43		33/36
50/0
 		3.7
5.36
 		36% He drop allow max
7.2% N2 increase
18% N2 increase
40from
to
diff		1.30
2.50
1.20		1.74
2.50
0.76		1.96
0.00
1.96		26/39
50/0
 		4.38
6.7
 		39% He drop allow max
7.8% N2 increase
15% N2 increase
50from
to
diff		1.30
3.00
1.70		2.21
3.00
0.79		2.49
0.00
2.49		22/42
50/0
 		5.07
8.04
 		42% He drop allow max
8.4% N2 increase
13% N2 increase
60from
to
diff		1.30
3.50
2.20		2.68
3.50
0.82		3.02
0.00
3.02		19/43
50/0
 		5.75
9.38
 		43% He drop allow max
8.6% N2 increase
12% N2 increase
70from
to
diff		1.30
4.00
2.70		3.15
4.00
0.85		3.55
0.00
3.55		16/44
50/0
 		6.43
10.72
 		44% He drop allow max
8.8% N2 increase
11% N2 increase
80from
to
diff		1.30
4.50
3.20		3.62
4.50
0.88		4.08
0.00
4.08		14/45
50/0
 		7.12
12.06
 		45% He drop allow max
9% N2 increase
10% N2 increase
90from
to
diff		1.30
5.00
3.70		4.09
5.00
0.91		4.61
0.00
4.61		13/46
50/0
 		7.8
13.4
 		46% He drop allow max
9.2% N2 increase
9% N2 increase
100from
to
diff		1.30
5.50
4.20		4.56
5.50
0.94		5.14
0.00
5.14		12/47
50/0
 		8.48
14.73
 		47% He drop allow max
9.4% N2 increase
9% N2 increase

Color visualization in O2 col indicates ppO2 limits and in N2 and He cols indicates partial pressure differences
Green: Partial pressure difference is below 0.5
Orange: Partial pressure difference is between 0.5 and 1.0
Red: Partial pressure difference is above 1.0

IANTD 5% rule aka IANTD rule of fifth

The 5:1 ratio indicates that a diver should not switch to a gas that has a helium drop of 5% (actual percent by volume) for every 1%(actual percent by volume) increase in nitrogen content.
[Tom Mount, Joseph Dituri: Exploration and Mixed Gas Diving Encyclopedia (The Tao of Survival Underwater)]

This can be rephrased into:
"Limit your rise in N2 to one fifth of the drop in Helium as you ascend."

This calculations might be incorrect and / or contain bugs. Taking this calculations for real might kill you.
Do not rely on this tool for your dive planning.
Do your own calculations.

You are most welcome to check the source code on GitHub and report bugs if you find some.

Copyright Cornelius Schmale 2025