diff --git a/core/CMakeLists.txt b/core/CMakeLists.txt
index 5121222cc..3a0806454 100644
--- a/core/CMakeLists.txt
+++ b/core/CMakeLists.txt
@@ -86,6 +86,8 @@ set(SUBSURFACE_CORE_LIB_SRCS
 	file.h
 	format.cpp
 	format.h
+	gas.c
+	gas.h
 	gas-model.c
 	gaspressures.c
 	gaspressures.h
diff --git a/core/dive.c b/core/dive.c
index 2d82f5565..a54c3aa00 100644
--- a/core/dive.c
+++ b/core/dive.c
@@ -866,13 +866,6 @@ static void update_min_max_temperatures(struct dive *dive, temperature_t tempera
 	}
 }
 
-int gas_volume(const cylinder_t *cyl, pressure_t p)
-{
-	double bar = p.mbar / 1000.0;
-	double z_factor = gas_compressibility_factor(cyl->gasmix, bar);
-	return lrint(cyl->type.size.mliter * bar_to_atm(bar) / z_factor);
-}
-
 /*
  * If the cylinder tank pressures are within half a bar
  * (about 8 PSI) of the sample pressures, we consider it
@@ -948,31 +941,6 @@ void update_setpoint_events(const struct dive *dive, struct divecomputer *dc)
 	}
 }
 
-void sanitize_gasmix(struct gasmix *mix)
-{
-	unsigned int o2, he;
-
-	o2 = mix->o2.permille;
-	he = mix->he.permille;
-
-	/* Regular air: leave empty */
-	if (!he) {
-		if (!o2)
-			return;
-		/* 20.8% to 21% O2 is just air */
-		if (gasmix_is_air(*mix)) {
-			mix->o2.permille = 0;
-			return;
-		}
-	}
-
-	/* Sane mix? */
-	if (o2 <= 1000 && he <= 1000 && o2 + he <= 1000)
-		return;
-	fprintf(stderr, "Odd gasmix: %u O2 %u He\n", o2, he);
-	memset(mix, 0, sizeof(*mix));
-}
-
 /*
  * See if the size/workingpressure looks like some standard cylinder
  * size, eg "AL80".
@@ -1059,23 +1027,6 @@ static void sanitize_cylinder_info(struct dive *dive)
 	}
 }
 
-/* Perform isobaric counterdiffusion calculations for gas changes in trimix dives.
- * Here we use the rule-of-fifths where, during a change involving trimix gas, the increase in nitrogen
- * should not exceed one fifth of the decrease in helium.
- * Parameters: 1) pointers to two gas mixes, the gas being switched from and the gas being switched to.
- *             2) a pointer to an icd_data structure.
- * Output:     i) The icd_data stucture is filled with the delta_N2 and delta_He numbers (as permille).
- *            ii) Function returns a boolean indicating an exceeding of the rule-of-fifths. False = no icd problem.
- */
-bool isobaric_counterdiffusion(struct gasmix oldgasmix, struct gasmix newgasmix, struct icd_data *results)
-{
-	if (!prefs.show_icd)
-		return false;
-	results->dN2 = get_he(oldgasmix) + get_o2(oldgasmix) - get_he(newgasmix) - get_o2(newgasmix);
-	results->dHe = get_he(newgasmix) - get_he(oldgasmix);
-	return get_he(oldgasmix) > 0 && results->dN2 > 0 && results->dHe < 0 && get_he(oldgasmix) && results->dN2 > 0 && 5 * results->dN2 > -results->dHe;
-}
-
 /* some events should never be thrown away */
 static bool is_potentially_redundant(const struct event *event)
 {
@@ -1464,29 +1415,6 @@ static void fixup_dive_pressures(struct dive *dive, struct divecomputer *dc)
 	simplify_dc_pressures(dc);
 }
 
-int find_best_gasmix_match(struct gasmix mix, const cylinder_t array[], unsigned int used)
-{
-	int i;
-	int best = -1, score = INT_MAX;
-
-	for (i = 0; i < MAX_CYLINDERS; i++) {
-		const cylinder_t *match;
-		int distance;
-
-		if (used & (1 << i))
-			continue;
-		match = array + i;
-		if (cylinder_nodata(match))
-			continue;
-		distance = gasmix_distance(mix, match->gasmix);
-		if (distance >= score)
-			continue;
-		best = i;
-		score = distance;
-	}
-	return best;
-}
-
 /*
  * Match a gas change event against the cylinders we have
  */
@@ -2031,17 +1959,6 @@ extern int get_cylinder_idx_by_use(const struct dive *dive, enum cylinderuse cyl
 	return -1; // negative number means cylinder_use_type not found in list of cylinders
 }
 
-int gasmix_distance(struct gasmix a, struct gasmix b)
-{
-	int a_o2 = get_o2(a), b_o2 = get_o2(b);
-	int a_he = get_he(a), b_he = get_he(b);
-	int delta_o2 = a_o2 - b_o2, delta_he = a_he - b_he;
-
-	delta_he = delta_he * delta_he;
-	delta_o2 = delta_o2 * delta_o2;
-	return delta_he + delta_o2;
-}
-
 /* fill_pressures(): Compute partial gas pressures in bar from gasmix and ambient pressures, possibly for OC or CCR, to be
  * extended to PSCT. This function does the calculations of gas pressures applicable to a single point on the dive profile.
  * The structure "pressures" is used to return calculated gas pressures to the calling software.
@@ -2052,7 +1969,7 @@ int gasmix_distance(struct gasmix a, struct gasmix b)
  *			divemode = the dive mode pertaining to this point in the dive profile.
  * This function called by: calculate_gas_information_new() in profile.c; add_segment() in deco.c.
  */
-extern void fill_pressures(struct gas_pressures *pressures, const double amb_pressure, struct gasmix mix, double po2, enum divemode_t divemode)
+void fill_pressures(struct gas_pressures *pressures, const double amb_pressure, struct gasmix mix, double po2, enum divemode_t divemode)
 {
 	if ((divemode != OC) && po2) {	// This is a rebreather dive where pressures->o2 is defined
 		if (po2 >= amb_pressure) {
@@ -2180,20 +2097,6 @@ void cylinder_renumber(struct dive *dive, int mapping[])
 		dc_cylinder_renumber(dive, dc, mapping);
 }
 
-static bool gasmix_is_invalid(struct gasmix mix)
-{
-	return mix.o2.permille < 0;
-}
-
-int same_gasmix(struct gasmix a, struct gasmix b)
-{
-	if (gasmix_is_invalid(a) || gasmix_is_invalid(b))
-		return 0;
-	if (gasmix_is_air(a) && gasmix_is_air(b))
-		return 1;
-	return a.o2.permille == b.o2.permille && a.he.permille == b.he.permille;
-}
-
 int same_gasmix_cylinder(cylinder_t *cyl, int cylid, struct dive *dive, bool check_unused)
 {
 	struct gasmix mygas = cyl->gasmix;
@@ -3922,13 +3825,6 @@ fraction_t best_he(depth_t depth, const struct dive *dive)
 	return fhe;
 }
 
-bool gasmix_is_air(struct gasmix gasmix)
-{
-	int o2 = gasmix.o2.permille;
-	int he = gasmix.he.permille;
-	return (he == 0) && (o2 == 0 || ((o2 >= O2_IN_AIR - 1) && (o2 <= O2_IN_AIR + 1)));
-}
-
 void invalidate_dive_cache(struct dive *dive)
 {
 	memset(dive->git_id, 0, 20);
diff --git a/core/dive.h b/core/dive.h
index c6b08abd4..042f825f4 100644
--- a/core/dive.h
+++ b/core/dive.h
@@ -27,13 +27,6 @@ extern const char *cylinderuse_text[];
 extern const char *divemode_text_ui[];
 extern const char *divemode_text[];
 
-struct icd_data { // This structure provides communication between function isobaric_counterdiffusion() and the calling software.
-	int dN2;      // The change in fraction (permille) of nitrogen during the change
-	int dHe;      // The change in fraction (permille) of helium during the change
-};
-
-extern bool isobaric_counterdiffusion(struct gasmix oldgasmix, struct gasmix newgasmix, struct icd_data *results);
-
 /*
  * Events are currently based straight on what libdivecomputer gives us.
  *  We need to wrap these into our own events at some point to remove some of the limitations.
@@ -63,35 +56,8 @@ struct event {
 
 extern int event_is_gaschange(const struct event *ev);
 
-/* Volume in mliter of a cylinder at pressure 'p' */
-extern int gas_volume(const cylinder_t *cyl, pressure_t p);
-extern double gas_compressibility_factor(struct gasmix gas, double bar);
-extern double isothermal_pressure(struct gasmix gas, double p1, int volume1, int volume2);
-extern double gas_density(struct gasmix gas, int pressure);
-extern int same_gasmix(struct gasmix a, struct gasmix b);
-
-static inline int get_o2(struct gasmix mix)
-{
-	return mix.o2.permille ?: O2_IN_AIR;
-}
-
-static inline int get_he(struct gasmix mix)
-{
-	return mix.he.permille;
-}
-
-struct gas_pressures {
-	double o2, n2, he;
-};
-
 extern void fill_pressures(struct gas_pressures *pressures, const double amb_pressure, struct gasmix mix, double po2, enum divemode_t dctype);
 
-extern void sanitize_gasmix(struct gasmix *mix);
-extern int gasmix_distance(struct gasmix a, struct gasmix b);
-extern int find_best_gasmix_match(struct gasmix mix, const cylinder_t array[], unsigned int used);
-
-extern bool gasmix_is_air(struct gasmix gasmix);
-
 /* Linear interpolation between 'a' and 'b', when we are 'part'way into the 'whole' distance from a to b */
 static inline int interpolate(int a, int b, int part, int whole)
 {
diff --git a/core/equipment.c b/core/equipment.c
index 325c57cb7..81033ace6 100644
--- a/core/equipment.c
+++ b/core/equipment.c
@@ -10,6 +10,7 @@
 #include <stdlib.h>
 #include <stdarg.h>
 #include <time.h>
+#include <limits.h>
 #include "equipment.h"
 #include "gettext.h"
 #include "dive.h"
@@ -101,6 +102,36 @@ const char *gasname(struct gasmix gasmix)
 	return gas;
 }
 
+int gas_volume(const cylinder_t *cyl, pressure_t p)
+{
+	double bar = p.mbar / 1000.0;
+	double z_factor = gas_compressibility_factor(cyl->gasmix, bar);
+	return lrint(cyl->type.size.mliter * bar_to_atm(bar) / z_factor);
+}
+
+int find_best_gasmix_match(struct gasmix mix, const cylinder_t array[], unsigned int used)
+{
+	int i;
+	int best = -1, score = INT_MAX;
+
+	for (i = 0; i < MAX_CYLINDERS; i++) {
+		const cylinder_t *match;
+		int distance;
+
+		if (used & (1 << i))
+			continue;
+		match = array + i;
+		if (cylinder_nodata(match))
+			continue;
+		distance = gasmix_distance(mix, match->gasmix);
+		if (distance >= score)
+			continue;
+		best = i;
+		score = distance;
+	}
+	return best;
+}
+
 bool weightsystem_none(const weightsystem_t *ws)
 {
 	return !ws->weight.grams && !ws->description;
diff --git a/core/equipment.h b/core/equipment.h
index 73e21b8ed..eb789ede8 100644
--- a/core/equipment.h
+++ b/core/equipment.h
@@ -2,7 +2,7 @@
 #ifndef EQUIPMENT_H
 #define EQUIPMENT_H
 
-#include "units.h"
+#include "gas.h"
 
 #ifdef __cplusplus
 extern "C" {
@@ -10,15 +10,6 @@ extern "C" {
 
 struct dive;
 
-// o2 == 0 && he == 0 -> air
-// o2 < 0 -> invalid
-struct gasmix {
-	fraction_t o2;
-	fraction_t he;
-};
-static const struct gasmix gasmix_invalid = { { -1 }, { -1 } };
-static const struct gasmix gasmix_air = { { 0 }, { 0 } };
-
 enum cylinderuse {OC_GAS, DILUENT, OXYGEN, NOT_USED, NUM_GAS_USE}; // The different uses for cylinders
 
 typedef struct
@@ -63,6 +54,8 @@ extern bool weightsystem_none(const weightsystem_t *ws);
 extern void remove_cylinder(struct dive *dive, int idx);
 extern void remove_weightsystem(struct dive *dive, int idx);
 extern void reset_cylinders(struct dive *dive, bool track_gas);
+extern int gas_volume(const cylinder_t *cyl, pressure_t p); /* Volume in mliter of a cylinder at pressure 'p' */
+extern int find_best_gasmix_match(struct gasmix mix, const cylinder_t array[], unsigned int used);
 #ifdef DEBUG_CYL
 extern void dump_cylinders(struct dive *dive, bool verbose);
 #endif
diff --git a/core/gas.c b/core/gas.c
new file mode 100644
index 000000000..9f67d0b92
--- /dev/null
+++ b/core/gas.c
@@ -0,0 +1,79 @@
+// SPDX-License-Identifier: GPL-2.0
+#include "gas.h"
+#include "pref.h"
+#include <stdio.h>
+#include <string.h>
+
+/* Perform isobaric counterdiffusion calculations for gas changes in trimix dives.
+ * Here we use the rule-of-fifths where, during a change involving trimix gas, the increase in nitrogen
+ * should not exceed one fifth of the decrease in helium.
+ * Parameters: 1) pointers to two gas mixes, the gas being switched from and the gas being switched to.
+ *             2) a pointer to an icd_data structure.
+ * Output:     i) The icd_data stucture is filled with the delta_N2 and delta_He numbers (as permille).
+ *            ii) Function returns a boolean indicating an exceeding of the rule-of-fifths. False = no icd problem.
+ */
+bool isobaric_counterdiffusion(struct gasmix oldgasmix, struct gasmix newgasmix, struct icd_data *results)
+{
+	if (!prefs.show_icd)
+		return false;
+	results->dN2 = get_he(oldgasmix) + get_o2(oldgasmix) - get_he(newgasmix) - get_o2(newgasmix);
+	results->dHe = get_he(newgasmix) - get_he(oldgasmix);
+	return get_he(oldgasmix) > 0 && results->dN2 > 0 && results->dHe < 0 && get_he(oldgasmix) && results->dN2 > 0 && 5 * results->dN2 > -results->dHe;
+}
+
+static bool gasmix_is_invalid(struct gasmix mix)
+{
+	return mix.o2.permille < 0;
+}
+
+int same_gasmix(struct gasmix a, struct gasmix b)
+{
+	if (gasmix_is_invalid(a) || gasmix_is_invalid(b))
+		return 0;
+	if (gasmix_is_air(a) && gasmix_is_air(b))
+		return 1;
+	return a.o2.permille == b.o2.permille && a.he.permille == b.he.permille;
+}
+
+void sanitize_gasmix(struct gasmix *mix)
+{
+	unsigned int o2, he;
+
+	o2 = mix->o2.permille;
+	he = mix->he.permille;
+
+	/* Regular air: leave empty */
+	if (!he) {
+		if (!o2)
+			return;
+		/* 20.8% to 21% O2 is just air */
+		if (gasmix_is_air(*mix)) {
+			mix->o2.permille = 0;
+			return;
+		}
+	}
+
+	/* Sane mix? */
+	if (o2 <= 1000 && he <= 1000 && o2 + he <= 1000)
+		return;
+	fprintf(stderr, "Odd gasmix: %u O2 %u He\n", o2, he);
+	memset(mix, 0, sizeof(*mix));
+}
+
+int gasmix_distance(struct gasmix a, struct gasmix b)
+{
+	int a_o2 = get_o2(a), b_o2 = get_o2(b);
+	int a_he = get_he(a), b_he = get_he(b);
+	int delta_o2 = a_o2 - b_o2, delta_he = a_he - b_he;
+
+	delta_he = delta_he * delta_he;
+	delta_o2 = delta_o2 * delta_o2;
+	return delta_he + delta_o2;
+}
+
+bool gasmix_is_air(struct gasmix gasmix)
+{
+	int o2 = gasmix.o2.permille;
+	int he = gasmix.he.permille;
+	return (he == 0) && (o2 == 0 || ((o2 >= O2_IN_AIR - 1) && (o2 <= O2_IN_AIR + 1)));
+}
diff --git a/core/gas.h b/core/gas.h
new file mode 100644
index 000000000..f5d948da0
--- /dev/null
+++ b/core/gas.h
@@ -0,0 +1,57 @@
+// SPDX-License-Identifier: GPL-2.0
+#ifndef GAS_H
+#define GAS_H
+
+#include "units.h"
+
+#ifdef __cplusplus
+extern "C" {
+#else
+#include <stdbool.h>
+#endif
+
+// o2 == 0 && he == 0 -> air
+// o2 < 0 -> invalid
+struct gasmix {
+	fraction_t o2;
+	fraction_t he;
+};
+static const struct gasmix gasmix_invalid = { { -1 }, { -1 } };
+static const struct gasmix gasmix_air = { { 0 }, { 0 } };
+
+struct icd_data { // This structure provides communication between function isobaric_counterdiffusion() and the calling software.
+	int dN2;      // The change in fraction (permille) of nitrogen during the change
+	int dHe;      // The change in fraction (permille) of helium during the change
+};
+
+extern bool isobaric_counterdiffusion(struct gasmix oldgasmix, struct gasmix newgasmix, struct icd_data *results);
+
+extern double gas_compressibility_factor(struct gasmix gas, double bar);
+extern double isothermal_pressure(struct gasmix gas, double p1, int volume1, int volume2);
+extern double gas_density(struct gasmix gas, int pressure);
+extern int same_gasmix(struct gasmix a, struct gasmix b);
+
+static inline int get_o2(struct gasmix mix)
+{
+	return mix.o2.permille ?: O2_IN_AIR;
+}
+
+static inline int get_he(struct gasmix mix)
+{
+	return mix.he.permille;
+}
+
+struct gas_pressures {
+	double o2, n2, he;
+};
+
+extern void sanitize_gasmix(struct gasmix *mix);
+extern int gasmix_distance(struct gasmix a, struct gasmix b);
+
+extern bool gasmix_is_air(struct gasmix gasmix);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif
diff --git a/packaging/ios/Subsurface-mobile.pro b/packaging/ios/Subsurface-mobile.pro
index 7d78ade94..02bb4076a 100644
--- a/packaging/ios/Subsurface-mobile.pro
+++ b/packaging/ios/Subsurface-mobile.pro
@@ -67,6 +67,7 @@ SOURCES += ../../subsurface-mobile-main.cpp \
 	../../core/deco.c \
 	../../core/divesite.c \
 	../../core/equipment.c \
+	../../core/gas.c \
 	../../core/membuffer.c \
 	../../core/sha1.c \
 	../../core/strtod.c \