diff --git a/core/dive.cpp b/core/dive.cpp
index acbeb0bcf..d5d393201 100644
--- a/core/dive.cpp
+++ b/core/dive.cpp
@@ -2342,12 +2342,12 @@ std::unique_ptr<dive> clone_make_first_dc(const struct dive &d, int dc_number)
 }
 
 //Calculate O2 in best mix
-fraction_t best_o2(depth_t depth, const struct dive *dive, bool in_planner)
+fraction_t dive::best_o2(depth_t depth, bool in_planner) const
 {
 	fraction_t fo2;
 	int po2 = in_planner ? prefs.bottompo2 : (int)(prefs.modpO2 * 1000.0);
 
-	fo2.permille = (po2 * 100 / dive->depth_to_mbar(depth.mm)) * 10;	//use integer arithmetic to round down to nearest percent
+	fo2.permille = (po2 * 100 / depth_to_mbar(depth.mm)) * 10;	//use integer arithmetic to round down to nearest percent
 	// Don't permit >100% O2
 	if (fo2.permille > 1000)
 		fo2.permille = 1000;
@@ -2355,12 +2355,12 @@ fraction_t best_o2(depth_t depth, const struct dive *dive, bool in_planner)
 }
 
 //Calculate He in best mix. O2 is considered narcopic
-fraction_t best_he(depth_t depth, const struct dive *dive, bool o2narcotic, fraction_t fo2)
+fraction_t dive::best_he(depth_t depth, bool o2narcotic, fraction_t fo2) const
 {
 	fraction_t fhe;
 	int pnarcotic, ambient;
-	pnarcotic = dive->depth_to_mbar(prefs.bestmixend.mm);
-	ambient = dive->depth_to_mbar(depth.mm);
+	pnarcotic = depth_to_mbar(prefs.bestmixend.mm);
+	ambient = depth_to_mbar(depth.mm);
 	if (o2narcotic) {
 		fhe.permille = (100 - 100 * pnarcotic / ambient) * 10;	//use integer arithmetic to round up to nearest percent
 	} else {
diff --git a/core/dive.h b/core/dive.h
index aa7715349..9ab5425eb 100644
--- a/core/dive.h
+++ b/core/dive.h
@@ -125,6 +125,8 @@ struct dive {
 	pressure_t un_fixup_surface_pressure() const;
 	depth_t gas_mod(struct gasmix mix, pressure_t po2_limit, int roundto) const;
 	depth_t gas_mnd(struct gasmix mix, depth_t end, int roundto) const;
+	fraction_t best_o2(depth_t depth, bool in_planner) const;
+	fraction_t best_he(depth_t depth, bool o2narcotic, fraction_t fo2) const;
 
 	bool dive_has_gps_location() const;
 	location_t get_gps_location() const;
@@ -162,9 +164,6 @@ struct dive_components {
 	unsigned int when : 1;
 };
 
-extern fraction_t best_o2(depth_t depth, const struct dive *dive, bool in_planner);
-extern fraction_t best_he(depth_t depth, const struct dive *dive, bool o2narcotic, fraction_t fo2);
-
 extern std::unique_ptr<dive> clone_make_first_dc(const struct dive &d, int dc_number);
 
 extern bool time_during_dive_with_offset(const struct dive *dive, timestamp_t when, timestamp_t offset);
diff --git a/qt-models/cylindermodel.cpp b/qt-models/cylindermodel.cpp
index 2b915888d..46e84c018 100644
--- a/qt-models/cylindermodel.cpp
+++ b/qt-models/cylindermodel.cpp
@@ -424,11 +424,11 @@ bool CylindersModel::setData(const QModelIndex &index, const QVariant &value, in
 			if (QString::compare(qPrintable(vString), "*") == 0) {
 				cyl.bestmix_o2 = true;
 				// Calculate fO2 for max. depth
-				cyl.gasmix.o2 = best_o2(d->maxdepth, d, inPlanner);
+				cyl.gasmix.o2 = d->best_o2(d->maxdepth, inPlanner);
 			} else {
 				cyl.bestmix_o2 = false;
 				// Calculate fO2 for input depth
-				cyl.gasmix.o2 = best_o2(string_to_depth(qPrintable(vString)), d, inPlanner);
+				cyl.gasmix.o2 = d->best_o2(string_to_depth(qPrintable(vString)), inPlanner);
 			}
 			pressure_t modpO2;
 			modpO2.mbar = prefs.decopo2;
@@ -440,11 +440,11 @@ bool CylindersModel::setData(const QModelIndex &index, const QVariant &value, in
 		if (QString::compare(qPrintable(vString), "*") == 0) {
 			cyl.bestmix_he = true;
 			// Calculate fO2 for max. depth
-			cyl.gasmix.he = best_he(d->maxdepth, d, prefs.o2narcotic, make_fraction(get_o2(cyl.gasmix)));
+			cyl.gasmix.he = d->best_he(d->maxdepth, prefs.o2narcotic, make_fraction(get_o2(cyl.gasmix)));
 		} else {
 			cyl.bestmix_he = false;
 			// Calculate fHe for input depth
-			cyl.gasmix.he = best_he(string_to_depth(qPrintable(vString)), d, prefs.o2narcotic, make_fraction(get_o2(cyl.gasmix)));
+			cyl.gasmix.he = d->best_he(string_to_depth(qPrintable(vString)), prefs.o2narcotic, make_fraction(get_o2(cyl.gasmix)));
 		}
 		type = Command::EditCylinderType::GASMIX;
 		break;
@@ -663,7 +663,7 @@ bool CylindersModel::updateBestMixes()
 	bool gasUpdated = false;
 	for (auto &cyl: d->cylinders) {
 		if (cyl.bestmix_o2) {
-			cyl.gasmix.o2 = best_o2(d->maxdepth, d, inPlanner);
+			cyl.gasmix.o2 = d->best_o2(d->maxdepth, inPlanner);
 			// fO2 + fHe must not be greater than 1
 			if (get_o2(cyl.gasmix) + get_he(cyl.gasmix) > 1000)
 				cyl.gasmix.he.permille = 1000 - get_o2(cyl.gasmix);
@@ -673,7 +673,7 @@ bool CylindersModel::updateBestMixes()
 			gasUpdated = true;
 		}
 		if (cyl.bestmix_he) {
-			cyl.gasmix.he = best_he(d->maxdepth, d, prefs.o2narcotic, cyl.gasmix.o2);
+			cyl.gasmix.he = d->best_he(d->maxdepth, prefs.o2narcotic, cyl.gasmix.o2);
 			// fO2 + fHe must not be greater than 1
 			if (get_o2(cyl.gasmix) + get_he(cyl.gasmix) > 1000)
 				cyl.gasmix.o2.permille = 1000 - get_he(cyl.gasmix);