Explosion　venting　technology　is　an　effective　means　of　reducing　explosive　hazards,　and　hydrogen　volume　fraction　(χ)　is　one　of　the　important　parameters　affecting　its　effectiveness.　The　vent　covers,　as　a　key　part　of　the　explosion　venting　application,　consist　of　two　types:　inertia-free　(e.g.,　membranes)　and　inertial　(e.g.,　panels).　χ　has　been　extensively　studied　using　inertial-free　vent　covers,　but　very　limited　work　has　been　done　using　an　inertial　vent　cover.　Hence,　the　effects　of　χ,　ranging　from　0　to　1.0,　on　the　vented　H2/CH4/air　explosion　were　studied　in　a　chamber　with　a　hinged　aluminum　panel,　and　the　explosion　overpressure　during　venting　was　simulated　by　FLACS　software.　The　results　show　that　the　flame　bubble　becomes　larger　and　brighter　with　increasing　χ.　However,　the　time　for　the　flame　to　travel　through　the　vent　(tout)　and　the　opening　angle　of　the　hinged　panel　at　the　time　of　tout　constantly　decreases　as　χ　increases　from　0　to　1.0.　In　the　tests　with　χ　≤　0.6,　p3　caused　by　acoustically　enhanced　combustion　becomes　the　pressure　peak　with　the　highest　amplitude　in　the　internal　pressure　profile,　but　the　pressure　peak　p2　induced　by　the　external　explosion　dominates　the　internal　pressure　trace　for　χ　＞　0.6.　In　comparison　to　the　H2/CH4/air　deflagration　experiments　using　an　inertialess　vent　cover,　the　shape　of　the　external　fireball　is　quite　similar　for　smaller　χ　in　the　current　study　with　an　inertial　vent　panel.　However,　the　use　of　the　inertial　vent　panel　results　in　a　more　flattened　external　fireball　for　larger　χ.　The　highest　amplitude　of　the　external　pressure　peak　(pext)　and　the　maximum　reduced　overpressure　(pred)　increase　with　increasing　χ.　Whether　the　studies　are　performed　with　inertial　and　inertialess　vent　covers　or　FLACS　simulations,　the　formation　time　(Δt)　of　pext　decreases　linearly　with　increasing　χ,　but　pred　increases　linearly　with　Sl2.　The　explosion　overpressure　simulated　by　FLACS　is　relatively　close　to　the　experimental　results,　and　in　particular,　the　simulated　pred　agrees　very　well　with　the　experimental　value.　©　2025　Elsevier　Ltd