diff --git a/src/gas_cylinder.py b/src/gas_cylinder.py
new file mode 100644
index 0000000..d73bea4
--- /dev/null
+++ b/src/gas_cylinder.py
@@ -0,0 +1,171 @@
+"""
+High-pressure gas cylinder model with configurable working fluid.
+
+The cylinder is a 0D lumped model. Its primary state is total mass and
+total internal energy; pressure, temperature, density, enthalpy, and other
+thermodynamic quantities are recovered from CoolProp on demand.
+"""
+
+import numpy as np
+import CoolProp.CoolProp as CP
+from CoolProp import AbstractState
+
+
+DEFAULT_FLUID = "Helium"
+DEFAULT_P_INIT = 18.031e6   # Pa, absolute
+DEFAULT_VOLUME = 20e-3      # m^3, 20 L
+DEFAULT_T_INIT = 78.0       # K
+
+
+class HighPressureGasCylinder:
+    """0D high-pressure gas cylinder.
+
+    Parameters
+    ----------
+    fluid : str
+        CoolProp fluid name, for example ``"Helium"`` or ``"Nitrogen"``.
+    P_init : float
+        Initial absolute pressure [Pa].
+    V : float
+        Cylinder volume [m^3].
+    T_init : float
+        Initial temperature [K].
+    backend : str
+        CoolProp backend name. ``"HEOS"`` is used by default.
+    """
+
+    def __init__(self, fluid=DEFAULT_FLUID, P_init=DEFAULT_P_INIT,
+                 V=DEFAULT_VOLUME, T_init=DEFAULT_T_INIT, backend="HEOS"):
+        if not fluid:
+            raise ValueError("fluid must be a non-empty CoolProp fluid name")
+        if P_init <= 0:
+            raise ValueError("P_init must be > 0")
+        if V <= 0:
+            raise ValueError("V must be > 0")
+        if T_init <= 0:
+            raise ValueError("T_init must be > 0")
+
+        self.fluid = fluid
+        self.backend = backend
+        self.V = V
+        self._state = AbstractState(backend, fluid)
+
+        self._state.update(CP.PT_INPUTS, P_init, T_init)
+        rho = self._state.rhomass()
+        self.mass = rho * V
+        self.U = self.mass * self._state.umass()
+
+    def _update_state(self):
+        rho = self.rho
+        u = self.specific_internal_energy
+        self._state.update(CP.DmassUmass_INPUTS, rho, u)
+        return self._state
+
+    @property
+    def rho(self):
+        """Gas density [kg/m^3]."""
+        return self.mass / self.V
+
+    @property
+    def specific_internal_energy(self):
+        """Specific internal energy [J/kg]."""
+        return self.U / self.mass
+
+    @property
+    def P(self):
+        """Absolute pressure [Pa]."""
+        return self._update_state().p()
+
+    @property
+    def T(self):
+        """Temperature [K]."""
+        return self._update_state().T()
+
+    @property
+    def h(self):
+        """Specific enthalpy [J/kg]."""
+        return self._update_state().hmass()
+
+    @property
+    def cp(self):
+        """Specific heat at constant pressure [J/(kg K)]."""
+        return self._update_state().cpmass()
+
+    @property
+    def cv(self):
+        """Specific heat at constant volume [J/(kg K)]."""
+        return self._update_state().cvmass()
+
+    @property
+    def gamma(self):
+        """Local heat capacity ratio cp/cv [-]."""
+        return self.cp / self.cv
+
+    @property
+    def R_specific(self):
+        """Specific gas constant [J/(kg K)]."""
+        state = self._update_state()
+        return state.gas_constant() / state.molar_mass()
+
+    @property
+    def compressibility_factor(self):
+        """Compressibility factor Z = P/(rho R T) [-]."""
+        return self.P / (self.rho * self.R_specific * self.T)
+
+    def ghost_state(self):
+        """Return a stagnant conservative state ``[rho, rho*u, rho*E]``.
+
+        This is useful as a boundary ghost cell for conservative flow models.
+        The state uses the real-fluid internal energy density. A pipe solver
+        that assumes an ideal-gas EOS must still be checked for EOS consistency
+        before coupling it directly to this real-fluid cylinder.
+        """
+        return np.array([self.rho, 0.0, self.rho * self.specific_internal_energy])
+
+    def apply_flux(self, mdot, edot, dt, sign):
+        """Update cylinder mass and energy from a boundary flux.
+
+        Parameters
+        ----------
+        mdot : float
+            Mass flow rate [kg/s].
+        edot : float
+            Energy flow rate [W].
+        dt : float
+            Time step [s].
+        sign : int
+            ``-1`` for outflow from the cylinder, ``+1`` for inflow.
+        """
+        if dt < 0:
+            raise ValueError("dt must be >= 0")
+        if sign not in (-1, 1):
+            raise ValueError("sign must be +1 or -1")
+
+        self.mass += sign * mdot * dt
+        self.U += sign * edot * dt
+
+        if self.mass <= 0:
+            raise RuntimeError(
+                f"Cylinder mass non-positive after apply_flux: mass={self.mass}, "
+                f"mdot={mdot}, edot={edot}, dt={dt}, sign={sign}"
+            )
+
+        # Force a thermodynamic validity check immediately after the update.
+        self._update_state()
+
+    def state_summary(self):
+        """Return common cylinder state quantities as a dictionary."""
+        return {
+            "fluid": self.fluid,
+            "V": self.V,
+            "mass": self.mass,
+            "rho": self.rho,
+            "P": self.P,
+            "T": self.T,
+            "U": self.U,
+            "u": self.specific_internal_energy,
+            "h": self.h,
+            "gamma": self.gamma,
+            "R_specific": self.R_specific,
+            "Z": self.compressibility_factor,
+        }
diff --git a/tests/test_gas_cylinder.py b/tests/test_gas_cylinder.py
new file mode 100644
index 0000000..469097a
--- /dev/null
+++ b/tests/test_gas_cylinder.py
@@ -0,0 +1,75 @@
+import numpy as np
+import pytest
+
+from gas_cylinder import (
+    DEFAULT_FLUID,
+    DEFAULT_P_INIT,
+    DEFAULT_T_INIT,
+    DEFAULT_VOLUME,
+    HighPressureGasCylinder,
+)
+
+
+def test_default_high_pressure_cylinder_state_matches_inputs():
+    cylinder = HighPressureGasCylinder()
+
+    assert cylinder.fluid == DEFAULT_FLUID
+    assert cylinder.V == DEFAULT_VOLUME
+    assert abs(cylinder.P - DEFAULT_P_INIT) / DEFAULT_P_INIT < 1e-8
+    assert abs(cylinder.T - DEFAULT_T_INIT) < 1e-8
+    assert cylinder.mass > 0
+    assert cylinder.U > 0
+    assert cylinder.gamma > 1.0
+    assert cylinder.compressibility_factor > 0.0
+
+
+def test_cylinder_accepts_configurable_fluid_pressure_volume_temperature():
+    cylinder = HighPressureGasCylinder(
+        fluid="Nitrogen",
+        P_init=2.0e6,
+        V=50e-3,
+        T_init=300.0,
+    )
+
+    assert cylinder.fluid == "Nitrogen"
+    assert abs(cylinder.P - 2.0e6) / 2.0e6 < 1e-8
+    assert abs(cylinder.T - 300.0) < 1e-8
+    assert abs(cylinder.V - 50e-3) < 1e-15
+    assert cylinder.mass > 0
+
+
+def test_cylinder_ghost_state_is_stagnant_conservative_vector():
+    cylinder = HighPressureGasCylinder()
+    ghost = cylinder.ghost_state()
+
+    assert ghost.shape == (3,)
+    assert abs(ghost[0] - cylinder.rho) < 1e-12
+    assert ghost[1] == 0.0
+    assert abs(ghost[2] - cylinder.rho * cylinder.specific_internal_energy) < 1e-6
+
+
+def test_cylinder_apply_flux_outflow_reduces_mass_energy_and_pressure():
+    cylinder = HighPressureGasCylinder()
+    mass_before = cylinder.mass
+    U_before = cylinder.U
+    P_before = cylinder.P
+
+    mdot = 1e-3
+    edot = mdot * cylinder.h
+    dt = 1.0
+    cylinder.apply_flux(mdot=mdot, edot=edot, dt=dt, sign=-1)
+
+    assert abs(cylinder.mass - (mass_before - mdot * dt)) < 1e-12
+    assert abs(cylinder.U - (U_before - edot * dt)) < 1e-6
+    assert cylinder.P < P_before
+
+
+def test_cylinder_rejects_invalid_inputs():
+    with pytest.raises(ValueError):
+        HighPressureGasCylinder(fluid="")
+    with pytest.raises(ValueError):
+        HighPressureGasCylinder(P_init=0.0)
+    with pytest.raises(ValueError):
+        HighPressureGasCylinder(V=0.0)
+    with pytest.raises(ValueError):
+        HighPressureGasCylinder(T_init=0.0)