[marian.nedelcu]

I think the gas tank discharging were highly studied. But I am still confused when I count the number of equations and the number of unknowns.
Problem: Having known pressure and temperature of gas into the tank find their variation versus time if the tank discharges in atmosphere through a small hole.
Equations:
1. Mass conservation (continuity equation)
2. Momentum conservation( Euler or Bernoulli equation)
3. Energy conservation
4. Gas state equation
Unknowns:
3 parameters inside the tank: pressure temperature and density
3 parameters at the hole; velocity, temperature and density
Note:
1. the pressure at hole is assumed to be equal with atmospheric pressure
2. The state equation is counted twice; inside the tank and at the hole

Conclusion; 5 equation and 6 unknowns
Could you tell me what I am misiing?

[Persia]

I think you are missing the equation which correlates the orifice (hole) discharge rate to the tank pressure.