oil well christmas trees in the nature of an assembly of valves, spools, and fittings that control the flow of oil or gas out of a well; fluid flow meters; pressure measuring apparatus in the nature of pressure meters; measuring instruments in the nature of densitometers; viscosity meters for measuring the viscosity of fluids; multiphase meters for measuring individual fluid phases; fluid flow totalizers; fluid flow analyzers; fluid flow controllers; liquid level monitoring systems comprised ...