fluid 15 is acceptable to be used the sensors 24 and 34 conduct a similar function of measuring either an amount of oil or amount that the plunger rod 30 or plunger 28 has moved to estimate the amount of fluid 15 that has been injected into an air conditioning system 22 in a vehicle the way the sensors 24 and 34 are used in making this estimation will be discussed further below with respect to fig3 and 4 turning now to fig2 fig2 is a schematic diagram of the cartridge 14 containing the fluid 15 the valve assembly 16 and the fitting 20 are shown the cartridge 14 and the fluid 15 are shown to be fluidly connected to the valve assembly 16 the valve assembly 16 may include a solenoid valve 46 that may be operably connected to a solenoid 50 the solenoid 50 may be receive its power from the battery 36 in embodiments that use a battery 36 in other embodiments that use a cord to plug the injector 10 into a line voltage the solenoid 50 may be actuated or provided power by the line voltage the solenoid 50 may have a biased position that does not provide a fluid connection between the fluid 15 and the fitting 20 however when the solenoid 50 is actuated the solenoid 50 actuates the solenoid valve 46 to provide a fluid connection between the fluid 15 and the fitting 20 in some embodiments of the invention the amount of fluid 15 injected into a vehicle may be estimated using time if the viscosity of the oil is known than the microprocessor 52 may calculate how long the solenoid 50 needs to remain open to let a desired amount of fluid 15 into.