There are a number of methods that can be used to accurately dispense a volume of fluid. The method used depends on the accuracy desired and the hardware available.
Tradeoffs between hardware needed and software complexity also impact how we finally decide to do it.

Perhaps the simplest approach to precision dispense is by timing. The fluid must be held at a constant pressure, and an electronic (solenoid) valve is opened for a preset time to dispense a known volume.
While this does require that fluid pressure be constant, this can be provided by pressure regulators. A common practice is to hold fluid in a reservoir that is pressurized by a regulated air supply. The regulator is responsible for maintaining air pressure, so timed dispenses will result in the same volume of fluid released each time.

We must assume that the required precision will not be significantly affected by the time the valve takes to open or close. The controlling computer can easily provide timing that is orders of magnitude more accurate than the valve transition time, so valve control is rarely a problem.

This method is often used in household appliances such as coffeemakers.