Liquid

For the measurement of liquid, many technologies exist.
At the simplest and cheapest one, we choose the float flowmeters that require no power supply. Reading is direct.
For fluids with conductivity, such as tap water, we will choose the electromagnetic technology, easy to use and very reliable, without pressure loss. A large choice of pipe diameters are available (from DN15 to DN600, even more on demand)
For high precision requirements, Coriolis flow meters are preferred. Accuracy reaches 0.1% reading with low pressure drop.
Finally, for non-contact applications with the fluid, ultrasonic flow meters are recommended. They exist with simple clamp on connection outside of the pipe or in line.