Partially filled pipes and open channels
A conventional full-pipe meter calculates flow using the full measuring cross-section. That assumption is wrong in a half-full gravity drain.
Consider a level measurement at a properly designed flume or weir, an area–velocity system, or a specialist partly filled pipe meter. The measurement must account for the actual wetted area and the relevant hydraulic conditions. Channel geometry, sediment, backwater and sensor location become part of the selection. Pulsar open-channel and area–velocity systems
A level sensor alone does not generally determine flow in an arbitrary drain. It needs a valid hydraulic relationship or complementary velocity measurement.
Thermal energy / BTU meters
A water-volume meter tells you how much water passed. A thermal energy meter combines flow with supply/return temperatures and a calculator to determine transferred heating or cooling energy.
Buy the complete compatible assembly: flow sensor, matched temperature sensors, calculator and required outputs. Specify water versus glycol mixture, concentration, flow-sensor location, temperature range, minimum temperature difference and condensation exposure. Cooling energy and water consumption are different quantities. Kamstrup thermal-meter product information
Coriolis, turbine, vortex and differential-pressure instruments
These may be appropriate for specialist industrial duties even when they are not the usual choice for a domestic water connection.
Coriolis instruments measure mass flow and may provide density. Industrial turbine meters use a rotor and need fluid/viscosity suitability checks. Vortex and differential-pressure instruments have their own flow, pressure-loss and installation constraints. Compare the complete process requirement, not just the word “water.” Manufacturer process portfolios distinguish these principles and their operating benefits. Endress+Hauser instrumentation overview
For ultrapure or demineralized water, qualified ultrasonic designs are among the options. Endress+Hauser demineralized-water application
Fire service, hygienic service and aggressive fluids
Treat these as specific engineered applications. A general-purpose meter is not accepted simply because its diameter matches.
For fire service, have the responsible designer validate emergency flow and head loss and specify the accepted assembly. Purpose-built fire-service meters have their own documentation. Neptune fire-service/turbine testing guide
For hygienic service, specify the required wetted construction, drainability and cleaning conditions. For saline or chemically aggressive water, obtain material compatibility for the complete wetted assembly, including seals. A stainless-steel housing does not establish that every wetted part is stainless steel or suitable for the fluid.