CHAPTER 2 - BASIC CONCEPTS RELATED TO FLOWING WATER AND MEASUREMENT
6. Other Examples of Velocity Flow Measurement Devices
Measuring devices not previously mentioned are dilution in the concentration of tracers, such as salts and dyes; acoustic or magnetic meters; pitot tubes; rotameters, which are tapered tubes with suspended flow indicators; and many others that are not commonly used. In the dilution method, discharge is calculated by determining the quantity of water necessary to dilute a known quantity of concentrated chemical or dye solution. Chemical analysis or color comparison is used to determine the degree of dilution of the injected or mixed samples. In transit time acoustic meters, the velocity of sound pulses in the direction of flow is compared to the velocity of sound pulses opposite to the direction of flow to determine the mean velocity and, thus, discharge. With Doppler acoustic meters, sound pulses are reflected from moving particles within the water mass, similar to radar. In the magnetic meter, the flowing water acts like a moving electrical conductor passing through a magnetic field to produce a voltage that is proportional to discharge. Pitot tubes relate velocity head, V2/2g, to discharge.