The principles of orifice and venturi meters are discussed in Section 14.16. Two other kinds of inferential (or momentum) meter are the Dall tube and the V cone venturi. In both, flow accelerates through a constriction and leads to a pressure drop. The pressure difference is measured in the Dall tube and the V cone venturi as an indicator of velocity (and so flow) in the same way as for an orifice. The V cone venturi design is claimed to have a turn-down ratio of 25:1 and to be less affected by conditions upstream and downstream and can be fitted into shorter lengths of straight pipe than is recommended for other meter types. Further types of momentum meter are indicated in Table 18.5.
An ultrasonic flow meter as shown in Fig. 16.11 measures the velocity of a fluid to calculate volume flow. The vortex flowmeter can measure the average velocity along the path of an emitted beam of ultrasound by averaging the difference in measured transit time between the pulses of ultrasound propagating into and against the direction of the flow or by measuring the frequency shift from the Doppler effect. Ultrasonic flow meters are affected by the acoustic properties of the fluid and can be impacted by temperature, density, viscosity and suspended particulates. They are often inexpensive to use and maintain because they do not use moving parts, unlike mechanical flow meters.
Insertion probe flow meters are installed for temporary measurement of flow for consumption surveys or for distribution networks analyses. These instruments are either the turbine or electromagnetic (EM) type, the latter becoming more common. Both are inserted into the pipe where flow measurement is required. The turbine type uses a small rotating vane at the end of a probe to record flow velocity. The vane is susceptible to damage, in which case the instrument has to be returned to the manufacturer for repair and recalibration. The turbine meter is inserted through a 40 mm diameter tapping in the pipe which has to be of at least 200 mm diameter. The EM probe (Plate 30(c)) uses an electromagnet at its end to apply a magnetic field to the water. Electrodes either side of the probe pick up the induced EMF in the water which is proportional to the velocity past the electrodes. The tapping for an EM insertion probe is 20 mm diameter and can usually be installed in pipes of diameter 150 mm and greater. EM probes are made up to 1 m long; therefore, they cannot be used for pipes of diameter greater than 900 mm and are restricted to flow with velocity less than about 1.75 to 2.0 m/s due to the flexibility of the probe.
Insertion probes measure the velocity at the position of the measuring device. This can be at the pipe centre line or at defined points along the diameter. The measured velocity has to be converted to mean pipe velocity of flow by relating the measured value to the average velocity across the whole pipe. For this a velocity profile for the pipe is used, determined by using the same instrument to record velocities at set points across the diameter from crown to invert. The recorded measurements are corrected to take account of the disturbance caused by the instrument itself (increased local velocity). The disturbance coefficients are unique to each instrument and are provided by its manufacturer. For the conditions usually encountered the ratio of the mean velocity to the centerline velocity is 0.83 but can range from 0.7 to 1.0. Values differing widely from 0.83 should be viewed with caution and the cause investigated. However, satisfactory results should be obtained if the internal diameter is measured accurately and if the number of flow profile readings is sufficient—five for pipes of DN 150, nine for DN 300 and 13 for larger pipes. The measurements should be repeated at least three times to ensure the ratio is consistent and repeatable; the flow must be relatively consistent during each profile run. In practice poor field conditions often make precise measurement difficult so that several attempts may be necessary. Once the profile is established satisfactorily the instrument is set at the pipe centre line and the data logger is attached. In pipes in poor condition the velocity profile changes with flow and can render very inaccurate measurements of flows significantly different from that at which the velocity profile was established.
