The following figure shows, among other things, an inductive transducer used to generate an electrical signal. Some designs use radio-frequency sensors, which have the advantage of not producing drag on the lower flows. In other models, the shaft rotation is transmitted via a mechanical counter gear. They reel-type counters or Woltmann.
These models require a considerable torque, resulting in startup problems and nonlinear behavior at low flow rates. Accountants domestic water consumption, for example, often have "fluid slides" to very low flows, such as originating a leaky faucet. Some accountants Woltmann with vertical-mounted winch have certain advantages, which has a lesser effect of the pivots and friction and enhanced sensitivity, which is achieved by a field of larger measurement values. It also contributes to the buoyancy effect of the fluid, which partly offset the weight of the rotor and reduces its effective mass. Figure 49 shows two models of this type. The first is used for liquids or gases, second only to liquids.
Figure: Examples of typical mechanical meters Woltmann. Left: axial rotor. Right: vertical rotor.
The typical manufacturing materials are austenitic steels for the body, stainless steel flanges and inside and durable materials such as tungsten carbide for prestressed steel pivots. For high pressure applications are available high voltage clamps (PN> 1,500 psi bar/21.750). In addition, both the body and the other components can be made of durable plastic for applications in the chemical industry. However, it is evident that each material has a temperature and pressure of work limits. It is therefore advisable to follow in all cases the manufacturer's advice.
For unusual applications have developed special models. A special model for measuring the flow of chemicals dispensed absolutely pivots and uses the thrust generated by the blades to the rotor spin freely with the fluid inside the body. In this model there are two rotors at both ends of a vertical axis. The mixed composition of forces pushing and pulling on the two blades at both ends kept the group floating freely inside the body. Induction sensors measure the speed of rotation. These models can be made of chemically resistant materials such as PVDF or PFA and to measure the flow of chemicals "dry" as acetone or SF 6.
Figure: Sample flow meter turbine for measuring very low flows. a = flow meter body, b = interchangeable adapter (for widening the scope of measured values), c = Opening for the measurements, d = Pivot, e = turbine rotor, f = Induction Sensor
For applications with very low flows, the inlet fluid is passed through a narrow opening to increase its speed (Fig. below). Ala outlet opening, the fluid jet impinges on a small rotor. Magnetic sensors (or sometimes optical) record their movement. These models use lightweight rotors mounted on pivots of precious stones that offer a minimum daily frictional resistance. The figure below shows a model with interchangeable adapters at the entrance opening. The adapters can be changed and can cover different areas of flow measurement values with the same counter. However, the linear behavior and repeatability of these flowmeters is not as good as in conventional turbine meters.
We have also developed some very specific models for hygienic applications. To measure flow of milk, wine or fruit juices should be used special pins, heads, coated and hygienic materials approved. Certainly, some models have the certification.
3A. These models can be sprayed regularly with cleaners and sterilizing fluids. However, these models may experience problems purge gases or vapors, because high speeds are reached can damage the rotor or the pivot. The insertion turbine meters have been widely used since the 1980's for monitoring water flow in industrial plants. Figure below showing that these flowmeters are, in fact, small "turbines at the end of a stick." These devices feature a local high sensitivity and repeatability. Some models are of a propeller, while others hide the small turbine inside a pillow. The overall experience with these devices is reasonably good flow monitoring applications in general water piping of large diameter.
Ill.: insertion turbine meter.
