This invention relates to a flow circuit for a flow meter which extends the operating range of a flow meter, particularly although not exclusively for gas metering, and to a flow meter incorporating such a circuit. The flow meter could be a fluidic flow meter, a vortex shedding meter, a turbine meter etc.

The useful operating range of a fluidic flowmeter can be limited by the specification of an upper limit to the pressure drop across the meter when operating at its maximum required flowrate. This effectively sets a limit on the maximum meter size. Because the flow meter has a lower limit of operating Reynolds number, below which the meter does not function, fixing the meter size sets a minimum operating flowrate for the meter for a given fluid. There is thus a conflict between the need for a large meter at high flow rates, and a small meter at low flow rates, and one proposed solution is described in GB 2172966 - to have two meters and a valve switching between them - but this is relatively expensive and impractical. A basic object of the present invention is to provide a flow circuit and a flow meter in which the operating range is significantly extended.

According to a first aspect of the present invention there is provided a flow circuit for a flow meter, the circuit comprising a fluid flow line having an upstream junction or a fluid reservoir from which the fluid flow is divided into an inlet stream to a fluidic meter and an inlet stream to a

laminar by-pass resistor, and a downstream junction or a fluid reservoir at which an outlet stream from the fluidic meter and an outlet stream from the by-pass resistor are recombined, in which the combined operating characteristics of the junctions and the by-pass resistor are selected and matched to the operating characteristics required of the flow meter, such that the relative magnitude of the two fluid streams changes over the required operating range, whereby, at high fluid flowrate, at the top of the required flow range, the greater proportion of the fluid passes through the by-pass resistor, while at low fluid flowrate, at the bottom of the flow range, the greater proportion of the fluid passes through the fluidic meter.

A second aspect of the invention is directed to a flow meter incorporating the above defined circuit. Hence, in accordance with this second aspect, there is provided a flow eter comprising a fluidic meter placed in parallel with a laminar flow by-pass resistor.

In contrast to prior art proposals of by-pass fluid flow metering in which a small fluidic meter is placed in parallel with a large meter and the small meter is used to infer the total flow, with the intention that both flows should increase proportionally if possible, the present invention departs from this teaching and achieves the opposite result by forcing the two flows (by-pass and meter) to change disproportionately, and the choice of two different types of resistance is essential to this principle. Thus, the flowmeter in accordance with the invention is, most

advantageously, automatically self-switching.

In practice, at the maximum specified flowrate t overall pressure drop across the circuit is still set by t maximum pressure drop specification. However, at the maxim flow condition, most of the fluid passes through the by-pa resistor, so the flow meter can be made smaller than equivalent independent meter taking all the flo Conversely, at lower flows, the by-pass resistor has relatively high resistance compared with the fluidic meter, that most flow passes through the meter. This syst effectively increases the measurement range of the flowmete This smaller meter size, combined with the fact that at l flows most of the fluid passes through the meter, effective reduces the minimum operating flowrate of the circuit at whi the meter still functions, below that for an . equivalen larger, independent meter. An alternative explanation of t circuit function is to consider a fixed size of flow mete At the minimum meter operating point the flow through t biasing circuit is slightly higher than that through the met alone, due to the small fraction of flow through the by-pa resistor. At the maximum pressure drop condition, howeve the circuit flow is much higher than for the meter alon because most of the flow now passes through the by-pas Provided the proportional gain in flow at the maxim operating point is greater than that at the minimum operati point, the biased by-pass circuit results in a net increase operating flow ratio.

To produce the desired effect, the resistance to fl

through the by-pass must be less than that through the meter at high flows but more than the meter at low flows. Consider flow resistance in terms of an Euler number, defined as pressure drop divided by fluid dynamic head within an element. The Euler number of a fluidic flow meter is typically close to unity at the maximum flow condition, rising somewhat as the flowrate decreases. A good by-pass resistor requires an Euler number which is low at high flow, but which rises quickly as flowrate decreases. Such a characteristic is displayed by parallel plates or by a laminar pipe resistor. Use of a bundle of multiple laminar pipe resistors enables the number, diameter and length of such pipes to be optimally selected and matched to the meter. Careful design at the inlet and outlet of the resistor to minimize inlet and exit losses at the maximum flow condition may be important. In this respect radial or axial flow diffusion may be employed to reduce exit losses.

The accompanying drawings, show, by way of example, in Figures la, lb, and lc three basic designs of the flow circuit in accordance with the invention and in Figures 2, 3, 4 and 5 end view, a front view, a plan view and an enlarged nozzle detail respectively of a flowmeter in accordance with the invention.

The flow circuit of Figure la comprises a fluid flow line 1 having an upstream junction Jl (or in Fig. lb, the reservoir R), where the flow is divided into two streams 2 and 3, whereby a fraction of the fluid flows as an inlet stream 2 to a fluidic flow meter 4, and the remainder flows as an inlet

stream 3 to a laminar by-pass resistor 5. Downstream of these two flow devices 4, 5 the two fluid streams 2A, 3A recombine at downstream junction J2 (or in Fig lc, the reservoir R) , prior to leaving the metering circuit. The resistor may for instance consist of a bundle of small tubes or a number of parallel plates to provide the desired laminar flow characteristics

The design of junctions Jl and J2 are also important in effecting the proportion of by-pass flow. The junctions can be designed to bias flow through the by-pass at high flows, by utilising the fluid momentum as either a directed jet or vortex. Any such biasing function will be much reduced at the low flows, where it is unfavourable, due to the increased viscous losses. The simplest design for Jl would be a full bore "T" piece, with the side connection to the meter. Forming Jl as a suitable jet pump would incur some extra pressure loss, but assist the bias action. At J2 the outflow from the resistor could tend to oppose meter flow across a "T"-piece, with the flow out through the side connection. Alternatively, a more sophisticated opposed jet with central radial diffusion could be used. Another option for J2 is as a form of vortex amplifier with the by-pass flow acting as control of the meter outflow.

To improve compactness the junctions, by-pass resistor and fluidic meter could be integrated into a single body.

In the embodiments of Figures lb and lc are illustrated the possibilities of supply being flow or to a reservoir R, e.g. the atmosphere, in contrast to the employment of two

junctions Jl and J2.

Tests have been conducted with a fluidic 'target meter ¹ oscillatory flowmeter of Figures 2 to 5 having a nozzle width of 1.4 mm, placed in parallel with a bundle of 57 laminar tubes of 1.8 mm diameter as a by-pass resistance. At 200 Pa pressure drop, the maximum flowrate was 5.1 m ³ /h) (so the system was not optimally sized for 6 m ³ /h) . Nevertheless, the minimum flow detected was 0.03 m ³ /h, giving a flow range of 170. For instance, this system showed great improvement over the meter alone (0.073 m ³ /h and range 82).