GB2254148A

GB2254148A - Flowmeter

Info

Publication number: GB2254148A
Application number: GB9206579A
Authority: GB
Inventors: John Andrew Bushman
Original assignee: Individual
Current assignee: Individual
Priority date: 1991-03-28
Filing date: 1992-03-25
Publication date: 1992-09-30
Anticipated expiration: 2012-03-25
Also published as: GB9106727D0; GB9206579D0; GB2254148B

Abstract

A device for the measurement of the rate of fluid flow through a conduit comprises a housing 1 having an inlet port 2 and an outlet port 3 and a rotor 4 driven by a motor disposed between the ports whereby a first channel 8 is provided for a subsidiary flow in one rotational sense and a second channel 9 is provided for a subsidiary flow in the opposite rotational sense, a first transducer 10 arranged on the housing and communicating with the first channel, a second transducer 11 arranged on the housing and communicating with the second channel, the transducers being arranged symmetrically on the housing. The surface of the rotor has perturbation means 12, 13 to cause a perturbation in the fluid flow around the rotor and the transducers are connected to means to amplify and record any pressure pulse produced as the perturbation means pass the transducers, thereby enabling the rate of fluid flow through the device to be measured. <IMAGE>

Description

TITLE Device for the measurement of the rate of Fluid Flow The present invention relates to a device for the measurement of the rate of flow of fluids, i.e. both gases and liquids. This has been primarily designed for use in anaesthesia but is not limited to such use. The design of the flow meter should make it particularly useful in the measurement of low flow rates of gases where the amount of energy in the slow moving gas stream may be negligible.

An intention of the invention is to be able to measure bi-directional gas flows in a range 0.5 to 100 1. min-l, the instrument having a fast response time so that it is able to faithfully record sudden changes in gas velocity. The preferred instrument has a linear response. An important application is for the measurement of gas flows to and from ventilated or spontaneously breathing patients.

Conventional means of measuring gas flows in anaesthesia are by:1) Variable orifice flow meters or rotameters. These are unidirectional and have a slow response time. The measurement is made by eye as they have no electrical output. A number of different sized rotarneters would be required to measure over a range 1 to 100 1. min-1.

2) Fixed-orifice flow meters or Fleisch tubes. These are bi-directional and have a fast response time but they require sensitive differential pressure gauges as the pressure drop across them is small. They are therefore expensive and relatively delicate.

3) The Drager volumeter. This is like a Roots blower which is rotated by the gas flow. It is not accurate at low flows and because of the mass of the moving parts is not good at registering rapid changes in gas flow.

4) Turbines. The "Wright Respirometer" and "Spiroflow". These are unidirectional instruments and are non-linear, particularly at low flows where they become very inaccurate.

All the above conventional devices are "passive", that is to say that the energy for making the measurement comes from the gas flow. If the gas flow is very low then the energy available is low and the error of the instrument increases. This invention relates to an instrument where the energy to make the measurement is supplied by the instrument.

In the conventional devices listed above the flow may be either laminar or turbulent. Any transition between these two states causes inaccuracies. In the device of the invention the fluid flow therethrough is intended to be maintained in a turbulent state.

According to the present invention there is provided a device for the measurement of the rate of fluid flow through a conduit, the device comprising a housing having an inlet port and an outlet port and a rotor disposed between the two ports whereby a first channel is provided between the two ports for a subsidiary fluid flow in one rotational sense and a second channel is provided between the ports for a subsidiary flow in the opposite rotational sense, a first transducer arranged on the housing and communicating with the first channel, a second transducer arranged on the housing and communicating with the second channel, the transducers being arranged symmetrically on the housing, the surface of the rotor having perturbation means to cause a perturbation in the fluid flow around the rotor, and the transducers being connected to amplifying means to amplify and record by recording means any pressure pulse produced as the perturbation means pass the transducers thereby enabling the rate of fluid flow through the device to be measured.

The two ports may be interchangeable. The first and second channels are preferably curved channels. The transducers may take the form of microphones. The perturbation means may comprise fins or depressions in the surface of the rotor or may comprise slots in the surface of the rotor. Preferably the perturbation means comprises two fins. Preferably the recording means are voltage recording means.

By way of example preferred embodiments of the invention will now be described with reference to the accompanying diagrammatic drawings, wherein: Figure 1 is a sectional view of a device for the measurement of the rate of fluid flow, Figure 2 is a circuit diagram applicable to the device of Figure 1, Figure 3 is a modified circuit diagram applicable to a modified form (not shown) of the device of Figure 1, and Figure 4 is a graph illustrating the obtention of voltage values employing the device of Figure 1.

Referring to Figure 1 the device shown comprises a housing 1 with an inlet port 2 and an outlet port 3. The ports 2 and 3 are connected to a fluid, e.g. gas, flow to be measured. Arrow 6 indicates the direction of the fluid flow to be measured. The fluid flow to be measured by the device could be in the opposite direction in which case the inlet port would be port 3 and the outlet port would be port 2; the ports 2 and 3 are interchangeable to correspond to the direction of fluid flow to be measured.

A rotor 4 with hub 5 is mounted in the housing 1.

The rotor 4 is driven by a motor (not shown) in a counter clockwise sense as shown by the curved arrow 7. The rotor 4 carries fins 12 and 13 (these being perturbation means to cause a perturbation in the fluid flow around the rotor 4). Instead of fins 12 and 13 as shown other perturbation means could be adopted, for instance slots or depressions. The actual number of fins 12, 13 (or other perturbation means) is not critical but would normally be even in number and arranged symmetrically around the circumference of the rotor 4. Four fins are recommended for a device to be used for anaesthetic applications, assuming a rotor speed of 2400 r. p. m.

There are two transducers 10, 11 (for instance microphones) symmetrically placed on the housing 1 and communicating with curved channels 8 and 9 formed between the housing 1 and the rotor 4.

Fluid entering port 2 can pass through the device towards the outlet port 3 either via channel 8 or channel 9. Fluid entering channel 9 will be travelling in the same rotational sense as fin 13 and the fluid pressure rise as fin 13 approaches transducer 11 will be minimal.

Fluid entering channel 8 will be travelling in the opposite rotational sense to that of fin 12 and there will result a pressure rise as fin 12 approaches transducer 10.

The transducers 10 and 11 may be connected in series to an AC coupled differential amplifier so that any difference in the pressure pulse produced will be amplified as the fins 12 and 13 pass the transducers 10 and 11. Figure 2 shows a suitable circuit diagram where an even number of fins 12, 13 is provided. 14 indicates a differential amplifier, 15 a rectifier and 16 a voltmeter. The output from the amplifier 14 is rectified by rectifier 15 and the recorded voltage shown by the voltmeter 16 can be represented graphically as shown by Figure 4 to demonstrate a linear relationship, i.e. a linear function of the flow rate through the device, output values (AC volts) being shown on the vertical axis and flow rate values (litres per minute) being shown on the horizontal axis.

Where a device is to be used which is similar to that shown in Figure 1 but is modified in that an odd number of fins (not shown) is provided, a suitable circuit to adopt would be as shown in Figure 3 where 20, 21 denote transducers, 24 denotes amplifiers, 25 denotes rectifiers and 26 denotes a voltmeter. The outputs from the transducers 20, 21 may thus be amplified, rectified and the DC outputs fed to a differential amplifier.

With reference to the device illustrated and particularly described with reference to Figures 1, 2 and 4, the output from the device is a linear function of the flow rate through the instrument. The instrument may employ inexpensive transducers which can be coupled to an amplifier and thus there will be no problems due to DC drift or offset.

Claims

1. A device for the measurement of the rate of fluid flow through a conduit, the device comprising a housing having an inlet port and an outlet port and a rotor disposed between the two ports whereby a first channel is provided between the two ports for subsidiary fluid flow in one rotational sense and a second channel is provided between the ports for a subsidiary flow in the opposite rotational sense, a first transducer arranged on the housing and communicating with the first channel, a second transducer arranged on the housing and communicating with the second channel, the transducers being arranged symmetrically on the housing, the surface of the rotor having perturbation means to cause a perturbation in the fluid flow around the rotor, and the transducers being connected to amplifying means to amplify and record by recording means any pressure pulse

produced as the perturbation means fluid flow through the device to be measured.

2. A device according to Claim 1, wherein the two ports are interchangeable.

3. A device according to Claim 1 or Claim 2, wherein the first and second channels are curved channels.

4. A device according to any preceding claim, wherein the transducers take the form of microphones.

5. A device according to any preceding claim, wherein the perturbation means comprise fins on the surface of the rotor.

6. A device according to Claim 5, wherein the perturbation means comprise two fins on the surface of the rotor.

7. A device according to any one of Claims 1 to 4, wherein the perturbation means comprise depressions in the surface of the rotor.

8. A device according to any one of Claims 1 to 4, wherein the perturbation means comprise slots in the surface of the rotor.

9. A device according to any preceding claim, wherein the recording means are voltage recording means.

10. A device for the measurement of the rate of fluid flow through a conduit substantially as herein described with reference to Figure 1 and one or more of Figures 2 to 4 of the accompanying drawings.

11. Use of the device as claimed in any one of Claims 1 to 10 for measuring the rate of flow of fluids.

12. Use of the device as claimed in any one of Claims 1 to 10 for measuring the rate of flow of gases.