GB2091886A

GB2091886A - Flow meter calibration

Info

Publication number: GB2091886A
Application number: GB8102256A
Authority: GB
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1981-01-26
Filing date: 1981-01-26
Publication date: 1982-08-04

Abstract

In a method for calibrating a fluid flowmeter, e.g. a venturi 10, the flowmeter is connected between a container 12 filled with fluid and a reservoir 17 which is kept at constant pressure. A piston 14 is then introduced at a steady rate into the container to displace liquid through the meter. <IMAGE>

Description

SPECIFICATION Fluid flowmeter calibration The present invention relates to the calibration of fluid flowmeters such as venturis.

There are many instances in science and technology where knowledge of a fluid flow rate is required.

One well known method of measuring such a rate is by passing the flow through a venturi and measuring the pressure drop between the maximum and minimum areas. Venturis can also be used to provide liquid flows at rates which are substantially independent of back pressure, by operation at cavitating condition so that the liquid partly vapourises at a venturi minimum area.

Whilst it should be possible to calculate fluid flow rates from venturi pressure drops using a knowledge of the venturi dimensions and the physical characteristics (such as temperature and viscosity) ofthe fluid, it is normal, in view of the accuracy of measurement usually required, to calibrate venturis.

Calibration of a venturi requires that fluid at known constant physical conditions be passed at a known steady rate through the venturi whilst the pressure drop across the venturi is measured. Measurements are taken at a plurality of flow rates and fluid physical conditions. Maintenance of the steady conditions and flow rate is a demanding task.

According to the present invention a method of calibrating a fluid flowmeter includes the steps of connecting the fluid flowmeter to an outlet of a container filled with fluid, introducing a piston into the container at a steady rate, and measuring the response of the fluid flowmeter.

Preferably fluid passes from the venturi into a sealed reservoir which is maintained at a constant pressure such that fluid can be returned to the container through the fluid flowmeter without the need to disconnect the fluid flowmeter. This is particularly useful when calibrating a fluid flowmeter using dangerous fluids.

It will be realised that this method of calibrating a fluid flowmeter, which is mainly of use for calibrating fluid flowmeters for liquid flows, will require the measurement of liquid properties, such as temperature and pressure, in the container. The flow rate over a given period of time can be calculated to high accuracy from the known physical dimensions of the piston, which can be measured accuratey, and a known steady rate of introduction of the piston.

Steady introduction of the piston can be effected by, for example, a screw drive driven by a constant speed electric motor.

The method of calibration is particularly suitable for a venturi flowmeter, where the response of the meter is the pressure drop across the venturi.

Apparatus and calibration for calibrating a venturi fluid flowmeter according to the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawing.

Aventuri 10 to be calibrated is connected to an outlet 11 of a container 12 filled with liquid 13. A piston 14 projects into the container 12 and is connected, after passing through seals 24 and a box 23, to a drive shaft 15 drivable by a constant speed drive 16. The box 23 feeds into a calibrated closed tube 25.

The outlet of the venturi 10 leads to a sealed reservoir 17 containing gas 18 maintained at a constant pressure via a constant pressure supply or exhaust valve 19. Liquid 13 pressure and temperature at the outlet 11 are measured by gauges 20, 21 respectively, and a differential pressure gauge 22 is connected between the maximum and minimum areas of the venturi 10.

In use the constant speed drive 16 drives, via the drive shaft 15, the piston 14 into the container 12, so displacing liquid 13 through the venturi 10 into the reservoir 17, in which the back pressure is kept constant by valve 19 and gas 18. The pressure drop across the venturi 10 is measured by the differential pressure gauge 22 and liquid 13 pressure and temperature on entry to the venturi 10 is measured by gauges 20, 21. The liquid flow rate can be calculated knowing the dimensions of piston 14 and the drive rate of drive 16 and drive shaft 15. If necessary, the liquid flow rate can be corrected for any leakage from container 12 past a seal 24 to the box 23 using the calibrated tube 25.

The process is repeated for a plurality of constant speed drive 16 rates and, if required, for a plurality of temperatures 20 and back pressures 18, 19, so giving a calibration for the venturi 10.

After full travel of piston 14 into the container 12 liquid 13 can be returned to the container, to continue the calibration, without removing venturi 10, by withdrawing the piston 14. The return process can be assisted by varying the pressure of gas 18 in reservoir 17. By suitable dimensioning of the apparatus it can be arranged that, with the piston 14 substantially withdrawn from container 12, liquid is withdrawn from the venturi 10 to leave it substantial- ly empty. These facilities are especially useful when a venturi is being calibrated for use with a dangerous liquid such as, for example, toxic or flammable liquids.

It will be realised that the apparatus as described above may in practice require the presence of several items such as filling means for container 12, flow on/off control in outlet 11, and perhaps a liquid/no gas valve in outlet 11. Such items are well known in the relevant arts, do not form part of the invention, and have therefore, in order to simplify understanding of the invention, not been illustrated or described herein.

It will also be realised that while the method and apparatus have been described as applied to a venturi flowmeter, they are also of use for the calibration of other types of flowmeter.

CLAIMS (Filed 8 Jan 82) 1. A method of calibrating a fiuid flowmeter including the steps of connecting the fluid flowmeter to an outlet of a container filled with fluid, introducing a piston into the container at a steady rate, and measuring the response of the fluid flowmeter.

2. A method as claimed in claim 1 wherein the fluid is a liquid.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Fluid flowmeter calibration The present invention relates to the calibration of fluid flowmeters such as venturis. There are many instances in science and technology where knowledge of a fluid flow rate is required. One well known method of measuring such a rate is by passing the flow through a venturi and measuring the pressure drop between the maximum and minimum areas. Venturis can also be used to provide liquid flows at rates which are substantially independent of back pressure, by operation at cavitating condition so that the liquid partly vapourises at a venturi minimum area. Whilst it should be possible to calculate fluid flow rates from venturi pressure drops using a knowledge of the venturi dimensions and the physical characteristics (such as temperature and viscosity) ofthe fluid, it is normal, in view of the accuracy of measurement usually required, to calibrate venturis. Calibration of a venturi requires that fluid at known constant physical conditions be passed at a known steady rate through the venturi whilst the pressure drop across the venturi is measured. Measurements are taken at a plurality of flow rates and fluid physical conditions. Maintenance of the steady conditions and flow rate is a demanding task. According to the present invention a method of calibrating a fluid flowmeter includes the steps of connecting the fluid flowmeter to an outlet of a container filled with fluid, introducing a piston into the container at a steady rate, and measuring the response of the fluid flowmeter. Preferably fluid passes from the venturi into a sealed reservoir which is maintained at a constant pressure such that fluid can be returned to the container through the fluid flowmeter without the need to disconnect the fluid flowmeter. This is particularly useful when calibrating a fluid flowmeter using dangerous fluids. It will be realised that this method of calibrating a fluid flowmeter, which is mainly of use for calibrating fluid flowmeters for liquid flows, will require the measurement of liquid properties, such as temperature and pressure, in the container. The flow rate over a given period of time can be calculated to high accuracy from the known physical dimensions of the piston, which can be measured accuratey, and a known steady rate of introduction of the piston. Steady introduction of the piston can be effected by, for example, a screw drive driven by a constant speed electric motor. The method of calibration is particularly suitable for a venturi flowmeter, where the response of the meter is the pressure drop across the venturi. Apparatus and calibration for calibrating a venturi fluid flowmeter according to the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawing. Aventuri 10 to be calibrated is connected to an outlet 11 of a container 12 filled with liquid 13. A piston 14 projects into the container 12 and is connected, after passing through seals 24 and a box 23, to a drive shaft 15 drivable by a constant speed drive 16. The box 23 feeds into a calibrated closed tube 25. The outlet of the venturi 10 leads to a sealed reservoir 17 containing gas 18 maintained at a constant pressure via a constant pressure supply or exhaust valve 19. Liquid 13 pressure and temperature at the outlet 11 are measured by gauges 20, 21 respectively, and a differential pressure gauge 22 is connected between the maximum and minimum areas of the venturi 10. In use the constant speed drive 16 drives, via the drive shaft 15, the piston 14 into the container 12, so displacing liquid 13 through the venturi 10 into the reservoir 17, in which the back pressure is kept constant by valve 19 and gas 18. The pressure drop across the venturi 10 is measured by the differential pressure gauge 22 and liquid 13 pressure and temperature on entry to the venturi 10 is measured by gauges 20, 21. The liquid flow rate can be calculated knowing the dimensions of piston 14 and the drive rate of drive 16 and drive shaft 15. If necessary, the liquid flow rate can be corrected for any leakage from container 12 past a seal 24 to the box 23 using the calibrated tube 25. The process is repeated for a plurality of constant speed drive 16 rates and, if required, for a plurality of temperatures 20 and back pressures 18, 19, so giving a calibration for the venturi 10. After full travel of piston 14 into the container 12 liquid 13 can be returned to the container, to continue the calibration, without removing venturi 10, by withdrawing the piston 14. The return process can be assisted by varying the pressure of gas 18 in reservoir 17. By suitable dimensioning of the apparatus it can be arranged that, with the piston 14 substantially withdrawn from container 12, liquid is withdrawn from the venturi 10 to leave it substantial- ly empty. These facilities are especially useful when a venturi is being calibrated for use with a dangerous liquid such as, for example, toxic or flammable liquids. It will be realised that the apparatus as described above may in practice require the presence of several items such as filling means for container 12, flow on/off control in outlet 11, and perhaps a liquid/no gas valve in outlet 11. Such items are well known in the relevant arts, do not form part of the invention, and have therefore, in order to simplify understanding of the invention, not been illustrated or described herein. It will also be realised that while the method and apparatus have been described as applied to a venturi flowmeter, they are also of use for the calibration of other types of flowmeter. CLAIMS (Filed 8 Jan 82)

1. A method of calibrating a fiuid flowmeter including the steps of connecting the fluid flowmeter to an outlet of a container filled with fluid, introducing a piston into the container at a steady rate, and measuring the response of the fluid flowmeter.

2. A method as claimed in claim 1 wherein the fluid is a liquid.

3. A method as claimed in claim 2 wherein the liquid passes into a sealed reservoir maintained at a constant pressure.

4. A method as claimed in claim 3 including the step of returning liquid from the reservoir to the container by withdrawing the piston from the container.

5. A method as claimed in any one of claims 2 to 4 wherein the temperature and pressure of the liquid in the container are measured.

6. A method as claimed in any one of claims 2 to 5 including means for measuring leakage from the container past the piston.

7. A method as claimed in claim 6 wherein the leakage is measured by a calibrated tube.

8. A method as claimed in any one of claims 1 to 7 wherein the piston is operated by a screw drive.

9. A method as claimed in any one of claims 1 to 8 wherein the piston drive means includes a constant speed electric motor.

10. A method of calibrating a venturi flowmeter as claimed in any one of claims 1 to 9.

11. Apparatus for calibrating a fluid flowmeter including a container, a piston, means for introducing the piston into the container at a steady rate, and means for connecting the flowmeter to an outlet from the container.

12. Apparatus as claimed in claim 12 including a reservoir, means for connecting the reservoir to a downstream end of a flowmeter and means for maintaining a constant pressure within the reservoir.

13. Apparatus as claimed in claim 11 or in claim 12 including temperature measuring means.

14. Apparatus as claimed in any one of claims 11 to 13 including pressure measuring means.

15. A method of calibrating a fluid flowmeter substantially as herein described.

16. Amethod of calibrating afluidflowmeter substantially as herein described with reference to the accompanying drawing.

17. Apparatus for calibrating a fluid flowmeter substantially as herein described.

18. Apparatus for calibrating a fluid flowmeter substantially as described with reference to the accompanying drawing.