GB2147107A - Gas flow measurement - Google Patents

Abstract

Apparatus for measuring a gas flow comprises a reservoir (36) and a calibrated chamber (35) interconnected together, the assembly of reservoir and calibrated chamber being rotatable to a first position in which liquid in the reservoir passes to the calibrated chamber. In second different position (as shown) gas is passed into the calibrated chamber via pipe (27) and aperture (42), the amount of gas being measured by the change in the level in the calibrated chamber with time. The apparatus is used to measure very low flow rates past a needle valve which may be adjusted to provide a predetermined flow in leak testing systems. <IMAGE>

Description

SPECIFICATION Measuring apparatus The present invention relates to measuring apparatus for measuring a flow of gas. The measurement of the gas flow may be used in turn to measure, for example, pressure, or the integrity of sealed containers, or, in connection with the last measured use, to calibrate standard "leaks".

Thus although the apparatus of the invention has widespread use we will hereafter confine the description to a particular preferred use.

The integrity of a sealed container may be measured by pressurising the container and detecting, by means of a pressure measuring device, any drop in the pressure or flow of gas to the container. In practice, some degree of leakage is allowable.

In order to check and calibrate the pressure measuring device, there may be added into the system a standard "leak". With the standard leak connected to the system, the operation of the pressure measuring device may be calibrated.

However there is a constant problem facing quality control engineers in the establishment and maintenance of suitable standard "leaks". In the past various attempts have been made to create standard leaks using very fine orifices in various forms or, alternatively, sintered or porous materials, such as ceramics. Whilst these products may be useful initially, they all suffer from two inherent defects, they require calibration at the point of manufacture against a master standard and they are damaged or destroyed by dirt or liquid. Thus they are, at best, only transfer standards requiring retesting and certification and they are not durable.

A further problem arises when different specifications are required since these will require different standard leaks.

The present invention provides, according to one aspect, means for measuring a gas flow comprising means connectable to the gas source, an assembly comprising a reservoir and a calibrated chamber interconnected together, said reservoir and calibrated chamber containing liquid, the reservoir and calibrated chamber being arranged with respect to each other so that when the assembly is moved from a first normal position to a second refill position liquid in the reservoir passes to the calibrated chamber, and when the assembly is returned to the first position the calibrated chamber is filled with liquid above a first predetermined level, means being provided to pass the gas into the calibrated chamber when said assembly is in said first position, a second predetermined level being provided below said first predetermined level, the volume in said calibrated chamber between said first and second predetermined levels being known, whereby, in use, said assembly is moved from said second position to said first position and gas is passed into said calibrated chamber and the time taken for the level of liquid in said calibration chamber to move from said first predetermined level to said second predetermined level is measured to determine the gas flow.

The means connectable to the gas source may include a flow restrictor, which may comprise a standard leak.

The standard leak may comprise a needle valve which may be adjusted to as to provide an adjustable standard leak.

The calibration chamber may have a transparent front surface and the first and second predetermined levels may be indicated on the transparent front.

The assembly may comprise a block of material machined, moulded or otherwise formed to provide the reservoir and calibrated chamber which may be interconnected at their lower ends.

The block may have a front surface of transparent material such as perspex.

The assembly may be pivotally mounted to a suitable support so as to be pivotable between the first and second positions. There may be provided a gas exhaust above the reservoir.

When said assembly is in said first position the gas may be passed to said calibration chamber below said second predetermined level in which case the liquid within the reservoir and calibrated chamber is preferably one which does not create any froth on its surface when bubbles are passed through it and may, for example, be of high surface tension.

A preferred arrangement of the invention will now be described by way of example only and with reference to the accompanying drawings in which: Figure 1 is a front perspective view of an apparatus according to the invention, Figure 2 is a rear view of the apparatus of Fig. 1, Figure 3 is a cross section through part of the apparatus of Fig. 1 on the line 3-3, Figure 4 is a diagram showing a system in which the apparatus of the invention may be used, and, Figure 5 is a perspective view of an alternative arrangement of part of the apparatus according to the invention.

Referring firstly to Fig. 4 there is illustrated by way of example an apparatus for testing the integrity of sealed objects, for example headlamps or gas appliances. The object under test is indicated at 10 and a pressure pipe 11 is connected to the object 10. In a manufacturing process the object 10 may be moved to the pressure pipe 11 automatically and connected with it. The pressure pipe connects the object 10 with a source of pressure 1 2 and a pressure measuring device 1 3 measures the pressure and hence the leakage gas flow rate in the system.

The manufacture will generally specify the extent to which the object under test is leakproof and by having a very sensitive pressure measuring device 1 3 any leak in the object 10 can be detected by a reduction of the pressure in the system in a very short time or by the gas flow rate (also detected by a suitable pressure measuring device 1 3). For example, in the case of testing the integrity of a gas appliance the criteria required might be that the leakage in the system should not be greater than 85 cc per hour under a pressure of 40 mb.

Clearly it is necessary to calibrate the pressure measuring device 13 when the apparatus is first installed and also at regular intervals thereafter. A preferred way of calibrating the apparatus is to connect to the pressure system a standard leak which complies with the maximum size of leak that the manufacturer of the object can tolerate. The leak is provided in the apparatus indicated at 14 and with the standard leak connected through the valve 16 to the system, the pressure measuring device 1 3 can be calibrated or checked.

The apparatus 14 is illustrated in more detail in Figs. 1, 2 and 3. Referring particularly to Figs. 1 and 2, the valve 16 is connected to a connector 21 and thence via a tube 22 to the standard leak which may be in the form of a taper needle valve 23. The taper needle valve may comprise a standard fine taper 1" needle valve. As is well known, the needle valve seats in a bore and by movement of the needle valve towards and away from the bore, the aperture through which gas may pass is varied. The movement is controlled by means of a screw vernier 24 with a scale 26 which indicates the extent to which the needle is moved towards and away from the bore.

The needle valve is thereafter connected by means of tube 27 to an integral assembly 28.

The various parts, including the needle valve 23 and integral assembly 28 are mounted on a support plate 29 which in turn is mounted on a base 31.

The assembly 28 is of a moulded plastics material or machined metal and comprises a calibrated chamber 35 and a reservoir 36 moulded into or cut from the solid block and being closed by a front transparent perspex (R.T.M.) plate 37 which sealingly engages with the block material. The calibrated chamber 35 and reservoir 36 are interconnected at their lower ends by an interconnection 38.

The depth (ie the distance from front to rear) of the reservoir 36 is much greater than that of the calibrated chamber 35 as is clear from Fig. 3. Also formed within the material of the assembly 28 is a bore 41 which passes from the top surface of the assembly 28 where it interconnects with the flexible tube 27 via a connector, the bore 41 passing down and connecting with a horizontal bore 42 which passes into the calibrated chamber 35 adjacent its lower end. Above the bore 42 there are lines marked on the perspex cover 37 across the calibrated chamber 35 which indicate a first predetermined level 43 adjacent the top of the calibrated chamber and a lower second predetermined level 44 towards the bottom of the calibrated chamber.

A liquid such as 3M's Fluorinert (RTM) is provided in the reservoir and calibrated chamber.

The upper end of the reservoir 36 includes a closable gas outlet 46.

The integral assembly 28 is mounted by means of a pin 47 to the support plate 29 in such a manner that it may rotate about the pin 47 from the first position shown in Fig. 1 to a second position at right angles thereto, that is, with the upper surface shown in Fig. 1 to the left.

A conventional stop watch 48 is also conveniently attached to the support plate 29.

In use of the apparatus of Figs. 1 to 3 in the system shown in Fig. 4 when it is desired to test the pressure measuring apparatus 13, the taper needle valve 23 is set by means of the vernier scale 26 to the desired standard leak, the valve 1 6 opened, and then gas under pressure from the system passes through the tube 22, needle valve 23, tube 27, through the chamber 35 and reservoir 36 and out of the outlet 46.

As mentioned before from time to time it is necessary to calibrate the taper needle valve 23 itself and in these circumstances, before applying the gas under pressure from the system in the above described manner the assembly 28 is rotated about the pin 47 so that the top surface is vertical and to the left in Fig. 1, in which circumstance liquid from the reservoir 36 passes into the calibrated chamber 35 and all of the air from the calibrated chamber 35 is exhausted into the reservoir 36. The assembly 28 is then rotated back to the position shown in Fig. 1 under which circumstance the calibrated chamber 35 will be full of liquid. When the gas pressure is applied through the taper needle valve 23 the gas will pass through the tube 27 down the bore 41, along the bore 42 and up through the calibrated chamber 35 so that the level of liquid in the calibrated chamber 35 begins to fall. The stop watch 48 may be used to time the interval between the level of the liquid passing from the first level 43 to the second level 44. The volume between these two levels is arranged to be a standard volume, for example 1 cc and the time measurement thereby provides an indication of the size of the standard leak provided by the taper needle valve 23.

The liquid in the assembly is of a type such that the bubbles will break very rapidly at the surface of the liquid so that a clean meniscus is formed at the transparent front window.

The invention is not restricted to the details of the foregoing example. As already described, the apparatus of Figs. 1 to 3 can be used in a variety of circumstances and the arrangement described with respect to Fig. 4 is given by way of example only.

Fig. 5 illustrates an alternative form of the block of the assembly 28, partly cut away, and in particular has a different shaped reservoir 36.

Claims (11)

1. Apparatus for measuring a gas flow comprising means connectable to the gas source, an assembly comprising a reservoir and a calibrated chamber interconnected together, said reservoir and calibrated chamber containing liquid, the reservoir and calibrated chamber being arranged with respect to each other so that when the assembly is moved from a first normal position to a second refill position liquid in the reservoir passes to the calibrated chamber, and when the assembly is returned to the first position the calibrated chamber is filled with liquid above a first predetermined level, means being provided to pass the gas into the calibrated chamber when said assembly is in said first position, a second predetermined level being provided below said first predetermined level, the volume in said calibrated chamber between said first and second predetermined levels being known, whereby, in use, said assembly is moved from said second position to said first position and gas is passed into said calibrated chamber and the time taken for the level of liquid in said calibration chamber to move from said first predetermined level to said second predetermined level is measured to determine the gas flow.

2. Apparatus as claimed in claim 1 in which the means connectable to the gas source may include a flow restrictor.

3. Apparatus as claimed in claim 2 in which the flow restrictor comprises a standard leak.

4. Apparatus as claimed in claim 3 in which the standard leak comprises a needle valve which may be adjusted to as to provide an adjustable standard leak.

5. ApparatusCas claimed in any of claims 1 to 4 in which the calibration chamber has a transparent front surface and the first and second predetermined levels are indicated on the transparent front.

6. Apparatus as claimed in any of claims 1 to 5 in which the assembly is formed from a block of material to provide the reservoir and calibrated chamber which are interconnected at their lower ends.

7. Apparatus as claimed in claim 6 in which the block has a front surface of transparent material.

8. Apparatus as claimed in any of claims 1 to 7 in which the assembly is pivotally mounted to a suitable support so as to be pivotable between the first and second positions.

9. Apparatus as claimed in any of claims 1 to 7 in which there is provided a gas exhaust above the reservoir.

10. Apparatus as claimed in any of claims 1 to 9 in which the liquid within the reservoir and calibrated chamber is of high surface tension so that it does not create any froth on its surface when bubbles are passed through it.

11. Apparatus for measuring gas flow as claimed in claim 1 substantially as hereinbefore described.