WO1988007712A1

WO1988007712A1 - Standard gas flow apparatus

Info

Publication number: WO1988007712A1
Application number: PCT/GB1988/000253
Authority: WO
Inventors: Joseph Terence Furness; Henri Rosenberg
Original assignee: Furness Controls Ltd
Priority date: 1987-04-02
Filing date: 1988-03-31
Publication date: 1988-10-06
Also published as: FI885567A; GB8707881D0; EP0310642A1; FI885567A0; JPH01503562A

Abstract

Apparatus for providing a standard fluid flow comprising a closed chamber (10), a fluid aperture (12) for fluid to pass out of said closed chamber (10), a piston means (17), the outer diameter of the part of the piston means (17) which extends into the closed chamber (10) having been measured at many points by means traceable to a National or International Standard, and means for moving said piston means (17) into said closed chamber (10) to thereby expel fluid through said fluid aperture (12), said means for moving said piston means (17) comprising motor means (23) controlled by a standard frequency or time period signal traceable to a National or International Standard.

Description

STANDARD GAS FLOW APPARATUS

The pr esent i nvent i on re l a t e s to s t andard f l u i d f low apparatus .

In the past, one way to provide a standard fluid flow has been by a piston and a cylinder apparatus, the piston being moved along the cylinder to provide a known fluid flow. Difficulties, however, have arisen, in that it is difficult to measure accurately the inside diameter of the bore of the cylinder to provide the volumetric measurements and it is difficult to measure accurately the rate of movement of the piston.

It is also important in establishing a primary standard, as opposed to a secondary or transfer standard that all critical measurements can in some way, be related to traceable standards, for example, standards provided by a National Laboratory, such as the UK National Physical Laboratory which are, in turn, traceable to known International Standards. Indeed, it is not unknown for standard fluid flow apparatus in different countries to provide different results because of the lack of traceability of the standard fluid flow apparatus back to National and International Standards. The National Standards are in fact compared with one another so as to maintain accuracy on an international basis.

The present invention provides a standard fluid flow apparatus comprising:

a closed chamber ; a first aperture in said closed chamber for fluid to pass into or out of said closed chamber;

a second aperture in said chamber;

a piston means extending through said second aperture;

the outer diameter of the part of the piston means which extends into the closed chamber having been measured at many points by means traceable to a National or International Standard;

the second aperture mounting a fluid seal means which seals with the piston;

means being provided to measure the rate of movement of the piston means in such a way that said movement is traceable to National or International Standards;

whereby movement of the piston means into the closed chamber expels a known quantity of fluid from the closed chamber via the first aperture and movement of the piston in a direction out of the closed chamber causes a known quantity o£ fluid to pass into the closed chamber via said first aperture.

Because the seal is carried by the closed chamber or cylinder, rather than by the piston, in order to measure the volume of fluid passing through the fluid aperture, it is necessary to measure the outer diameter of the piston means rather than, as in previous instances the internal diameter of the cylinder. Measuring internal diameters is difficult and is less accurately traceable to national standards. In the present instance, the outer diameter of the piston can be measured simply by measuring means such as micrometer gauges which can be checked against a national standard by means of slip gauges. The diameter of the piston at each point along its length and at each point around its circumference can be measured accurately and traced back to the national standards .

According to a further aspect the invention also provides apparatus for providing a standard fluid flow comprising a closed chamber, a fluid aperture for fluid to pass out of said closed chamber, a piston means, the outer diameter of the part of the piston means which extends into the closed chamber having been measured at many points by means traceable to a National or International Standard, and means for moving said piston means into said closed chamber to thereby expel fluid through said fluid aperture, said means for moving said piston means comprising motor means controlled by a standard frequency or time period signal traceable to a National or International Standard. The standard frequency or time period signal is preferably a nationally transmitted radio frequency signal, and in the UK, comprises a standard 200 kHz radio signal from Droitwich. In this way movement of the piston is controlled by a traceable national standard.

In a preferred arrangement, the standard frequency is received by a radio receiver, is divided by a computer, and the output pulses of the computer are passed to an electric stepping motor which drives the piston means through an accurate lead screw. Preferred arrangements of the invention will now be described by way of example only and with reference to the accompanying drawings in which

Figure 1 shows in diagra atic form a standard fluid flow apparatus in which a cylinder and piston parts are stiown in axial section, and

Figure 2 shows a modified version of the apparatus of Figure 1.

The apparatus comprises a chamber in the form of a cylinder 10, one closed end wall 11 of which includes a fluid inlet/outlet 12. Mounted within the cylinder 10 is a piston 13 the outer diameter of the piston 13 being slightly smaller than the inner diameter of the cylinder 10 and the piston 13 is sealed with respect to the cylinder 10 by means of a seal 14 mounted in the cylinder 10 adjacent its end opposite the closed end wall 11.

The piston 13 comprises a hollow cylinder having closed end walls 17,18, the closed end wall 18 disposed outside the cylinder 10 mounting a threaded nut 19 which engages with a lead screw 21» The lead screw 21 can be rotated by means of a gear box 22 driven by an electric stepper motor- 23. The stepper motor 23 is controlled by means of electric pulses along line 24 and provided by a stepper motor control 26, the stepper motor control being controlled by a computer 27 via line 28. The computer 27 receives pulsed inputs from an RF (radio frequency) receiver 28 along line 29, the radio receiver 28 receiving an RF carrier frequency of a known frequency from a transmitter 31.

The seal 14 itself comprises, within an annular groove 32 in the cylinder wall, two axially spaced O-rings 33,34 with an annular spacer 36 mounted therebetween, the gap between the 0-rings 33,34 and the adjacent wall of the piston 13 being filled with oil. A screw bore extends vertically upwards through the cylinder wall and is closed by a nut 37 through which the level of oil within the seal 14 can be noted or alternati ely the nut 37 may be replaced by a sight glass. A seal check unit 31 is a pressure sensor connected to the oil filled cavity to detect any ingress of fluid from the cylinder or egress of oil from the cavity.

The lead screw 21 and nut 19 are accurately manufactured to a known measurable tolerance. A good quality lead screw has an accuracy of better than +/- 0.025mm per 300mm length which is less than 0.01% error.

Before use, the outer diameter of the piston 13 is measured at various points along its length and at various points around its periphery. This can be done by means of a micrometer which has in turn been checked, at the size of the diameter of the piston, against slip gauges. The slip gauges themselves have been measured against a national standard, in the case of the UK, the standard provided by the National Physical Laboratory which in turn is traceable to an International Standard.

It is thus possible to very accurately determine the cross sectional area of the piston at all points along its length by means of a simple set of measurements with a readily available micrometer.

Piston/cylinder assemblies of various sizes are required to cover a flow range of from O.lcc/ in to typically 20L/min although larger flow ranges can equally be covered. The diameter of these different piston sizes can be a measured to a very high degree of accuracy. Typically for smaller pistons, say of 6mm diameter the accuracy is plus or minus 0.0025mm and for pistons of, say, 25mm diameter the accuracy is plus or minus 0.005mm. Greater accuracy can be achieved. The maximum cross sectional errors can easily be maintained to the following figures:

+/-0.08% for a 6mm piston +/-0.04% for a 12mm piston +/-0.02% for a 24mm piston +/-0.02% for a 50mm piston +/-0.01% for a 100mm piston +/-0.01% for a 200mm piston

As has already been mentioned, it is desired to control motion of the piston 30 in accordance with a traceable national standard. One way of doing this is to utilise the 200 kHz (1500m) radio signal transmitted by the BBC from Droitwich. The carrier frequency of this signal is maintained at 200 kHz to an accuracy of better than +/- 10 . This carrier signal is received by the radio receiver 28 and is passed to the computer 27. The computer 27 includes a dividing apparatus which divides the frequency of the received radio signal down to provide a pulsed output being a multiple of the received radio frequency, the pulsed output being passed along line 28 to the stepper motor control 26 and thence along line 24 to the stepper motor 23.

Thus the stepper motor 23 moves at a speed which is determined accurately by the frequency of the 200 kHz radio signal from the BBC. The stepper motor drives through a gear box the lead screw 21 (although it may in some circumstances be possible to eliminate the gear box) and thus, from an accurate measurement of the lead screw, one is able to determine the rate of movement of the piston 13 in accordance with the frequency received from the BBC. The exact volume at any time passing out of the cylinder 10 via the fluid outlet 12 can be measured, since not only is the speed at which the piston 13 is moving known, but also its exact diameter at any particular time as predetermined by measuring its diameter.

The barometric pressure change can be measured by measuring the pressure within the cylinder 10.

The best accuracy achievable by this apparatus so far is +/- 0.11% for flows of the range of 1 to 10 cc/min and +/- 0.04% for lOL/min.

Apparatus was provided for flow checking up to 20L/min and this comprised of a single stepper motor 23 driving 6 lead screw/piston units. The lead screw length in each case was lm long. The table below shows a flow capability for various motor speeds. Piston Piston movement per min Diameter

mm 50 100 200 400

6 1.414 2.83 5.65 11.31 cc/mi n

12 5 .655 11.31 22 .62 45.24 cc/min

24 22 .619 45 .24 90 .48 180 .96 cc/min

50 98.175 196.35 392 . 70 785 .40 cc/min

100 392 . 70 785 .40 1570 .80 3141 . 60 cc/mi n

200 1570.8 3141 .6 6283 . 2 12566.4 cc/mi n

To check for loss of fluid, particularly gas, into the seal 14, the oil between the two oil rings 33,34 is regularly checked for gas bubbles. That can be done by removing the screw 37 or by examining a sight glass.

The apparatus may be used for providing a measured rate of flow- of fluid, but in the case of the fluid being gas, can conveniently be attached to apparatus described in our copending British Patent Application 8616157.

Other frequency standards may be used instead of receipt of the radio frequency from the transmitter 29. For example, standard frequencies or periods of time provided over a telephone line may be utilised.

The effect of temperature can be easily controlled by mounting the complete apparatus in a sealed environment. The ambient temperature can be measured to +/- 0.2°C. Such a small change of temperature does not particularly affect the pressure changes of the fluid, because of the large thermal mass of the cylinder and piston.

Figure 2 shows an alternative arrangement of apparatus. In this case, the apparatus thus far described is mounted within a sealed compartment 50. The sealed compartment may be made of a material resistent to pressure, for example, thick metal such as steel and includes a port 52 through which the pressure within the closed chamber may be varied. To provide a closed system, the fluid outlet 12 passes out through a closed bore in the sealed compartment 51, and is passed to a remote location, and the fluid is then returned to the sealed compartment 51 via a port 53. In this way, by the use of a closed system, gases other than air, such as argon might be used.

With the arrangement described, the pressure within the sealed compartment may be varied and may be reduced or increased as desired for a particular measurement.

The i nven t i on i s not l i m i t ed to the det a i l s of the foregoing example.

Claims

1. A standard fluid flow apparatus comprising:

a closed chamber;

a first aperture in said closed chamber for fluid to pass into or out of said closed chamber;

a second aperture in said chamber;

a piston means extending through said second aperture;

the second aperture mounting a fluid seal means which seals with the piston;

whereby movement of the piston means into the closed chamber expels a known quantity of fluid from the closed chamber via the first aperture and movement of the piston in a direction out of the closed chamber causes a known quantity of fluid to pass into the closed chamber via said first aperture.

2. Apparatus as claimed in claim 1 in which the closed chamber is provided by a cylinder.

3. Apparatus as claimed in claim 1 or claim 2 in which motor means is provided to move said piston means into or out of said closed chamber.

4. Apparatus as claimed in claim 3 in which said motor means is controlled by a standard frequency or time period signal.

5. Apparatus as claimed in claim 4 in which said motor means is controlled by a standard radio signal.

6. Apparatus as claimed in any of claims 4 or 5 in which the motor means comprises a stepper motor controlled by said standard frequency signal.

7. Apparatus as claimed in claim 6 in which there is provided a divider means for dividing the standard frequency signal before applying said frequency signal to said stepper motor.

8. Apparatus as claimed in claim 6 or claim 7 in which said stepper motor drives said piston means through a gear box and lead screw.

9. Apparatus as claimed in any of claims 1 to 8 in which said closed chamber, said piston, and said motor means is mounted within a sealed compartment.

10. Apparatus as claimed in any of claims 1 to 9 in which said seal comprises an annular groove in said closed chamber around said second aperture, said annular groove mounting two spaced "0" rings with liquid therebetween.

11. Apparatus as claimed in claim 10 in which means is provided to indicate loss of liquid between said "O" rings.

12. Apparatus for providing a standard fluid flow comprising a closed chamber, a fluid aperture for fluid to pass out of said closed chamber, a piston means, the outer diameter of the part of the piston means which extends into the closed chamber having been measured at many points by means traceable to a National or International Standard, and means for moving said piston means into said closed chamber to thereby expel fluid through said fluid aperture, said means for moving said piston means comprising motor means controlled by a standard frequency or time period signal traceable to a National or International Standard.

13. Apparatus as claimed in claim 12 including a radio receiver for receiving the standard frequency, computer means to decide the received standard frequency signal and means to pass the output pulses of computer to an electric stepping motor which drives the piston means through an accurate lead screw.