GB2082778A

GB2082778A - Volume Measuring Apparatus

Info

Publication number: GB2082778A
Application number: GB8125554A
Authority: GB
Original assignee: Analytical Instruments Ltd
Current assignee: Analytical Instruments Ltd
Priority date: 1980-08-22
Filing date: 1981-08-21
Publication date: 1982-03-10
Also published as: GB2082778B

Abstract

A measure of an unknown volume V of, for example, a cavity is achieved utilising a pressure transducer 36 which monitors the rate of pressure rise in the unknown volume when gas from pressure regulator 30 is introduced into that volume, via valve 34. Where the flow of gas can be kept constant, by constant mass flow controller 32, an output signal representative of the unknown volume V is achieved by differentiating and inverting the output of the pressure transducer. Alternatively, a variable gas flow can be measured and an output signal representative of the unknown volume achieved by dividing the measured flow by the rate of pressure increase as monitored by the transducer. <IMAGE>

Description

SPECIFICATION Volume Measuring Apparatus The present invention relates to apparatus for measuring volume and in particular for measuring the internal volume of a component, tank or cavity.

A known technique for measuring an unknown internal volume of a cavity, tank or component involves the filling of two identical reservoirs to the same pressure and subsequently connecting one of these reservoirs to a known reference volume and the other to the unknown volume.

The charge pressure is shared in each case between the reservoir and the connected volume.

If the unknown and references volumes are identical, then the final pressures are also identical. However, if one exceeds the other then the difference in pressure can be used to provide a measure of the difference between the known and unkown volumes. For example, the difference in pressure can be measured by a differential pressure transducer whose output provides an indication of the unknown volume.

A problem in practice with the latter technique is that the total test cycle takes many seconds and possibly minutes to perform, firstly as a result of the time necessary to fill the reservoirs and secondly to reach equilibrium after the known and unknown volumes have been connected to the reservoirs. Such a technique is therefore not suited to production line testing, as for example in an internal combustion engine production line for measurement of cylinder volume.

An object of the present invention is to provide a technique for volume measurement, and an apparatus for carrying out that technique, which can be performed much more rapidly than the above described known method.

In accordance with the present invention, a measure of an unknown volume is achieved utilising the rate of pressure rise in the unknown volume when a gas flow is introduced into that volume under conditions such that a negligible temperature change occurs and that the pressure rise in the unknown volume is small during the period of the test.

The invention provides an apparatus for measuring the internal volume of a component, tank or cavity in which a flow of gas is introduced into the volume and the rate of pressure increase is measured by a pressure transducer and wherein either (a) the flow of gas is kept constant and the output of the pressure transducer is differentiated and inverted to provide an output signal representative of said volume or (b) the flow is not held constant but is measured and divided by the differentiated output of the transducer (the rate of pressure increase) to provide the output signal representative of said volume.

In one embodiment wherein the flow rate is not maintained constant, the flow rate is measured by a first differential pressure transducer across the linear restriction and the rate of change of pressure is measured by the differential output of a second differential pressure transducer connected between the unknown volume and a reference pressure, such as atmospheric.

In another embodiment, the pressure upstream of the linear restriction is maintained constant, the flow rate being measured by the direct output of a differential pressure transducer across the linear restriction and the rate of change of pressure being measured by the differential output of that transducer.

Preferably, however, the flow is maintained constant so that its magnitude need not be measured at all.

The principal advantage of the present technique is that it can be performed extremely quickly, in a matter of a few seconds, and independently of any reservoirs or reference volumes. It is therefore readily suited to production line measurement of volume, as in the above quoted example of cylinder volume measurement in internal combustion engine production.

The invention is described further hereinafter, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a schematic diagram denoting a constant flow into a volume through a small restrictor and used to explain the technique in accordance with the invention; Fig. 2 is a schematic diagram illustrating one embodiment of a volume measuring apparatus in accordance with the present invention; Fig. 3 is a schematic diagram illustrating a second embodiment of a volume measuring apparatus in accordance with the present invention; Fig. 4 is a schematic diagram illustrating a third embodiment of a volume measuring apparatus in accordance with the present invention; Fig. 5 is a schematic diagram of the electronic operating system for the embodiment of Fig. 4; and Fig. 6 is a schematic diagram of a still further embodiment in accordance with the invention.

The explanation of the operation of an apparatus in accordance with the present invention is based on Boyle's Law which states that, for a given mass of gas, the product of pressure and volume is constant, i.e.

P. V.=constant (i) Differentiating equation (i) gives dv dp P +V- =O (ii) dt dt Hence dv V=-P dt (iii) dp dt With reference to Fig. 1, a constant flow F is introduced through a very small restrictor r into a volume V, the pressure upstream of the restriction r being P, and downstream of the restriction r being P, both of which are a function of time. The restriction r can be arranged so that the pressure drop (P1-P) is linearly related to flow, i.e.

F=K (P,-P) (iv) where K is a constant. If the correct units are chosen then dv (v? dt Substituting (v) into (iii) gives P.F.

V= dp (vi) dt and substituting F form (iv) gives P.K. (P1-P) V= (vii) dp dt If at time t=0, the pressure within the volume V is always atmospheric and if the test measurement is performed very rapidly after connection of the flow to the volume by measuring both (P,-P) and dp dt within a fraction of a second of switching the flow into the unknown volume, and if the sensitivity of the equipment used is sufficiently high that the pressure does not increase by more than 1 m.bar during the test, then P always lies between atmospheric and atmospheric plus 1 m.bar. This may be approximated to constant atmospheric pressure with an accuracy of better than 0.1%.

Let K'=K.A. where A is atmospheric pressure.

Substituting into (vii) gives: K' (P1-P) V= (viii) dp dt Fig. 2 shows diagrammatically an apparatus in accordance with the present invention in which a flow of gas is provided either from a constant flow controller or a pressure regulator 10, together with a high restriction 12, and this flow is connected via a "linear" restriction r, to a threeway switching valve 1 6 which causes the flow to be exhausted to the atmosphere or to be connected to the unknown volume V, depending upon its selected switching position. The valve 1 6 is used to ensure that before the start of a test measurement the volume V is connected to atmosphere. A first differential pressure transducer 1 8 is connected across the restriction r,.One side of a second differential pressure transducer 20 is connected to the flow line between the restriction r, and the volume V. The other side of the transducer 20 is connected to atmosphere.

The output from the second differential pressure transducer 20 is differentiated by an electronic differentiator (not shown) and the resulting signal is fed, together with the output of the transducer 18, representative of (P,-P), into an electronic divider circuit (not shown) whose output, e, provides a measure of P.-P dp dt Thus, substituting into equation (vii) one gets: V= K"e where K" is a constant which may be adjusted by varying the gain of the electronic dividing circuit.

In the second embodiment in accordance with the invention illustrated in Fig. 3, the pressure P, is maintained accurately constant by means of a pressure regulator 22. A single differential pressure transducer 24 responds to the pressure drop across a "linear" restriction r2, the restriction r2 being connected to the unknown volume V via a three-way switching valve 28 arranged in an identical manner to the valve 1 6 of the first embodiment. The output from the transducer 24 is used to provide a voltage output proportional to the pressure drop (P1-P) across the restrictions r2 and the output voltage is differentiated. Since, however, P, is constant in this embodiment, the= differential is proportional to dp dt The direct output and differentiated output of the transducer 24 are connected to a divider as before to give a measure of the volume V.

The transducer 1 8 of the Fig. 2 embodiment is used to provide a measure of the flow, which has previously been shown to be proportional to (P1-P). This transducer may be repiaced by any other convenient means of electronic flow measurement.

In a further embodiment of the invention (not illustrated), the single tran'sducer of the Fig. 3 embodiment may be used in two sequential steps, first to determine the flow and, subsequently, to determine the rate of rise of pressure in the test volume. The two results can be held in a store and then divided as above to give an indication of the unknown volume.

Thus, the aforegoing embodiments can be used to provide a measurement of an unknown volume by determining the quotient of two easily detectable quantities, namely the flow rate to the unknown volume and the rate of charge of pressure in the unknown volume, the constants of the system being selected in the manner described.

In the aforegoing embodiments, it has been assumed that the flow rate has been a variable quantity. However, if the flow rate is arranged to remain constant at all times, then there is no need to measure that flow and a simpler embodiment can be achieved by maintaining the mass flow M of gas constant.

The mass flow M of gas is related to the volumetric flow by the expression: M.C.=P.F. (ix) where P is the gas pressure and C is a constant.

Substituting the above expression in equation (vi) gives M.C.

V= (x) dp dt and since M and C are constants, the volume may be read directly by measuring the inverse pressure differential dp dt This gives rise to the preferred embodiment illustrated in Fig. 4. Gas is supplied from a pressure regulator 30 to a constant mass flow controller 32 of proprietary design. This maintains a constant flow for all values of output pressure to within the specified pressure limits of the device.

The gas flow passes through a changeover valve 34 and normally vents to atmosphere. The unknown volume V is also connected to the changeover valve 34 and a gas path passes through the valve to atmosphere, thus ensuring that the unknown volume remains at atmospheric pressure until the test begins. A pressure transducer 36 is connected in the line between the unknown volume V and the valve 34 and measures the pressure in the volume V.

At the start of the test the valve 34 is switched over and the gas flow passes through the valve into the volume V. The pressure immediately begins to rise in the volume V and the rate of pressure increase dp dt, is computed by an electronic differentiator (not shown in Fig. 4) connected to the output of the pressure transducer 36. A short time is allowed (typically T second) for any transient, due to the action of the valve 34, to die away, then the rate dp dt is measured, inverted and presented to an output device such as a meter or digital voltmeter display. The output is proportional to the unknown volume and, after suitable calibration, may be arranged to read volume directly.

Fig. 5 shows an electronic operating system for the system of Fig. 4. The system of Fig. 5 includes a pressure transducer 38 (corresponding to the transducer 36 of Fig. 4) of the capacitance bridge type although other types of transducer could equally well be used. A supply 40 of highfrequency current is applied to the capacitance bridge circuit 38 from the supply 40. The output of the bridge 38 is rectified and amplified by a signal amplifier 42 and then differentiated by an electronic differentiator 44. The differential signal is then inverted in an inverter 46 and the resulting signal displayed on a digital voltmeter 48. All the latter circuit elements are known per se.

Calibration is achieved by testing a known volume, the flow controller 32 (Fig. 1) being adjusted on test until the correct reading of the volume is obtained. The internal volume of the apparatus is small, but can be eliminated by first of all obtaining an approximate reading on the known volume, then zeroing the reading with no added volume. This may be repeated in an iterative process until an accurate measurement of volume is achieved.

The range of volumetric measurement attainable by this technique can be extremely large ranging from the order of 1 ml up to several cubic meters, dependent on the flow rate chosen and the sensitivity of the pressure transducer.

In a more complex embodiment of the invention, the mass flow rate can be measured and introduced into the calculation in the manner described in connection with the embodiments of Figs. 2 and 3. This eliminates the need to check the flow and for calibration of the instrument on a regular basis and provides greater accuracy than the simplest embodiment described above.

The apparatus for the more complex arrangement can be substantially the same as that shown in Fig. 2. This can be marginally improved if (as shown in Fig. 6) the transducer 24a is connected to the line between the valve 1 6a and the volume V. The system operates in the same way as described in connection with Fig. 2, the volume being computed by dividing the output from the transducer 1 8a by the differential output from the transducer 20a.

Other ways of measuring the flow may be employed in place of transducer 18 (1 8a).

Claims

1. An apparatus for measuring the internal volume of a component, tank or cavity in which a flow of gas is introduced into the volume and the rate of pressure increase is measured by a pressure transducer and wherein either (a) the flow of gas is kept constant and the output of the pressure transducer is differentiated and inverted to provide an output signal representative of said volume or (b) the flow is not held constant but is measured and divided by the differentiated output of the transducer (the rate of pressure increase) to provide the output signal representative of said volume.

2. An apparatus as claimed in claim 1 wherein the gas flow is maintained constant by passing it from a pressure regulator through a constant mass flow controller which acts to maintain a constant flow for all values of output pressure within predetermined limits.

3. An apparatus as claimed in claim 2 wherein the gas flow is directed to the unknown volume by way of a valve which, prior to a test, connects said volume to a known pressure, such as atmospheric.

4. An apparatus as claimed in claim 2 or 3 wherein the pressure transducer is of the capacitance bridge type, the output of the bridge being amplified, differentiated and inverted to provide said output signal.

5. An apparatus as claimed in claim 1 wherein the flow rate is measured by a first differential pressure transducer across a linear restriction located in the gas flow, the rate of pressure increase being measured by the differentiated output of a second differential pressure transducer connected between the unknown volume and a known pressure, such as atmospheric.

6. An apparatus as claimed in claim 1 wherein the gas pressure upstream of a linear restriction in the gas flow is maintained constant, the gas flow rate being measured by the direct output of a differential pressure transducer across the linear restriction and the rate of increase of pressure being measured by the differential output of that transducer.

7. An apparatus for measuring the internal volume of a component, tank as cavity, substantially as hereinbefore described with reference to Figs. 1 to 3 of the accompanying drawings.

8. An apparatus for measuring the internal volume of a component, tank as cavity, substantially as hereinbefore described with reference to Figs. 4 to 6 of the accompanying drawings.