GB2176016A

GB2176016A - Determining volume

Info

Publication number: GB2176016A
Application number: GB08611877A
Authority: GB
Inventors: Geoffrey Alan Wilkin
Original assignee: International Research and Development Co Ltd
Current assignee: International Research and Development Co Ltd
Priority date: 1985-05-30
Filing date: 1986-05-15
Publication date: 1986-12-10
Also published as: GB8513586D0; GB2176016B; GB8611877D0

Abstract

The volume of a material sample is determined by locating the sample 70 within a first chamber 62, 66 of two chambers 62, 66 and 64, 68 having respective volumes equal to one another; decreasing the volume of each chamber 62, 66 and 64, 68 by a predetermined volume; and using the resultant pressure differential between the chambers 62, 66 and 64, 68 owing to the presence of the sample 70 in the first chamber 62, 66 to derive the volume of the sample 70. The pressure differential can be determined by pressure transducer 60. <IMAGE>

Description

SPECIFICATION Volume determination The invention relates to the determination of the volume of a material sample.

It has been proposed in US Patent No. 4112738 to use apparatus for the determination of the volume of a material sample which has two substantially equal-volume, cross-vented chambers in each of which a piston is reciprocable and to each of which is connected a differential pressure transducer. One of the chambers (into which the sample is inserted) has an auxiliary piston incrementally movable whereby a change in volume can be determined. Initially the free space volume of the chambers is equalised. That is achieved by moving the pistons to compress the gas in the chambers, detecting any pressure difference between the chambers, adjusting the auxiliary piston to remove the difference, cross-venting the chambers, and decompressing the gas in the chambers. This cycle is repeated until a zero pressure difference is obtained.The sample is then introduced into the one chamber and the free space volumes are again equalised. The difference between the two positions of the auxiliary piston represents the volume of the sample.

The repeated cycling of the apparatus which is necessary to establish equal free volume spaces in the chambers means that the cycle time required to determine the volume of a sample is relatively long, e.g.

of the order of several minutes.

It is an object of the present invention to provide a method of and apparatus for determining the volume of a material sample which enables the volume of the sample to be determined relatively quickly.

According to one aspect of the present invention, a method of determining the volume of a material sample comprises introducing said sample into a first one of two chambers having respective known volumes, decreasing the volume of each chamber by a respective predetermined volume and using the resultant pressure differential between said chambers owing to the presence of said sample in said first chamber to derive the volume of said sample.

According to another aspect of the present invention, apparatus for determining the volume of a material sample comprises first and second wall means defining first and second chambers, respectively, having respective known volumes, said first chamber being adapted to receive a material sample the volume of which is to be determined, first and second piston means in sealed, slidable relationship with respective parts of said first and second wall means, respectively, whereby the respective volume of each chamber is variable by a respective predetermined volume by relative movements of said piston means and said respective wall means, and differential pressure transducer means connected to each chamber thereby to determine, following a decrease in the respective volume of each chamber by said predetermined volume, the resultant pressure differential between said chambers owing to the presence of a material sample in said first chamber, said resultant pressure differential being proportional to the volume of said sample.

Preferably, said respective volumes of said chambers are equal to one another.

Preferably, said respective predetermined volumes are equal to one another.

Preferably, said piston means are movable simultaneously with one another.

An application of the apparatus and the method lies in the determination of the density of an article, particularly irregularly-shaped articles. Typically, it is a requirement to recycle materials from used articles, the recycling process being dependent upon the materials involved. Conveniently, such articles can be classified prior to recycling by differences in the densities of the articles. For example, articles such as metal cutting tools, rock drills and wear resistant machine parts which incorporate tungsten, tantalum and titanium carbides (which differ appreciably in density and this is reflected in the density of the article) can be readily classified for recycling by density.

However, the invention is not to be limited to the determination of the volume of an article or other material sample for that purpose alone.

Methods of and apparatus for determining the volume of a material sample are described below to illustrate the invention by way of example only with reference to the accompanying drawings, in which: Figure 1 is a schematic front elevation, partly in vertical section, of the apparatus; Figure 2 is a schematic end elevation, partly in section, of the apparatus shown in Figure 1; and Figure 3 is a schematic view similar to Figure 1 showing some possible modifications to the apparatus.

Firstly, however reference is made to the following examples.

Example I When the respective volumes of each chamber are equal to one another and when the respective predetermined volumes by which the respective volumes of the chambers are decreased are equal to one another, the volume of the material sample is determined using the following formula:

in which: Vl = the volume of each chamber after the compression stroke of the respective piston means.

V2 = the predetermined volume by which the volume of each chamber is decreased by said predetermined amount.

AP = the resultant pressure differential between said chambers owing to the presence of the material sample in said first chamber.

Example lI The volume Va of the material sample is determined by comparing the resultant pressure differential with reference data established using range of reference samples of known volumes.

The determination of Va by comparison avoids the inaccuracies which may be introduced when meas uring V, and Viz for input into the formula quoted in Example I above. It will be appreciated from the formula quoted in Example I above, that for fixed values of V, and V2 and with Vl AP small compared with Po V2, Va is nearly linear with respect to AP. Accordingly, the AP values-for relatively few reference samples covering the volume range of interest need be determined to establish the relevant reference data, the volume Va of the unknown sample being determined by relating the AP value therefor to the nearest pair of reference values (one above and one below) using a standard linear interpolation routine.

Reference is now made to the drawings.

The apparatus (see Figures 1 and 2) has two identical cylinders 10, 12 in which reciprocate respective idential pistons 14, 16 which are driven in unison by a motor 18 via a scotch crank arrangement 20 by equal amounts. The latter comprises an eccentric crank 22 carrying a rolling bearing element 24, the outer race-of which cooperates with a channel 26 formed in a block 28 uniting the piston rods 30, 32 of the pistons 14, 16, respectively. The output shaft of the motor 18 cooperates with optical switches or proximity switches, for exam pie, to control the location ofthe pistons 14, 16.

Bearings 34, 36 are provided in the ends of the cylinders 10, 12 to guide the piston rods 30, 32, respec tively, to ensure parallel reciprocation of the rods 30, 32 in the direction of the arrows 38. Passages (not shown) are provided in the walls of the cylinders 10, 12 adjacent to bearings 34 36 to vent the variable volumes contained within the upper regions of the cylinders 10, 12 between the upper surface of pistons 14, 16 and the cylinder end walls adjacent to bearings 34,36, thus minimising the unwanted pressure variations in such variable volumes which would otherwise occur upon reciprocation of pistons 14, 16.

A support block 40 slidably supports the cylinders 10, 12 in bearings 42, 44, respectively, whereby the cylinders 10, 12 can be raised and lowered, also in a direction parallel to the arrows 38, from and to a block 46. The block 46 has two annular recesses 48, 50 accommodating annular seals 52, 54, respectively, the lower ends of the cylinders 10, 12 being in sealing engagement with the seals 52, 54 when the cylin ders 12 are in their lower position The cylinders 10, 12 are raised or lowered relatively to the block 46 by a pivoted lever 56, operable by a pneumatic cylinder (not shown) for example, which passes through an aperture in a crosshead 58 se cured to the upper ends of the cylinders 10, 12.

The block 46 contains a differential pressure transducer 60 which is connected by respective galleries 62, 64 to the interiors 66, 68 of the cylinders 10, 12 in their lower positions, thereby forming variable volume chambers 62, 66 and 64, 68, respectively.

In operation, with the cylinders 10, 12 in their upper position, an article 70 is placed under the cylinder 10; cylinder 10 being the measuring cylinder and cylinder 12 being the reference cylinder.

The cylinders 10, 12 are then lowered to their lower position to seal against the seals 52, 54. The voi- umes of the chambers 62, 66 and 64, 68 are then reduced to a minimum by movement of the pistons 14, 16 which start from an upper predetermined position (e.g. top dead centre), and travel to a lower prede termined position to reduce the volume of the chambers 62, 66 and 64, 68 by a predetermined volume thereby compressing the gas (which conveniently can be air) in the chambers 62, 66 and 64, 68. The pressure differential (caused by the presence of the article 70 in the chamber 62, 66) between the cham bers 62, 66 and 64, 68 is sensed by the transducer 60.

The signal output from the transducer 60 is then used to derive the volume of the article 70. The value of the volume of the article can be determined as an actual value which is recorded, or, alternatively, it may be used in subsequent operatiqns to determine other parameters of the article 70, e.g. the density of the article 70.

Although the apparatus can be operated manually, preferably it forms part of automated equipkent for handling relatively large numbers of articles. In that instance, the articles can be indexed between an input station and an output station between which is positioned at least one further station at which the apparatus according to the invention is located. Such indexing apparatus is indicated in part at 72 in Figure 1.

An operating cycle for the apparatus described with reference to the drawings is of the order of 7 sec onds duration.

For practical purposes, it is essential for the presure differential to be determined under substantially isothermal conditions.

Isothermal conditions can be achieved by decreasing the respective volumes of the chambers 62, 66 and 64, 68 relatively slowly.

Alternatively, following the decrease in the respective volumes of the chambers 62, 66 and 64, 68, the resultant pressure differential is determined only after a delay period. The delay period can be selected to be of a duration sufficient to ensure isothermal conditions will prevail. Alternatively, the delay period can be a period during which successive pressure differential values, taken at small predetermined time intervals, are compared with the previous value until substantially no difference or a difference less than a predetermined value (based on the accuracy of volume measurement required) is detected between two successive values and the final value, or, alternatively the penultimate value or an averaged value (say averaged over the final three values), is used to derive the sample volume Va.

The speed with which isothermal conditions are achieved and the accuracy with which the volume Va can be determined can be influenced by the selection of materials having good heat transfer and thermal diffusivity properties such as copper and aluminium for the construction of the apparatus and gases such as dry helium or nitrogen for introduction into the chambers 62, 66 and 64, 68 during the determination cycle.

The accuracy of the determination can be affected by other factors also. For example, the size of the chambers 62, 66 and 64,68 and the volumes thereof after they have been decreased are chosen to suit the range of sizes of the material samples to be assessed. The sensitivity of the apparatus may be optimised by selecting the stroke of the pistons 14, 16 to give a maximum pressure differential value substantially equal to the full scale of the transducer 60.

In an apparatus in which air fills the chambers 62, 66 and 64, 68, the value of Po will vary and has to be compensated for. The simplest form of compensation is to determine the volume of a reference sample and to formulate a correction factor based on the ratio of the determined volume to the known volume of the sample. However, the correction factor has to be updated at frequent intervals. Alternatively, when using the formula quoted in Example I, Po can be measured each cycle and inputed into the equation along with AP.

To minimise possible differences between the pressure Po in each chamber 62, 66 and 64, 68, the chambers can be vented to atmosphere, e.g. through valves 74, 76 (see Figure 3) or through a common valve 78 (see Figure 3) or to each other. In the latter instance, or if gases other than air are used, a common valve 80 (see Figure 3) which is a three way valve can connect the chambers 62, 66 and 64, 68 to one another and to a pressure transducer 82 for determining Po.

It will be appreciated that neither the respective volumes of the two chambers nor the respective predetermined volumes by which the volumes of the chambers are decreased need be equal. However, in that instance, when determining the volume of the sample by calculation, the formula quoted in Example I would have to be modified to accommodate the differences in such volumes.

Claims

1. A method of determining the volume of a material sample comprising introducing said sample into a first one of two chambers having respective known volumes, decreasing the volume of each chamber by a respective predetermined volume and using the resultant pressure differential between said chambers owing to the presence of said sample in said first chamber to derive the volume of said sample.

2. A method according to claim 1, in which said respective volumes of said chambers are equal to one another.

3. A method according to claim 1 or claim 2, in which said respective predetermined volumes are equal to one another.

4. A method according to claims 1, 2 and 3, in which the volume of said sample is derived using the formula hereinbefore described in Example I.

5. A method according to any one of claims 1 to 3, in which the volume of said sample is derived by comparing said resultant pressure differential with reference data derived from samples of known volumes.

6. A method according to any one of the preceding claims, in which the volume of each chamber is decreased simultaneously.

7. A method according to any one of the preceding claims, in which, following said decrease in volume of each chamber, said resultant pressure differential is used to derive the volume of said sample only after a predetermined delay thereby to allow isothermal conditions to be established. (4

8. A method according to any one of claims 1 to 6, in which, following said decrease in volumeuof each chamber, monitoring said resultant pressure differential at predetermined intervals and comparing each successive pressure differential value with the immediately preceding value until there is substantially no difference or a difference less than a predetermined value between two successive values.

9. A method according to any one of the preceding claims, in which, following introduction of said sample into said first chamber, the pressures in said chambers are equalised one with the other prior to the step of decreasing the volumes of said chambers.

10. Apparatus for determining the volume of a material sample comprising first and second wall means defining first and second chambers, respectively, having respective known volumes, said first chamber being adapted to receive a material sample the volume of which is to be determined, first and second piston means in sealed, slidable relationship with respective parts of said first and second wall means, respectively, whereby the respective volume of each chamber is variable by a respective determined volume by relative movements of said piston means and said respective wall means, and differential pressure transducer means connected to each chamber thereby to determine, following a decrease in the respective volume of each chamber by said respective predetermined volume, the resultant pressure differential between said chambers owing to the presence of a material sample in said first chamber, said resultant pressure differential being proportional to the volume of said sample.

11. Apparatus according to claim 10, in which said respective volume of said chambers are equal to one another.

12. Apparatus according to claim 10 or claim 11, in which said respective predetermined volumes are equal to one another.

13. Apparatus according to any one of claims 10 to 12, in which said first and second wall means comprises first and second cylinder portions, respectively, in which said first and second piston means, respectively, are reciprocable and a support block therefor defining first and second galleries which form part of said first and second chambers, respectively, and which connect the interiors of said respective cylinder portions to said transducer means, said cylinder portions being movable into and out of sealed engagement with said block to permit a material sample to be placed on said block under said first cylinder portion.

14. Apparatus according to any one of claims 10 to 13, in which said first and second piston means are reciprocable simultaneously with one another to vary the respective volume of each chamber.

15. Apparatus according to any one of claims 10 to 14, comprising valve means which are operable to allow the pressures in said chambers to be equalised one with the other prior to operation of said piston means to decrease the volumes of said chambers.

16. A method according to claim 1 substantially as hereinbefore described with reference to any one of the Examples and to the accompanying drawings.

17. Apparatus according to claim 10 substantially as hereinbefore described with reference to any one of the Examples and to the accompanying drawings.