GB2237651A - Calibration of vessel weighing systems - Google Patents

Abstract

In order to calibrate a vessel weighing system without having to add material to the vessel, a calibration load is applied by a screw-and-nut mechanism (7) (or hydraulically) through the weighing load cell (8) and the calibration load cell (11) to the vessel (1). Three weighing load cells (8) and three calibration load cells (11) are spaced around the vessel. Each calibration load cell (11) is pivoted at (22), and is received in a bore (20) in a mounting block (15) which supports the vessel (1) by bearing on the weighing load cell (8). The calibration load cells (11) and mechanisms (7) are removable. <IMAGE>

Description

CALIBRATION OF VESSEL WEIGHING SYSTEMS This invention relates to calibration of vessel weighing systems.

Vessel weighing systems are used to weigh vessels containing bulk or liquid components, and are commonly used in the food and pharmaceutical industries, where it is important that the weighing systems are accurate. Known systems normally use load cells to measure and indicate the load on the vessel, and the cells have to be calibrated initially. They may also need to be checked subsequently. Calibration normally has to be done on site, because of the size of the vessel, and because the pipework to which it is connected causes a reaction load, which affects the readings obtained from the load cells. Calibration is performed by obtaining a reading with the vessel empty, and setting the indicator to zero, and then obtaining a reading with the vessel filled with a known weight of water or a bulk substance, and setting the indicator to the correct load.If possible, the vessel is emptied again to re-check the zero reading. This technique has two main disadvantages; firstly it is time-consuming, and secondly that the substance used for calibration can contaminate the vessel. It can also be time-consuming and disruptive to check the calibration subsequently.

According to a first aspect of our invention, apparatus for calibrating or verifying the calibration of a vessel weighing system having vessel load measuring and indicating means comprises reference load measuring means, reference load indicating means, and load applying means for applying a calibrating load to the vessel through the reference load measuring means, to enable the vessel load measuring and indicating means to be calibrated to the reference means.

Using a calibration force applied through the reference means has the advantage that the system can be calibrated without filling the vessel, and it can be a relatively rapid procedure. Checking of the calibration subsequently can also be done easily.

Where the vessel load measuring means comprises one or more separate members, the reference load measuring means may also comprise a corresponding number of separate members. Preferably each vessel load measuring means is provided on a vessel load member mounted from a support for the vessel, with the corresponding reference load measuring means being provided on respective reference load members mounted between the or each vessel load member and the vessel.

The reference load members are conveniently detachably mounted, so that they can be removed when not required. This enables them to be used as reference load members for more than one vessel.

Preferably each reference load member is attached to the support by a bracket means, which includes a pivot point for the member at an intermediate point in its length. The load applying means then acts on one end of the member, and the calibrating load is applied to the vessel by the other end of the member. The location of the pivot point is chosen to provide a suitable mechanical advantage. The bracket means and the reference load member may be arranged so that the pivot point is movable along the length of the member.

The pivot point is also arranged so that it is aligned horizontally with the point at which the member applies the calibrating load, in order to minimise horizontal movement of the application point caused by the pivoting action of the reference load member.

The load application means may be provided by mechanisms operated in any suitable way, such as mechanically, hydraulically, electrically or pneumatically. One arrangement has a screw jack mechanism acting between the support and the reference load member. Alternatively, an hydraulically-operated jack could be employed.

Preferably, each of the vessel and reference load members comprises a load cell. The output from the vessel load cells goes to a vessel control means and indicator; similarly the output from the reference load cells goes to a reference control means and indicator.

For initial calibration of the vessel load cells the vessel indicator reading is obtained with the vessel empty, and the vessel indicator is set to zero.

The reference load cells are attached, the reference indicator set to zero, and the load applying means operated to apply a calibration load to the vessel through the reference load cells, the calibration load being indicated on the reference indicator. The reading on the vessel indicator, which will be different because of the pipework effects, can then be set to the true load given by the reference indicator.

To check the calibration, the calibration load is reduced gradually to zero, and the readings of the two indicators are compared; if the load cells are functioning properly the readings should be the same.

The reference load cells are then removed. The calibration can be checked subsequently in the same way while the vessel is in use, and it is not necessary to empty the vessel to check the calibration. A second aspect of the invention is concerned with the method of calibration.

According to a second aspect of our invention, a method of calibrating a vessel weighing system having vessel load measuring and indicating means comprises the steps of obtaining a reading on the vessel load indicating means for the empty vessel, and setting this to zero, applying a calibration load to the vessel through reference load measuring, the calibration load being indicated by the reference load indicating means, and setting the reading on the vessel load indicating means to the reading on the reference load indicating means.

According to a third aspect of our invention, a method of verifying the calibration of a vessel weighing system having vessel load measuring and indicating means comprises the steps of obtaining a first reading on the vessel load indicating means and associating this with a corresponding reading of reference load indicating means, applying a calibration verifying load to the vessel through reference load measuring means, and taking a second reading on the vessel load indicating means, the calibration verifying load being indicated by the reference load indicating means and confirming that the change in the vessel load indicating means corresponds to the change in the reading indicated by the reference indicating means within predetermined values.

Various embodiments of our invention are shown, by way of example only, in the accompanying drawings, in which: - Figure 1 shows schematically a vessel weighing system including calibration apparatus; Figure 2 is a side view of part of the weighing and calibration apparatus; Figure 3 is an end view of part of the apparatus of Figure 2; Figure 4 is a graph of loads on the weighing and calibration apparatus during calibration with a positive load effect from pipework; Figure 5 shows schematically the forces on the apparatus; Figure 6 is in part a top view of a second calibration apparatus and in part a horizontal cross-section on line 6-6 of Figure 7; Figure 7 is a side view of the apparatus of Figure 6; and Figure 8 is in part an end view of the apparatus of Figure 6, and in part a vertical cross section on line 8-8 of Figure 7.

The vessel weighing system and calibration apparatus shown in Figure 1 is for a vessel 1 for storing bulk or liquid components. The vessel 1 is attached to relatively stationary supports 2 by pipes 3, and also rests on relatively stationary supports 4. The weighing and calibration apparatus is located between the vessel 1 and the supports 4, and comprises vessel load measuring and indicating means 5, reference load measuring and indicating means 6, and load applying means 7. The vessel load measuring and indicating means 5 comprises three load cells 8 (only two of which are shown) mounted on the supports 4, whose output goes to a control vessel junction box 9 and then to a vessel digital display 10 to indicate the load.The reference load measuring and indicating means 6 also comprises three load cells 11, each mounted between the vessel 1 and a corresponding vessel load cell 8, together with a control reference junction box 12 and a reference display 13. The load applying means 7 comprises three mechanisms, each of which acts between the support 4 and a corresponding reference load cell 11.

The arrangement of the load cells 8, 11 and the load applying means 7 is shown in more detail in Figures 2 and 3. The vessel load cell 8, which is of a known type, is attached at its outer end to the support 4 by bolts or other suitable fasteners (not shown) and at its inner end has a button 14 to which the load is applied from the vessel 1, through a mounting block 15. The mounting block 15 has lower and upper parts 16,17 respectively fastened by bolts or other suitable fasteners (not shown). The lower part 16 has a depending portion 18 with a groove 19 in which the button 14 is adapted to engage. The groove 19 ensures that the load cell 8 is oriented correctly, in line with the centre of vessel 1. The upper part 17 of the block 15 is adapted to support the vessel 1 on its upper surface, and has a through bore 20 in which the reference load cell 11 is located.

The reference cell 11 is similar in construction to the vessel load cell 8, but is mounted oppositely, so that the load button 14 engages in a depression 21 in the lower surface of the bore 20, the depression 21 being formed to ensure that the buttons 14 are aligned vertically. The reference load cell 11 is removably mounted on the support 4 by bracket means in the form of a pair of brackets 22, one on each side of the cell 11. As best seen in Figure 3, each bracket 22 has upper and lower parts 22,23 respectively, with a pivotal connection 25 between them. Each upper part 23 is rigidly connected to the load cell 11 by a pair of bolts 26, while each lower part 24 has a slot 27, into which fits a horizontal pin 28 on an upstanding pillar 29 provided on the support 4.The pivotal connection 25 comprises a nut and bolt assembly 30 passing through aligned openings 31 in the upper and lower parts 23,24. The connection 25 forms a pivot point for the load cell 11 and the pivot point is arranged so that it is aligned horizontally with the load application point of the button 14 of the load cell 11, and midway between the load applying means 7 and the button 14. This ensures that horizontal movement of the load application point is minimised.

The load applying means 7 acts between the outer ends of the reference load cell 11 and the vessel load cell 8. It is attached to the reference load cell 11, and acts on the vessel load cell 8 through a button 32 located on a pillar 33 provided on the cell 8. The load applying means 7 comprises a differential screw jack mechanism 34, having a pair of screws 35,36 working in a nut 37. The upper screw 35 has the larger pitch, say 1.75mm. It has an upper part which is adapted to be located in a correspondingly countersunk hole 38 in a mounting plate 22a for the reference cell 11, and locked by a nut 39 in order to attach the mechanism 34 to the cell 11, and a lower part which works in the nut 37. The lower screw 36 has a smaller pitch, say 1.5mm, and its head engages the button 32.

Coarse adjustment of the mechanism is provided by rotating the screw 36, and fine adjustment by rotating the nut 37, which gives an effective pitch of 0.25mm.

The control vessel junction box 9 and display 10 (not shown in Figures 2 and 3) are fixed to a suitable point on the support 4. The reference junction box 12 and display 13 are adapted to be removable, with the reference load cells 11 and the load applying means 7 providing a portable reference unit which can be used for more than one vessel 1.

For initial calibration of the vessel cells 8 the vessel 1 is empty, and an initial reading is obtained of the empty or dead weight of the vessel 1 from each vessel load cell 8. These readings are set to zero on the display 10. The reference load cells 11 and load applying means 7 are then attached to the apparatus, with the reference cells 11 chosen to have a similar range to the vessel cells 8. Each load applying means 7 is operated in turn (or simultaneously) to apply a maximum load to the vessel 1, through the reference load cell 8 and the mounting block 15. The load applied is measured by the reference cell, and indicated on the display 13. The reading obtained from the corresponding vessel load cell 8, which will be different from that on the reference cell 11 because of the reaction load from the pipes 3, can then be set to the same as the reading of the reference cell 11.The lower and upper limits of the vessel load cell 8 are therefore set, with the upper limit having eliminated the pipework reaction load. The calibration can be checked by reducing gradually to zero the load applied by the load applying means; if the calibration is correct the readings on the vessel and reference displays 10 and 13 will decrease together. Following calibration, the reference cells 11 and load applying means 7 may be removed, for use elsewhere.

The apparatus described can be used to check the calibration of a load cell that has already been calibrated, perhaps some months before. The reference load cells 11 and load applying means 7 are attached to the vessel and a base reading is taken from the reference and vessel load cells 11 and 5. A verification load is applied by the load applying means 7 and a second set of readings is taken from the reference and vessel load cells. Preferably a third set (or further sets) of readings at a different verification load is taken. The change in the indicated vessel load is then compared with the change in the indicated reference load to ensure that they react to the verification load in the same manner, within predetermined values. For example a delibrate offset may be required.

There is thus no need to empty the vessel in order to determine that the vessel load cell is still recording correct changes in weight. This can be particularly useful if metered weights of material are to be delivered from the vessel, but the vessel is not to be emptied. The change in weight, rather than the absolute indicated weight, is then of importance.

Figure 4 shows graphically the loads on the reference and vessel cells during the calibration procedure. The upper line V on the graph represents the load on the vessel load cell 8, starting at a dead load D and rising to a maximum A, representing the load L applied by the reference cell 11, less the reaction load P. The reference cell load is represented by the lower line R, starting at zero and going up to a maximum of L. Figure 5 shows the loads in the apparatus. F1 is the calibration load generated in the mounting block 15 (shown in two parts in the diagram), by the reference load F2 applied by the reference cell 11. F1 is equal to F2. F3 is the load seen by the vessel load cell 8, which is equal to the load applied, F2, minus the reaction load F4.The calibration procedure effectively sets F4 to zero, so that then F2=F3; that is the readings on the two cells 8 and 11 are the same.

It will be appreciated that the reference apparatus can be attached to the vessel 1 again, in order to check the calibration of the vessel load cells 8. A similar procedure to the initial calibration is followed, except that, if the vessel 1 is not empty, the reference display 13 is initially set to a value corresponding to that on the vessel display 10.

In a modification, such as that shown in Figure 5, the load applying means may comprise an hydraulic jack mechanism or any other suitable mechanism rather than a mechanical jack. The load applying means may act between the reference load cell 11 and the support 4, rather than the cell 8.

In a further modification, not shown, the pivot point 25 may be moved towards or away from the reference load cell button 14 in order to provide a mechanical advantage or disadvantage as appropriate.

The pivot point 25 may even be movably mounted if necessary.

In yet another modification, the lower surface of the bore 20 may not have the depression 21. The reference load cell therefore finds its own position, and is free from extraneous forces arising from the locating means.

A second vessel weighing calibration apparatus is shown in Figures 6 to 8 and comprises a cantilever load cell 50 fixed to a support 51 for a vessel 52 by bolts 53; a lower calibration unit member 54; an upper calibration unit member 55; and a load spacer 55'.

In its normal, weighing, use the vessel 52 is supported by support lugs 56 which have a self-centering connection 57 to a button on respective load cells 50. The load cells 50 are of the cantilever kind and have a projecting arm 58 connected to a mounting portion 59 which is bolted by the bolts 53 to the support 51 which bears the weight of the vessel transmitted to it via lugs 56 and load cells 50.

Variations in the weight of the vessel are detected by the load cells 50.

The support 51 has at each load cell mounting point a pair of columns 60. Each column 60 has an upper vertically movable end 61 which is extended to provide a column 60 of relatively long length during installation of the vessel 52 on the support 51. The long columns 60 contact the mounting lug 56 of the vessel before the connection 57 of the load cell does; preventing the load cell 50 from experiencing loads during installation of the vessel. When the vessel is installed the ends 60 are retracted to provide columns 60 of short lengths and allowing the load cell 50 to take the load of the vessel 52. The columns 60 may also be used to relieve the load on the load cell 50 for repair or replacement of the load cell 50, or during attachment of the calibration equipment and prevent damage to the load cell 50.

The calibration unit is secured to the support 51 when a load cell 50 is to be calibrated or checked.

The lower calibration unit member 54 comprises a pair of spaced wall plates 62 joined together by a loading cross-member 63, and two spacing cross-members 64 and 65. Each plate 62 is provided with a bearing 66 (best seen in Figure 8). The lower unit member 54 is releasably bolted to the support 51 for calibration or checking of the load cell 50 by bolts 67 which secure lugs 68 on the plates 62 to the support 51. The cross-member 65 serves as a carrying handle for the lower member 54 during transport of the member 54 to the site of a load cell 50. A man can carry the lower member 54 with one hand, and can also carry the upper member 55 with one hand.

Once the lower member 54 has been secured to the support 51 the upper member 55 of the calibration unit is mounted on the lower member 54.

The upper member 55 comprises a pair of spaced opposed side plates 69 connected at their upper end to a transverse plate 70, a calibration load cell 71 secured to the plate 70, and hydraulic load-applying means 72 mounted on the plate 70. The pair of side plates 69 are braced by a spacing bar 73 and are each provided with a bearing 74 of the same diameter as bearing 66 of the plates 62 of the lower member 54.

The calibration load cell 71 has a load transmitting button 71' and is of the kind known as a pancake load cell, and is secured to the underside of the transverse plate 70. The hydraulic load-applying means 72 comprises a hydraulic cylinder 75 bolted to the upper surface of the plate 70 and having a hydraulic ram 76 which passes through a hole in the plate 70. A force-transmitting frame 77 is also provided associated with the hydraulic cylinder and ram and serves to provide a reaction force to the test forces produced by the hydraulic cylinder 75. The transverse plate 70 is also provided with a carrying handle 78.

The calibration apparatus also includes a connecting pin 79 and the load spacer 55'. The pin i9 is best shown in Figure 8 and has a handle portion 80 and a shaft portion 81 of complementary diameter to the bearings 66 and 74.

The upper member 55 is secured to the lower member 54 (which is bolted to the support 51) by the pin 79. The bearings 66 and 74 are aligned, with the pair of plates 62 being disposed within the pair of side plates 69, and the handle 80 of the pin 79 held by the user who thrusts the pin through the two pairs of bearings. A retaining clip or pin, such as a cotter pin, may be applied to the forward end of the pin 79 to retain it in position. The user then places the load spacer 55' between the load-transmitting button of the calibration load cell 71 and the upper surface of a portion of the lug 56 of the vessel. The size of the load spacer 55' which is used depends upon the dimensions of the lug 56 and the relative mounting position of the lower member 54.

It will be appreciated that indicating means 90 for indicating the load experienced by the calibration cell 71 is also provided.

In use of the calibration apparatus hydraulic pressure is applied to the cylinder 75 by a convenient source (for example a hand pump) and the load-transmitting button 71' of the calibration load cell 71 has its engagement with the spacer 55' in the horizontal plane which includes the central axis of the shaft 81 of the pin 79. This ensures that the force transmitted through the calibration load cell 71 is substantially all in the vertical direction and minimises problems associated with forces being transmitted in a direction inclined to the vertical.

An advantage of this second embodiment is that the distance between the underside of the lug 56 on the vessel and the connection 57 to the load cell 50 is particularly small. This reduces the effect of any horizontal forces applied to the vessel by the components between the lug and the load cell. If the distance between the lug 56 and the connection 57 to the load cell 50 is too long any horizontal forces experienced by the lug 56 are magnified at the connection 57 by the "lever" effect of having a force applied at a position transversely (vertically) spaced from the force (horizontal). Minimising the vertical distance concerned minimises such problems.

It will be appreciated that different calibration load cells can be used and that this may require different sized spacers 55', as may different vessels.

The spacer 55' enables the calibration apparatus to be modified for use with different vessels easily.

In one particular modification which we envisage the hydraulic cylinder 75 is connected to a powered source of hydraulic fluid and a servo control system is provided which takes the signals from the calibration load cell and adjusts the hydraulic pressure to keep that signal constant during the calibration procedure.

The system also preferably includes the facility to set a desired pressure, or calibration load cell signal, and then automatically to adjust the hydraulic pressure to achieve it. A series of test values may be input into the system which may proceed automatically from one calibration point to the next. There may be automatic report producing means. The whole operation of the calibration test may be automatically controlled by an electronic controller.

It will be appreciated that although the use of the apparatus described envisages a test engineer bringing the lower and upper members 54 and 55 (and the source of hydraulic pressure) with him, and taking them away with him when the calibration or test is completed, the customer paying for the calibration service, the vessel owner may own his own apparatus and may perform his own calibrations. If the vessel owner has enough vessels for the time taken to set up the upper and lower members 54 and 55 to be inconvenient he may keep lower members 54 permanently affixed at each of his load cells 50 so that the calibration engineer has only to attach the upper member 55 to the permanently fixed lower members.This can save a considerable amount of time, and the lower members 54 do not include any expensive components: the lower members 54 are relatively cheap in comparison with the upper members 55. A user could thus have many lower members and fewer, or evenjust one, upper member.

It will also be appreciated that if the vessel requires frequent calibration, or cannot be easily reached (for example if it is in a nuclear reactor) then it may be desirable to have the calibration apparatus permanently in position, with the ram 76 being withdrawn out of the way for normal weighing use of the load cell 50.

The reference or calibration cell 71 need not be provided adjacent a lug 56 on the vessel, it could for example be provided in any weight bearing part of the support structure for the vessel, such as the leg of the vessel, but we prefer to provide it near to the connection of the vessel to its support structure.

It will also be appreciated that although in the embodiments described the reference load measuring means is provided directly in line with the vessel load measuring means this need not be essential. The reference load measuring means could, for example, apply the reference load to the vessel at a position adjacent the vessel load measuring means.

Claims (22)

1. Apparatus for calibrating or verifying the calibration of a vessel weighing system having vessel load measuring and indicating means comprising reference load measuring means, reference load indicating means, and load applying means for applying a calibrating load to the vessel through the reference load measuring means, to enable the vessel load measuring and indicating means to be calibrated to the reference means.

2. Apparatus according to claim 1 in which the vessel load measuring means comprises one or more separate members.

3. Apparatus according to claim 2 in which the reference load measuring means comprises a corresponding number of separate members to those of the vessel load measuring means.

4. Apparatus according to any one of claims 1 to 3 in which the or each vessel load measuring means is provided on a vessel load member mounted from a support for the vessel, with the corresponding reference load measuring means being provided on respective reference load members mounted between the or each vessel load member and the vessel.

5. Apparatus according to claim 4 in which the reference load members are detachably mounted so that they can be removed when not required.

6. Apparatus according to claim 4 or claim 5 in which each reference load member is attached to the support by a bracket means, which includes a pivot point for the member at an intermediate point in its length.

7. Apparatus according to claim 6 in which the load applying means acts on the member to one side of the pivot point and the calibrating load is applied to the vessel by a portion of the member to the other side of the pivot point.

8. Apparatus according to claim 6 or claim 7 in which the pivot point is arranged so that it is aligned horizontally with the point at which the member applies the calibrating load.

9. Apparatus according to any preceding claim in which a signal is fed from the or each of the vessel load measuring means to vessel control or display means, and a signal is fed from the or each reference load measuring means to reference control or display means

10. Apparatus according to claim 9 in which the signals from the vessel and reference load measuring means are fed to a common control or display means.

11. Apparatus according to claim 9 or claim 10 in which the control means comprises servo control means which maintains the calibrating load such as to keep the reference signals steady during a calibration operation.

12. Apparatus according to claim 11 in which the control means has input means to enable the user to input predetermined calibrating forces, or predetermined output reference signals, and which automatically carries out a calibration operation at the predetermined value.

13. Apparatus according to any preceding claim which comprises an operational member having the load applying means and the reference load measuring means, and a mounting member adapted to be secured to the vessel or to a support for the vessel and upon which the operational member is adapted to be mounted.

14. Apparatus according to any preceding claim in which spacer means is provided between the reference load measuring means and the vessel, modification of the spacer means modifying the apparatus for use with vessels of different vessel to support configurations.

15. Apparatus for calibrating a vessel weighing system substantially as herein described and illustrated with reference to Figures 1 to 3 of the accompanying drawings.

16. Apparatus for calibrating a vessel weighing system substantially as herein described and illustrated with reference to Figures 6 to 8 of the accompanying drawings.

17. A method of calibrating a vessel weighing system having vessel load measuring and indicating means comprising the steps of obtaining a reading on the vessel load indicating means for the empty vessel, and setting this to zero, applying a calibration load to the vessel through reference load measuring and indicating means, the calibration load being indicated by the reference load indicating means, and setting the reading on the vessel load indicating means to the reading on the reference load indicating means.

18. A method of verifying the calibration of a vessel weighing system having vessel load measuring and indicating means comprising the steps of obtaining a first reading on the vessel load indicating means and associating this with a corresponding reading of reference load indicating means, applying a calibration verifying load to the vessel through reference load measuring means, and taking a second reading on the vessel load indicating means, the calibration verifying load being indicated by the reference load indicating means, and confirming that the change in the reading indicated by the vessel load indicating means corresponds to the change in the reading indicated by the reference indicating means within predetermined values.

19. A method according to claim 17 or claim 18 comprising using apparatus for calibrating a vessel weighing system in accordance with any one of claims 1 to 16 in which the apparatus comprises an operational member having the load applying means and the reference load measuring means, and a mounting member for the operational member; the method further comprising the steps of securing the mounting member to the vessel or a support for the vessel, and securing the operational member to the mounting member.

20. A method according to claim 19 in which the mounting member and operational member are removed after calibration or verification of the calibration.

21. A method according to claim 19 in which the mounting member is left secured to the vessel or support for the vessel after calibration to facilitate re-attachment of the operational member for subsequent checking of the calibration- of the vessel load indicating means.

22. A method of calibrating, or checking the calibration of, a vessel weighing system having vessel load measuring and indicating means substantially as described herein with reference to the accompanying drawings.