GB2509725A - Apparatus for checking weighing apparatus - Google Patents

Abstract

An apparatus for checking a personal weighing scale 20 having a display 25 to show the weight of a load applied to a platform of the scale. To support the scale the apparatus comprises a base 2; fixed to the base is an arch structure 3 that extends above the scale and a threaded clamping screw 5 is mounted in the arch structure. A calibrated shear beam load cell 21 is located at the lower end of the screw and the screw can be rotated to make the lower end approach the platform of the scale and apply a vertical load to the platform. A display means 19 displays the load applied to the load cell. There may be a pressure distribution plate 22 having an area at of least 150cm2 and possessing an aspect ratio of no greater than 1.5 and where such a plate would be located between the load cell and the platform of the scale to be checked. The pressure distribution plate may have an outline corresponding to a pair of adjacent foot prints.

Description

APPARATUS FOR CHECKING A PERSONAL WEIGHING SCALE

This invention relates to apparatus for checking a personal weighing scale.

There is a very wide variety of personal weighing scales in use. Such scales may be based on a mechanical spring suspension system, or rely on strain gauges or load cells and electronics. In all cases, a loss of accuracy can occur due to damage to the apparatus, error in reference weight, friction in moving components, drift in electronic circuits, temperature changes, misaligned mechanical components due to thermal expansion or contraction, magnetic fields acting on ferrous components, forces from electrostatic fields and condensation of atmospheric water on cold items. Apart from personal weighing scales for casual home use, there are regulatory requirements for accuracy to be checked. This is important in medical facilities as an out of tolerance scale can lead to under-or over-prescribing, thus conferring a negative health impact on the patient, since many medicine doses are calculated with reference to the weight of the patient.

The Non-automatic Weighing Instruments (NAWI) European Directive controls the technical and performance characteristics and also specifies which tasks requirethe useof "controlled" NAWI. Oneof those specified tasks is "Weighing patients for the purposes of monitoring, diagnosis and medical treatment". All new weighing instruments used for medical weighing have to comply with the requirements of the Directive. Under these Regulations, the design of medical weighing instruments must be approved by a Notified Body (in the UK this is the National Weights and Measures Laboratory) and all product from the production line must be individually verified for conformity and accuracy by a Trading Standards Officer or other approved verifier. Each instrument must be covered by a Declaration of Conformity and bear the green M' label indicating conformity with the Directive and the Regulations.

Accordingly, weighing apparatus used in clinical environments for monitoring, diagnosis and treatment, such as hospitals, hospital-associated medical centres, ante/post natal clinics, medical practice treatment rooms, GP consulting rooms, mobile/visiting health care and nursing homes are subject to a legal requirement that the apparatus weighs accurately and that any deviations from accuracy are monitored and either compensated for or the apparatus replaced. Generally, "calibrated" weighing equipment is required to be re-certified at least annually.

The classic approach to checking such weighing apparatus is to calibrate them with traceable weights, i.e. place a calibrated standard known weight on the apparatus and see whether the apparatus records that weight correctly. In the United Kingdom, all scales in professional use must be certified, sealed and labelled "Traceable to National Standards" under 1S09001, 1S017025 and other industry standards. In most other countries, governmental authorities regulate the requirements for using such apparatus analogously. In the case of personal weighing apparatus, which is designed to weigh persons standing on it, this means the use of weights similar to the weight of a person, and current practice, for example within the United Kingdom National Health Service, is for weighing apparatus to be checked in this way using a set of standard weights which in total weigh 150 kilograms.

Such accuracy checking must be done in situ. The Earth's gravity varies by over 0.5% so scales must be calibrated at the specific location, because scales measure weight, not mass. While the smaller weights can be relatively easily handled, the more substantial ones naturally weigh a lot and are accordingly difficult to handle. This entails a risk of injury, for example trapped digits or crushed toes, as well as the possible back injuries caused by lifting heavy weights improperly. Transporting weights from one location to another adds to the difficulty and is both inconvenient and energy consumptive.

GB-A-2478792 discloses apparatus which can be used to check a personal weighing scale without the need to use a set ot heavy discrete weights tor checking. As described in this specification, apparatus for checking a personal weighing scale consists of a base of adequate extent to support the scale, an arch structure fixed to the base to extend above the scale, a threaded clamping screw mounted in the arch structure which can be rotated to make its lower end approach the platform of the scale, and apply a vertical load to that platform, a calibrated load cell located at the lower end of the screw, and means associated with the calibrated load cell to display the load applied to that load cell.

In the use of such apparatus, the load applied to the scale being checked can be seen on the load cell display and compared with the indication given by the display on the scale itself. If they are the same, or within a predetermined allowable tolerance, the scale is functioning properly. If, on the other hand, they deviate from one another, then the degree of deviation can be determined and, it greater than the allowable tolerance, remedial steps taken or the scale discarded.

GB-A-2478792 suggests that in order to ensure even pressure, the lower end of the clamping screw carries a pressure plate connected, for example, via a ball joint with the end of the threaded screw. The use of a ball joint in this way is stated to provide an advantage in that the forces to which the load cell itself are subjected are essentially linear, i.e. there is no torque applied which might cause the load cell to give a false indication. However, it has been found in practice that using a standard load cell as described in this specification does not provide reliable and repeatable results.

According to the present invention, there is provided apparatus for checking a personal weighing scale having a display to show the weight of a load applied to a platform of such a scale, the checking apparatus consisting of a base of adequate extent to support the scale, an arch structure fixed to the base and extending above the scale, a threaded clamping screw mounted in the arch structure which can be rotated to make its lower end approach the platform of the scale, and apply a vertical load to the platform, a calibrated load cell located at the lower end of the screw, and means associated with the calibrated load cell to display the load applied to that load cell, and characterised in that the load cell is a shear beam load cell.

In a shear beam load cell, the load applied is converted by the geometrical arrangement of the components of the cell into a shearing load applied to a beam component extending perpendicular to the direction of the load.

Preferably between the shear beam load cell and the platform, there is located a pressure distribution plate having an area of at least 150 cm2 and an aspect ratio not greater than 1.5. The pressure distribution plate simulates applying a load over the contact areas of a pair of feet of a person standing on the scale with their feet together and substantially centrally located on the scale platform. The pressure distribution plate may have an outline corresponding to a stylised pair of adjacent footprints to facilitate its being placed by the tester, when the apparatus according to the present invention is in use, in the position corresponding to the position of a person when the scale is being used to weigh that person. It is also found that this assists in placing the scale under test reliably centrally on the base plate of the apparatus.

The invention is illustrated by way of example with reference to the accompanying drawings in which: Figure 1 is a perspective view of an apparatus according to the present invention; in use and seen from above; Figure 2 is a perspective view of the apparatus shown in Figure 1 from below; and Figures 3 to 7 are plan, underneath plan, side, perspective from above and perspective from below views of the apparatus when assembling for transport in both cases as set up for use when testing a personal weighing scale.

Referring first to Figures 1 and 2 of the drawings, the apparatus according to the invention consists of a plate 1 held by screws 15 on to a rectangular base member 2 shown formed of welded rectangular section steel tube, and having four support feet 4. An upstanding post is located halfway along two sides of the base member to support an arch 3 made of three sections of rectangular section tube welded together. The lower ends of the two vertical side pillars of the arch 3 fit over the upstanding posts and may be held on to the posts by means of two pins 6 which pass through holes in the arch and the upstanding posts. An aperture 12 is formed in plate 1 where it projects past base member 2, which acts as a handle grip to assist in positioning or carrying the apparatus.

Set in the centre of the horizontal portion of the arch is a threaded shaft 5.

This preferably has a so-called square thread, analogous to those used in clamping apparatus such as vices and G-cramps. At the bottom of shaft 5 there is a head member 8 mounted on the threaded shaft via a ball joint. At the top, there is a crossbar 9 enabling the threaded shaft 5 to be turned.

Crossbar 9 is a sliding fit in a head 10 fast to shaft 5; beads 11 at its ends retain it in head 10.

A shear beam load cell 21 of generally S-shaped construction is set between the head member 8 at the bottom of the threaded shaft 7, and a footprint-shaped rigid plate 22, which rests on the platform of a scale 20 to be tested.

Scale 20 is set centrally on base plate 1 between the vertical parts of the arch, and moved in a direction perpendicular to the plane of the arch so that the cell 21 is located in the middle of the footprint-shaped plate 22.

The scale 20 has a display panel 25 showing the weight on its platform. The checking apparatus includes a handheld load cell display unit 19 connected to load cell 21 by a flexible lead (not shown in Figures 1 and 2 but shown in Figures 3, 5 and 6) which terminates in a standard RS232 connector which is plugged into unit 19 when the apparatus is in use and which can be held in a bracket 23 mounted on arch 3 when the apparatus is being carried, as noted in more detail below.

When the apparatus is to be used for checking the accuracy of weighing scale 20, it and the scale are assembled as shown in the drawings. The scale (if electronic) is switched on and the assembly of threaded shaft 7 then rotated to apply, via the load cell 21 and plate 22, a load to the top of the weighing scale 20. The load applied to the weighing scale 20 is indicated by its display 25 and the display on the display unit 19. Any deviation between the two readings can thus be detected and, if greater than a permitted tolerance, appropriate action taken. By rotating shaft 7 between taking pairs of readings, the accuracy of the scale 20 across its range of operation can be determined.

The load cell 21 itself should, of course, be regularly checked for accuracy by an appropriate standards testing laboratory or similar organisation.

When not required for use, and particularly to assist transportation between one testing location and another, the arch 3 may be quickly and easily detached from base 2 by withdrawing pins 6 and lifting the arch from its locating posts. The base, arch, plate 22, load cell 21 and display unit 19 can then be placed in a suitable bag or case. Alternatively, as shown in Figures 3 to 7, using three pins 17 and retaining pins 18 (illustrated in Figure 1), the footplate 22 (which has a hole in it) and the arch 3 (using the holes through which pins 6 were inserted) can be held flat against plate 1 with each pin 17 passing through one of the holes in the plate 1. The entire assembly can then simply be lifted and carried using handle 12.

Claims (3)

1. CLAIMS1. Apparatus for checking a personal weighing scale having a display to show the weight of a load applied to a platform of such a scale, the checking apparatus consisting of a base of adequate extent to support the scale, an arch structure fixed to the base and extending above the scale, a threaded clamping screw mounted in the arch structure which can be rotated to make its lower end approach the platform of the scale, and apply a vertical load to the platform, a calibrated load cell located at the lower end of the screw, and means associated with the calibrated load cell to display the load applied to that load cell, and characterised in that the load cell is a shear beam load cell.
2. 2. Apparatus according to Claim 1 and including, for location between the shear beam load cell and the platform of the scale to be checked, a pressure distribution plate of area at least 150 cm2 and aspect ratio not greater than 1.5.
3. 3. Apparatus according to Claim 2 wherein the pressure distribution plate has an outline corresponding to a pair of adjacent footprints.