Laboratory manual for the evaluation of non-automatic weighing devices
Table of contents
- Procedure for bench, counter or hanging scales, other platform scales, equal arm scales and forklift or hand-truck scales with load receiving elements having no more than four points of support.
- Platform scale—More than four support points
- Maximum concentrated load
- Vehicle scale mounted side by side
Sections 9, 10, 11, 12 and 14 of the Specifications Relating to Non Automatic Weighing Devices (1998)
Eccentricity tests are performed on complete devices and weighing elements tested separately. Shift tests are also performed on single point hanging scales if the load receptor is relatively large.
The purpose of this test is to reveal the ability of load cell(s), load cell mounting and check systems of a weighing element to resist or compensate for the torsion effects of non axial loads. This test also ensures that the load cells of electronic scales, the levers of mechanical scales, or both in the case of electromechanical scales, are adequately "balanced" in order to obtain accurate weighing.
The device must be capable of weighing accurately in spite of changes of position of the test load over the load receiving element.
- AZT should be disabled (OFF) or the scale must be prevented from returning to ZERO during the test. When practical, this test can be performed while performing Increasing/Decreasing load tests.
- If the IZSM range does not exceed 20% of the device capacity, tests will be performed with the IZSM set at the upper limit of its range.
- If the IZSM range exceeds 20%, tests will be performed twice. A first series of tests using the lighter platter provided by the applicant, and the second series with the IZSM set to the upper limit of its range.
Note: In the case of a multi-range device, it is 20% of Max of the lowest range; in the case of a multi-interval device, it is 20% of max of the first range.
- The device must be set for the maximum capacity and smallest verification scale interval for which the approval is sought.
- If so equipped, the "enhance resolution feature" of the device will be used during the tests. If this feature is not available, use the small weight method to determine the device errors before rounding.
- This test is performed at ambient temperature (≈20°C); or at the mid point of the temperature range if significantly different from 20°C.
Procedure for bench, counter or hanging scales, other platform scales, equal arm scales and forklift or hand-truck scales with load receiving elements having no more than four points of support.
- Stabilize the device at nominal conditions; zero the device.
- Apply a test load equal to 1/3 Max on the center of the platter (Position #1). Record the indication.
- Apply the same test load on the device in such a manner that the center of gravity of the test load lies approximately at the center of one of the numbered target boxes in the following illustrations. Record the indication.
- Proceed in the same manner with the other points of application. Avoid sliding weights on the LRE. Instead, weights must be picked and placed back down on the LRE. Record the indications.
Note: 25% of Max placed on the LRE over the load cell may also be use to perform corner tests on platform, floor or bench scales having 4 support points.
Platform scale—More than four support points
Larger platform scales and other scales with more than four points of support
- Zero the device.
- Apply a test load equal to 1/(n-1) of Max to the center of the platter (Position number 1). Record the indication. (n = number of support points)
- Divide the surface area of the LRE into 1/n segments, each over one of the load support points.
- Apply the same test load on the device in such a manner that the center of gravity of the test load lies at the center of each segment. (n = number of support points). Record the indication.
- Proceed in the same manner with each of the other segments. The test load should not overhang the edge of the LRE. Record the indication.
Forklift or hand-truck scales
- Using the appropriate loading pattern below, perform the same tests as above. Weights should be placed upon the largest pallet typically lifted by the forklift or hand-truck. If the forks are adjustable, repeat test at both minimum and maximum fork spacing.
- Due to the nature of these pieces of equipment, extreme care must be taken to ensure that the stability of the forklift or hand-truck is not compromised during the test. Testing should always be done with the forks in the lowered position.
- Zero the device.
- Apply a rolling load corresponding to the usual rolling load, the heaviest and the most concentrated one which may be weighed, but not exceeding 80% of Max at different points of the load receiving element.
- Record the indications.
During this test, observe the rail to detect any inappropriate motion, deflection, binding or friction that could adversely affect the scale performance.
Tank or hopper scales
- Zero the device.
- Use a load of at least 10% of Max without exceeding 25% of Max. Apply the load to each point of support. Care must be taken to keep the center of gravity of the load between the supporting points to prevent cantilevering the scale.
- Record the indications.
Note: The ratio between the test load and e must be such that it will be possible to establish the error and apply the limits of error.
Section / Shift tests on vehicle scales
Maximum concentrated load
The axle (or set of axles) weight of the vehicle used must not be greater than 75% Max in the case of a two section scale, and not greater than 50% Max in the case of scale with more than two sections.
- Zero the device
- Drive the loaded vehicle onto the weighbridge and position the center of the heaviest set of axles over the first section; record the indication.
- Move the concentrated load over the second section. Record the indications.
- Repeat the procedure for each of the other sections.
- Enter the weighbridge in the opposite direction and test each section again. At least two complete sets of shift tests should be conducted over each section of the scale. This is to determine the repeatability of the scale.
Note: The last section in each direction, before leaving the scale, cannot be adequately loaded with a typical test truck. The final stopping position should be just before the first set of axles leaves the weighbridge.
The concentrated load must also be placed between the sections to determine if any deflection of the deck or understructure is causing inaccuracies.
Note: When using a loader to conduct a shift test, the center-of-gravity (CoG) should be position over the load cell(s). This position will change depending upon the configuration of the machine. In addition, loaders, when used as strain or shift test loads should have their buckets / grapples in lowered positions if possible. This lowers the CoG and reduces the "sail" effect from the wind.
Shift test—Wide deck
If the width of the weighbridge exceeds 3 meters (or when the inspector deems it necessary), perform a first series of tests with the vehicle shifted on the right side of the deck and then shifted to the left side.
Warning: Wide Deck shift test should not be attempted on wooden deck scales if the test means driving the vehicle off of the longitudinal timbers intended to support the tires. The transverse mounted decking may not have adequate strength to support the concentrated load of the vehicle.
Shift test—Modular scales
In the case of weighbridge made of modules (multi-deck vehicle scale), shift tests must also be conducted by placing the load so that it straddles the connection between the modules. At least one shift test is to be conducted on the scale with the test load place on one side of the connection line of the module, then on the other side of the connection line.
Use of hydraulic jack
The use of weight truck hydraulic jack beam is prohibited for safety reasons.
Vehicle scale mounted side by side
Vehicle Scales mounted in non-traditional fashion. Often used for weighing of off-road mining equipment or large logging equipment. These scales usually consist of two scale decks mounted side by side, however other configurations are possible including a "T" configuration.
- Zero the device.
- If possible, test eccentricity on each scale separately using the loading patterns for vehicle scales as above. Then continue with the following tests.
- Drive a loaded vehicle onto the weighbridge and position the center of the heaviest set of axles over the first support point/section; record the indication.
- Move the concentrated load over the second load point/section. Record the indications.
- Repeat the procedure for each of the other load points/sections.
- If possible, enter the weighbridge in the opposite direction and test each load point/section again.
- Testing should approximate normal use loading patterns as much as possible.
Interpretation of the results
The difference between the results for different positions of the load must not exceed the absolute value of the In-Service Limit of Error for that load.
Each individual result must also be within the applicable limits of error for the test.
- Rev. 1 (November 2004)
- Change reference from "load cell" to "support point" to cover all scale types.
- Added eccentricity drawing for Forklift, single support.
- Corrected number of support points reference in platform scales (≤ 4).
- Added procedure for platform scales with more than four support points.
- Added procedure for "side by side" deck vehicle scales.
- Rev. 2 (April 2005)
- Modified & simplified forklift/hand truck loading requirements.
- Rev. 3 (September 2009)
- Modified test weight to 1/3 capacity
- Removed diagram for bench/counter scales with a single support point (test point now same for all devices up to 4 support points)
- Rev. 4 (March 2010)
- AZSM renamed to AZT
- Formatting changes
- Date modified: