Field Inspection Manual — Automatic Weighing Devices

Part 2, Section 1 a: Type 2-31, 3-11, 7-11: Automatic catch weighing device—Dynamic weighing

Table of contents


Reference

Weights and Measures Regulations – limits of error from sections 176,177 and 185, as appropriate. Product Test Load Development procedure.

Purpose

Weighing of discrete loads on an overhead rail scale or belt scale (not including automatic continuous totalizing weighing systems, which are commonly referred to as conveyor belt scales). Typical applications include carcass weighing on overhead rail scales in meat processing plants and individual package weighing across in-motion belt scales in meat and cheese processing plants as well as shipping and courier establishments.

Requirements

The device under test (DUT) must be tested for performance in the static mode (excluding motion detection) using the standard test procedures / inspection procedure outlines from the Specifications Relating to Non-automatic Weighing Devices. The limits of error applicable to automatic scales must be applied. If static testing is not possible, consult your gravimetric specialist, as additional tests may be required. The following requirements are in addition to static testing.

General

Automatic catch weighing devices (ACWDs) that weigh dynamically must be tested dynamically using product test loads which are representative of the types of products intended to be weighed by the system. In order to use test loads, the weight and uncertainty of the test loads must be determined. The separate Product Test Load Development procedure will assist the inspector in assuring that the intended test loads are suitable for use.

Procedures

Creating test loads

  1. Selecting appropriate test loads.
    1. Select suitable test loads representative of the product typically weighed on the device. The number of test loads and test runs required may be determined from the table below (typically ten for belt scales and five for overhead rail scales). The total number of individual test loads may be increased in order to facilitate testing, but the minimum number of runs must be respected at all times. Test loads must be stable and should be representative of the actual product to be weighed.
    2. If the DUT is used over a range of weights, then the test loads must be selected so that they span the intended usage range of the device (light — medium — heavy).
  2. Refer to the Product Test Load Development procedure to determine the acceptable upper and lower indicated values for each of the product test loads.

Dynamic test procedure

  1. Determine the belt or overhead rail speed and ensure that it is within the limits stipulated in the Notice of Approval. Refer to the procedure for determining belt speed below.
  2. Conduct dynamic tests using the previously established test loads. Refer to the table below for minimum number of weighments required.
  3. For overhead rail scales, known loads should be interspersed amongst the unknown loads (start — middle — end). To facilitate testing, known test loads may also be used in place of the unknown loads.
  4. For each test load, the indicated weight must be within the appropriate range or tolerance as previously established using the Product Test Load Development procedure.

Note: If the belt or overhead rail speed is operator adjustable, the weighments shall be conducted at the lowest and highest speeds (half at the lowest speed, half at the highest speed). Otherwise, test at as found speed.

Minimum number of weighments required – Dynamic weighing
Scale type Capacity Weighments Consisting of:
Automatic catch weighing device—Belt Scale ≤ 60 m⁄min (≤200 ft⁄min) 60 weighments 10 test loads × 6 runs
> 60 to 75 m⁄min (> 200 to 250 ft⁄min) 70 weighments 10 test loads × 7 runs
> 75 to 90 m⁄min (>250 to 300 ft⁄min) 80 weighments 10 test loads × 8 runs
> 90 to 106 m⁄min (>300 to 350  ft⁄min) 90 weighments 10 test loads × 9 runs
> 106 m⁄min (> 350 ft⁄min) 100 weighments 10 test loads × 10 runs
Automatic catch weighing device—Overhead Track Scale All Devices 15 weighments at each speed.
(5 known test loads × 3 runs = 15 weighments)
minimum of:
5 known test loads & 5 unknown loadsFootnote 1 = 10 loads ⁄ run
Interpretation of results

The DUT is deemed to comply if all results are within the appropriate limits of error.

Note: If one test load consistently causes problems, the inspector should determine if the problem is with the load and not the scale. If the load is defective, the test results for that load should be discarded. This is sometimes the case when a defective trolley is used to suspend a load, but it may also be due to a poorly selected test object.

Repeatability test (conduct at as found speed)

These two test loads may be run and used as part of the dynamic test.

Over length package test (conduct at as found speed)

Do not conduct this test if it will damage the system. Run a package that exceeds the length of the scale platter. The device should not display or transmit an incorrect weight, or should go into an error mode of some kind. This test may not apply to some device types (e.g. overhead rail). If a problem is found, the device should be rejected or the usage of the device restricted.

Off centre load test (scales with a belt only)

With the belt in motion, run a test load (0.5 Max) down each side of the scale and in the centre. The device must remain accurate within prescribed limits of error regardless of the location of the package on the belt.

Power failure test (initial inspection only)

Systems which store cumulative totals for subsequent trade transactions must have power failure safeguards in place. Prior to proceeding with the power failure test, the inspector must ensure that a loss of power will not adversely affect the ancillary equipment associated with the DUT.

While the system is in operational mode, interrupt the power to the DUT or, if so equipped, to the uninterruptible power supply. If an uninterruptible power supply is used, do not disconnect the DUT from the UPS to conduct the power failure test.

After a sufficient length of time has elapsed (i.e. 1-2 min), return power to the system and complete the transaction. All items which have passed over the load receiving element must be accounted for in the system memory or on a printed ticket.

Interpretation of results

The DUT is deemed to comply if all results of repeatability test, over length package test, off centre load test and power failure test are within the acceptable limits of error.

Determining belt speed

Belt speed may be determined directly from the DUT if so equipped with this feature. The accuracy of the DUT belt speed indication must be checked. If the DUT does not have a built in belt speed indication, belt speed must be determined as part of the test procedure. If belt speed is adjustable, the as tested speed must be entered on the inspection form and the speed control sealed. If the speed control is intended to be operator controlled or it cannot be sealed, the DUT must be tested at both the lowest and highest possible speeds.

Portable tachometer

Using a suitable contact or non-contact tachometer, follow the manufacturer's instructions for determining the speed of one of the belt pulleys in rotations per minute (rpm). If the tachometer is used to measure the speed of a pulley directly driving the belt, the inspector must ensure that there is no slippage between the belt and the drive wheel. A better option is to measure the speed of an idler or a non-driven pulley. The belt speed may be calculated using one of the following formulas:

belt speed (m/min) = [diameter (cm) × π × rpm] ÷ 100

or

belt speed (ft/min) = [diameter (in) × π × rpm] ÷ 12

Where:

If using a belt sensing tachometer capable of direct readings in feet or metres per minute, follow the manufacturer's instructions to determine the belt speed. In most cases, it can be measured directly from the belt with no further calculations.

Stop watch and tape measure

Using a stop watch and tape measure, belt speed may be calculated by measuring the total length of the belt and the time required for X revolutions of the belt. If the belt revolutions cannot be obtained to the nearest full revolution, add or subtract the appropriate fraction of the over or under run to the number of revolutions.

Use a piece of tape on the belt and a fixed reference on the belt frame to count number of revolutions.

The final belt speed is then calculated using the following formula:

belt speed (m/min) = [belt length (m) × number of belt revolutions(χ)] ÷ time (min)

Example

You are measuring the belt speed of a 12 metre long belt by timing ten revolutions of the belt. It takes an additional 3 metres of overrun before the belt comes to a complete stop. The extra 3 metres must be added into your calculations if the time is taken until the belt stops. Acceleration and deceleration of the belt may be ignored for the purposes of determining the average belt speed.

Actual revolutions = 10 + (3/12) = 10,25

Belt speed (m/min) = (12 m × 10,25)÷1,5 = 82 m/min

Revisions

Rev. 3 (Oct 2016)

Rev. 2 (June 2013)

Rev. 1 (May 2008)


Footnotes

Footnote 1

All loads may be known test loads if desired. The unknown loads are used simply to evaluate the interactions between individual loads usually used while the system is in operation.

Return to footnote 1 referrer

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