M-25—Load Cells and Force Transducers used in Trade Devices
Bulletin: M-25 (rev. 1)
Table of Contents
- 1.0 Purpose
- 2.0 Scope
- 3.0 Background
- 4.0 Policy
- 5.0 Compatibility
- 6.0 Other
- 7.0 Revisions
- 8.0 Additional Information
The purpose of this bulletin is to clarify when an approved load cell is required and to define the parameters necessary to ensure a selected load cell meets all of the requirements for use in a trade device.
This bulletin applies to all weighing devices for which the Notice of Approval, Notice of Class Approval or other Notice of Approval documents, including type C and D conditional approvals, stipulate that approved load cell(s) must be used in the device.
Non-modular devices, which have been tested in the mass approvals laboratory and hold a valid Notice of Approval for a complete device, do not require the use of separately tested and approved load cells.
Measurement Canada is phasing in the requirement for using approved load cells in Heavy Capacity Weighing Devices. The Notice of Approval (NOA), Notice of Class Approval (NOCA), or other Notice of Approval documents will indicate which devices must be equipped with approved load cells. Devices installed before the implementation date stipulated in the NOA’s are exempt from these requirements.
Approved load cells are considered to be load cells with a US NTEP Certificate of Conformance (CoC) or an OIML MAA Certificate of Conformity. In addition, the chosen load cell must meet the requirements for the weighing device as defined in the Compatibility section below. Measurement Canada will provide worksheets to assist in assessing the suitability of selected load cells.
4.1 Weighing Devices Installed after the Implementation of the Requirement for Use of Approved Load Cells
4.1.1 Initial inspections of devices identified in the scope above shall only be performed if the device is equipped with appropriate approved load cells. The use of other load cells will mean the device is not considered as meeting the conditions specified in the NOA/NOCA.
4.1.2 Replacement load cells used in these devices must be approved.
4.2 Weighing Devices Installed Prior to the Implementation of the Requirement for Use of Approved Load Cells
4.2.1 Replacement load cells for devices initially inspected prior to the implementation date and identified in the scope above, need not be approved at this time. However, as it is likely a sunset date will be established for these devices, the use of approved load cells is highly recommended in these devices as well.
Note: It is likely that all heavy duty devices will eventually be subject to the requirements for approved cells. For this reason, it is highly recommended that approved load cells be used in these devices, whenever possible.
Load cells must be fully compatible with the device they are intended to be used with. In addition, the device must be configured within the approved parameters of the load cell. Measurement Canada has identified those parameters which are considered mandatory and those which are considered to be optional. In order to facilitate the selection of a suitable load cell, Measurement Canada has developed a Load Cell Compatibility Worksheet. Completion of the worksheet will ensure that the selected load cell is appropriate for the application. This worksheet is available in an automated Excel format or in a manual paper version from Measurement Canada.
5.1 Mandatory Parameters
The following manufacturer specified load cell parameters must be appropriate for the device configuration.
- Temperature Limits
- Apportioning Factor – default values will be used unless otherwise requested
- Number of verification scale intervals (nind)
- Maximum capacity of the load cell (Emax)
- Minimum dead load of the load cell (Emin)
- Maximum number of load cell intervals (nLC)
- Minimum load cell verification interval (vmin)
- Electrically compatible with the indicator
- Data marking on the load cell and/or the accompanying documentation
5.2 Optional Parameters
- Input resistance of a load cell (RLC)
- Connection cable type (6 wire with sense required to change cable length)
- Connection cable length (should not be altered from manufactured length unless 6-wire implemented)
- Wire gauge appropriate for cable length
- Excitation voltage through Intrinsically Safe (IS) barriers and lightning suppressors.
- Humidity Testing of NTEP load cells (NH, SH, CH)
Where the use of approved load cells is mandatory, the following also applies.
6.1 Repaired or Re-manufactured Load Cells
The original Certificate of Conformance/Conformity (approval) no longer applies to a repaired load cell if that load cell is repaired by other than the original manufacturer or authorized agent. Companies claiming to be authorized agents must provide adequate written documentation from the original load cell manufacturer. A copy of all such documentation must be forwarded to Measurement Canada if the re-manufactured load cells are to be used in a device for which the use of approved load cells is mandatory.
6.2 Substitution of Metrologically Equivalent Load Cells
Load cells from the same or a different manufacturer may be substituted into a scale provided that the load cells to be substituted:
- have a valid Certificate of Conformance/Conformity (approval);
- are of the same, equivalent, or better Accuracy Class;
- have as many, or more, verification scale intervals (nmax) as the load cell originally used in the scale;
- have a minimum verification scale interval (vmin) that is suitable for the application;
- are of the same basic type and design as the cell(s) being replaced;
- can be placed in the scale without modifying the basic design of the load cell mounting assembly;
- have a capacity that is greater than, or equal to the capacity of the original cell;
- meet all other requirements for use of an approved load cell in a trade device;
- (recommended) are of the same wiring configuration as the force transducers being replaced without adding jumper wires, connecting sense wires to excitation wires, or by removing the sense leads.
In a system with multiple load cells, the replacement of all load cells in the system with approved and compatible load cells that have a type of output (e.g. analog, digital or hydraulic) different than the original force transducer is considered a metrologically equivalent replacement provided all requirements in sections 1 through 8 above are met.
The replacement of a load cell resulting in a combination of analog, digital or hydraulic load cells in one system is not considered a metrologically equivalent replacement and is not acceptable. All load cells in a multiple load cell system must have the same type of output (e.g. all analog, all digital or all hydraulic).
6.3 Approved Load cells
OIML Load cells must have been approved under the R60 Mutual Acceptance Arrangement. These may be identified by the OIML logo with the stylized letters MAA in the logo (either on the cell or the accompanying certificate).
NTEP Load cells may be identified through the NTEP logo on the load cell and/or accompanying certificate.
The purpose of Revision 1 is to modify sections 6.3 to reflect the fact that the NTEP logo may not be present on all NTEP approved load cells.
8.0 Additional Information
For additional information regarding this bulletin, please contact the undersigned.
Senior Program Officer, Gravimetric
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