M-25—Load Cells and Force Transducers used in Trade Devices

Category: Mass
Issue Date:
Effective Date:
Supersedes: M-25 (rev. 1)


Table of Contents


1.0 Purpose

The purpose of this bulletin is to clarify when an approved load cell is required and to define the parameters necessary to ensure that a load cell meets all of the requirements for use in a trade device.

2.0 Scope

This bulletin applies to all weighing devices whose Notice of Approval (NOA) stipulates that one or more approved load cells must be used in the device.

Currently, all high capacity weighing devices (Class IIIHD) require the use of approved load cells.

All other devices that hold a valid NOA do not require the use of separately tested and approved load cells unless specifically required in the NOA.

3.0 Background

Measurement Canada phased in the requirement for using approved load cells in high capacity weighing devices. The NOA, Notice of class approval (CLASS), or other approval documents indicate which devices must be equipped with approved load cells. Devices installed before the implementation date stipulated in these documents are exempt from these requirements.

4.0 Policy

Approved load cells are considered to be load cells with:

  • a US NTEP (National Type Evaluation Program) Certificate of Conformance (CC);
  • an International Organization of Legal Metrology (OIML) a mutual acceptance arrangement (MAA) Certificate of Conformity; or
  • an OIML Certification System (CS) Certificate of Conformity.

The chosen load cell must also 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

Initial examinations of newly installed devices identified in the scope above are only to be performed if the device is equipped with appropriate approved load cells. If other load cells are used, the device is not considered as meeting the conditions specified in the NOA or Notice of CLASS Approval.

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

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 cut-off 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 high capacity devices will eventually be subject to the requirements for approved load cells. For this reason, it is highly recommended that approved load cells be used in these devices, whenever possible.

5.0 Compatibility

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 mandatory and optional parameters. 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 from Measurement Canada in an automated Excel format or in a paper version to be completed manually.

5.1 Mandatory Parameters

The following manufacturer-specified load cell parameters must be appropriate for the device configuration.

  • Class
  • 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)
  • Electrical compatibility 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 (six-wire cable with sense required to change cable length)
  • Connection cable length (should not be altered from manufactured length unless a six-wire cable is used)
  • Wire gauge appropriate for cable length
  • Excitation voltage through intrinsically safe (IS) barriers and lightning suppressors

Humidity testing of NTEP load cells (humidity test codes NH, SH or CH)

6.0 Other

Where the use of approved load cells is mandatory, the following also applies.

6.1 Repaired or remanufactured load cells

The original Certificate of Conformance or Certificate of Conformity (MAA, CC or CS) no longer applies if the load cell is repaired by someone 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 remanufactured 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

The load cells of a device may be substituted for loads cells from the same or a different manufacturer provided that the new load cells:

  • have a valid Certificate of Conformance or Certificate of Conformity (MAA, CC or CS);
  • 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 device;
  • 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 device 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.

It is also recommended that they be of the same wiring configuration as the force transducers being replaced without adding jumper wires, connecting sense wires to excitation wires, or removing the sense wires.

In a system with multiple load cells, the replacement of all load cells in the system with approved and compatible load cells that don't have the same type of output (e.g. analog, digital or hydraulic) as the original force transducer is considered a metrologically equivalent replacement provided all the above requirements 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 be approved under OIML's R 60. The load cells or the accompanying certificate may be marked with the OIML logo, which could include the stylized letters MAA or CS.

NTEP load cells or the accompanying certificate may be marked with the NTEP logo.

7.0 Revisions

The purpose of revision 2 was to:

  • clarify the scope (section 2.0);
  • add load cells approved under the OIML-CS;
  • modify the French text to match the English text.

The purpose of revision 1 was to modify section 6.3 to reflect the fact that the NTEP logo may not be present on all NTEP-approved load cells.