Sequential Proficiency Testing Scheme for Load Cell Calibration

(ISO 376:2011 compliant | ISO/IEC 17043:2024 – Clause 7.2.1.3 a–u)

1. Introduction

Proficiency Testing (PT) is a key tool for evaluating the technical competence of calibration laboratories and for demonstrating the validity and comparability of calibration results. This article describes the design and implementation of a Sequential Proficiency Testing Scheme for Load Cell Calibration, developed in accordance with ISO 376 :2011 and fully compliant with the requirements of ISO/IEC 17043:2024, Clause 7.2.1.3 (a) to (u).

The scheme uses a sequential circulation design, where a single calibrated load cell is circulated among participating laboratories, allowing accurate assessment of laboratory performance while minimizing variability associated with multiple artifacts.

2. Objective of the PT Scheme

(ISO/IEC 17043:2024 – Clause 7.2.1.3 a)

The objectives of this proficiency testing scheme are to:

• Evaluate the technical competence of participating laboratories in the calibration of load cells in accordance with ISO 376 : 2011.
• Assess laboratories’ ability to correctly determine: indication values , repeatability , Reproducibility , relative error , Loadcell classification .
• Verify the consistency and comparability of calibration results obtained under sequential circulation conditions.

3. Scope and Applicability

(Clause 7.2.1.3 b)

This PT scheme is applicable to:

• Calibration laboratories performing load cell calibration in accordance with ISO 376 :2011.
• Laboratories accredited or seeking accreditation under ISO/IEC 17025.
• Load cell calibrations performed in compression and/or tension modes, within force ranges specified in the PT scheme announcement.

4. PT Scheme Design Type

(Clause 7.2.1.3 c)

The scheme is designed as a Sequential Proficiency Testing Scheme, characterised by the following features:

• One calibrated load cell is circulated sequentially among all participants.
• Measurements are performed one after another in a predefined circulation order.
• Reference laboratory measurements are carried out before and after participant circulation.

Justification of Design

• Load cells are mechanically stable artifacts when properly handled.
• High measurement accuracy is required for ISO 376:2011 classifications.
• Sequential circulation minimizes inter-artifact variability and allows effective monitoring of stability and drift.

5. Participants and Number of Participants

(Clause 7.2.1.3 d)

• Minimum number of participants: as PTP
• Maximum number of participants: as PTP
• One Reference Laboratory designated by the PT provider

The identities of participating laboratories are kept strictly confidential throughout the scheme.

6. Roles and Responsibilities

(Clause 7.2.1.3 e)

PT Provider

The PT provider is responsible for:

• Designing and coordinating the PT scheme
• Preparing, inspecting, and circulating the PT artifact
• Collecting, analyzing, and evaluating participant data
• Preparing and issuing the final PT report
• Ensuring confidentiality and impartiality

Reference Laboratory

The reference laboratory shall:

• Perform load cell calibration before and after circulation
• Provide reference values and associated measurement uncertainties

Participants

Participating laboratories shall:

• Perform calibration strictly in accordance with ISO 376 :2011
• Follow all PT instructions issued by the PT provider
• Submit results and uncertainty data within specified timelines.

7. Description of the PT Item (Load Cell)

(Clause 7.2.1.3 f)

The PT item is a load cell suitable for calibration in accordance with ISO 376.

Each load cell is:

• Uniquely identified
• Clearly labeled
• Traceable to national or international standards

8. Preparation, Handling, and Transportation

(Clause 7.2.1.3 g)

• The load cell is inspected and functionally verified prior to dispatch.
• Transportation is carried out using shock-resistant and temperature-controlled packaging.
• Detailed handling and mounting instructions are provided to participants.
• Participants are required to record relevant environmental conditions during calibration.

9. Homogeneity Assessment

(Clause 7.2.1.3 h)

Homogeneity is ensured by:

• The use of a single PT item in a sequential design.
• Verification of consistency through pre- and post-circulation reference measurements.

Acceptance Criterion - | Xrefpost -Xrefpre |<= UStab

10. Stability Assessment

(Clause 7.2.1.3 i)

Stability of the load cell is assessed by:

• Reference measurements conducted before and after circulation
• Evaluation of drift at each force point

Stability Criterion - | Xrefpost -Xrefpre |<= 0.3 x Uref

If instability is detected, appropriate corrective actions are initiated and documented.

11. Measurement Method

(Clause 7.2.1.3 j)

• Calibration is performed strictly in accordance with ISO 376.
• Typical force steps include: 10 %, 20 %, 40 %, 60 %, 80 %, and 100 % of the load cell capacity.
• Loading sequences follow ISO 376 requirements.
• Repeat measurements are performed at each force step to evaluate repeatability.

12. Assigned Value Determination

(Clause 7.2.1.3 k)

Assigned values are determined from reference laboratory measurements using the mean of pre- and post-circulation results:

X assigned = (Xref. pre + Xref.post)/2

13. Uncertainty of Assigned Value

(Clause 7.2.1.3 l)

The uncertainty associated with the assigned value includes:

• Reference laboratory uncertainty
• Stability component
• Repeatability component (where applicable)
• U assigned = k x sqrt(sumsq(uref,ustab))

14. Performance Evaluation Criteria

(Clause 7.2.1.3 m)

Participant performance is evaluated using the En number:

Performance is evaluated using En number:

En = (Xlab-Xassigned)/sqrt(sumsq(Ulab,Uassigned)

En Value Interpretation

|En| < 1 - Satisfactory

|En| > =1 - Unsatisfactory

15. Statistical Techniques

(Clause 7.2.1.3 n)

The following statistical techniques are applied:

• Descriptive statistical analysis
• En number evaluation
• Trend analysis to detect sequential drift

Robust outlier removal techniques are not applied, due to the sequential single-artifact design.

16. Handling of Outliers and Invalid Results

(Clause 7.2.1.3 o)

• Results are not excluded automatically.
• Suspect or inconsistent results are investigated.
• Technical review is performed before any exclusion decision.

17. Confidentiality

(Clause 7.2.1.3 p)

• Each participant is assigned a unique identification code.
• Participant identities are known only to the PT provider.
• All results are reported anonymously.

18. Impartiality and Conflict of Interest

(Clause 7.2.1.3 q)

• The reference laboratory is independent of the participants.
• Impartiality risk assessments are conducted.
• Conflict-of-interest declarations are obtained and maintained.

19. Data Handling and Records

(Clause 7.2.1.3 r)

• Secure electronic and physical data storage is maintained.
• Controlled spreadsheets and calculation sheets are used.
• Records are retained in accordance with the Research Consultant Quality Procedures.

20. Reporting of Results

(Clause 7.2.1.3 s)

The final PT report includes:

• Assigned values and associated uncertainties
• Participant results
• En number evaluation
• Homogeneity and stability assessment summaries
• Technical comments and conclusions

21. Communication with Participants

(Clause 7.2.1.3 t)

• PT instructions are issued prior to circulation.
• Technical clarifications are provided in writing.
• The final PT report is issued to all participants.

22. Corrective Actions and Follow-up

(Clause 7.2.1.3 u)

• Unsatisfactory results trigger technical investigation.
• Root cause analysis is recommended to participants.
• Corrective actions are documented and reviewed.