Does a 5 kg Step Weighbridge Really Need OIML C4, C5 or C6 Load Cells?
How to verify load cell accuracy class with the OIML R76 compatibility formula — vmin, Y and the difference between display division and verification scale interval — for an 8 × 30t weighbridge.
8 load cells x 30t d = 5kg vmin ≤ 1.768kg Y ≥ 17,000 OIML R60 C4/C5/C6
CURIOTEC NDSB-A 30T OIML C6/IP68 compression load cell — a typical choice for 8 × 30t weighbridges.
I. “5 kg resolution” — what the customer says vs what the standard means
Customers frequently specify that a truck scale indicator shall display weight in 5 kg increments. This requirement is often called a “5 kg resolution”. From a legal metrology viewpoint, however, the engineer must separate two different parameters before selecting any load cell:
| Symbol | Meaning | Practical role |
| d | Display resolution | The smallest step shown on the indicator display. |
| e | Verification scale interval | The value used for accuracy class evaluation and legal metrology verification. |
Key point: a weighbridge may display d = 5 kg while being legally verified at e = 10 kg or 20 kg. If legal verification is also required at e = 5 kg, the load cell specification becomes significantly more demanding.
II. System configuration used in this example
| Item | Value |
| Number of load cells (N) | 8 |
| Capacity per load cell (Emax) | 30,000 kg |
| Typical total scale capacity | 80 t |
| Required display step | 5 kg |
| Load transmission ratio (R) | 1 |
III. The OIML R76 compatibility check: vmin and Y
OIML R76 requires that the minimum load cell verification interval vmin is compatible with the verification scale interval of the complete instrument:
vmin ≤ e × R / √N
Substituting e = 5 kg, R = 1 and N = 8 load cells:
vmin ≤ 5 / √8
vmin ≤ 1.768 kg
The same requirement can be expressed as a minimum Y value (Y = Emax / vmin) for each 30 t load cell:
Y = 30,000 / 1.768
Y ≥ 17,000 (rounded)
Practical check: a 30 t load cell certified with vmin = 1.5 kg gives Y = 30,000 / 1.5 = 20,000, which comfortably satisfies the requirement for legal verification at e = 5 kg.
IV. So is OIML C4, C5 or C6 mandatory?
Not necessarily. The OIML class designation alone does not determine suitability. The class defines a performance envelope, but the decision must be made against the actual certified parameters on the OIML certificate:
- vmin — minimum load cell verification interval
- Y = Emax / vmin
- nLC — maximum number of load cell intervals
- Z — output-to-dead-load ratio
- Temperature performance, creep, and return to zero
| Class | Verdict for this configuration |
| C4 | May be acceptable — if the OIML certificate demonstrates vmin ≤ 1.768 kg for the 30 t model. |
| C5 | Recommended engineering choice — provides additional metrological margin for temperature, creep and long-term drift. |
| C6 | Even greater performance, but not mandatory based on this calculation alone — worth it only when higher performance is specifically required. |
KALA note: two load cells of the same class from different manufacturers can have different certified vmin values. Always read the certificate — the class is the label, vmin and Y are the answer.
V. Practical engineering recommendation — two very different cases
Case A — 5 kg is only the display step (d = 5 kg, e = 10 or 20 kg).
The legal verification interval remains 10 kg or 20 kg, so the vmin requirement relaxes considerably. Many C4 or C5 products will comply, and the selection can be driven by mechanical fit, IP rating and price.
Case B — legal verification at e = 5 kg.
Each 30 t load cell must satisfy vmin ≤ 1.768 kg and Y ≥ 17,000. A 30 t load cell with vmin = 1.5 kg (Y = 20,000) is an excellent engineering choice. Note that 80 t / 5 kg also means n = 16,000 scale intervals — beyond the usual 10,000 intervals of an ordinary Class III instrument, so the whole instrument file must support this special configuration, not just the load cells.
VI. Conclusion
| Question | Answer |
| Should load cells be selected by OIML class alone? | No. Verify the certified vmin and Y values against the required verification interval and number of load cells. |
| Is C4 suitable? | Possibly — only if the certificate satisfies vmin ≤ 1.768 kg (Y ≥ 17,000). |
| Which class does KALA recommend? | C5 — the balanced engineering choice for an 8 × 30 t weighbridge verified at e = 5 kg. |
| Is C6 necessary? | It adds margin but is generally unnecessary unless higher performance is specifically required. |
VII. Load cells KALA supplies for this class of application
CASC / South-Ocean and CURIOTEC both offer 30 t compression load cells with OIML certificates suitable for high-interval weighbridges. KALA checks the certified vmin, Y and nLC values of each model against the customer’s verification requirement before quoting.
Learn more about KDS 30tf / GF-1 OIML load cell on KALA’s English website.
VIII. References
- OIML R60 — Metrological Regulation for Load Cells: oiml.org/en/files/pdf_r/r060-e00.pdf
- OIML R76-1 — Non-automatic Weighing Instruments: oiml.org/en/files/pdf_r/r076-1-e06.pdf
- WELMEC — European Cooperation in Legal Metrology: welmec.org
- NIST Handbook 44 (United States): nist.gov/pml/owm/handbooks
IX. Useful internal references
- Load cell KDS 30tf / GF-1 OIML
- South-Ocean (CASC)
- CURIOTEC load cells
- CURIOTEC NDSB-A 30T load cell
- Weighing Accuracy
- How to choose the best load cell for your needs
KALA Technical Notes
This article discusses engineering selection principles for weighbridge load cells. Final product selection should always be verified against the official OIML certificate and the applicable national legal metrology regulations. If you need to check whether a specific load cell model satisfies your verification interval, contact KALA for technical support.
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