When it comes to weighing accuracy, your scale is only as reliable as the weights you use to test and calibrate it. Calibration weights whether cast iron or bullion (brass/stainless steel) are the foundation of accurate measurement in every lab, factory, or industrial unit. But choosing the wrong type of calibration weight can silently introduce errors, affect product quality, and even violate industry compliance standards.
In this guide, you’ll learn:
- The difference between cast iron and bullion calibration weights
- Their respective applications and accuracy classes
- How to select the right one for your weighing scale or industry
- How proper calibration saves time, money, and reputation
What Are Calibration Weights?
Calibration weights are standard reference masses used to check, adjust, or verify the accuracy of weighing scales. They ensure your scale measures correctly by comparing its readings against a known, certified standard.
Every calibration weight is manufactured under strict tolerance limits and classified according to OIML (International Organization of Legal Metrology) standards from Class M1 (general industrial use) to Class E1 (high-precision laboratories).
The Two Main Types of Calibration Weights
In most industrial and laboratory environments, you’ll encounter two primary types of calibration weights:
- Cast Iron Weights – Durable, cost-effective, ideal for high-capacity industrial scales.
- Bullion Weights (Stainless Steel or Brass) – Precision-engineered, corrosion-resistant, designed for analytical or lab applications.
Both serve the same purpose accuracy verification but their construction, accuracy levels, and usage differ significantly.
Cast Iron Calibration Weights
Overview
Cast iron weights are the most widely used calibration weights in factories, warehouses, and large-capacity weighing systems.
They’re made from high-grade cast iron and often coated with anti-corrosive paint or powder finish for durability.
Applications
- Platform and floor scales
- Weighbridges and truck scales
- Crane scales and heavy-duty industrial weighing systems
- Manufacturing, logistics, and raw material handling
Features
- Rugged and long-lasting
- Available in large capacities (1 kg – 500 kg and above)
- Cost-effective for industrial use
- Can be stackable for mass calibration
Accuracy Class
Typically manufactured to OIML M1 or M2 class, suitable for moderate-precision requirements.
Advantages
- High strength and stability
- Ideal for outdoor and heavy-duty environments
- Lower cost compared to stainless-steel weights
Limitations
- Requires periodic re-coating to prevent rust
- Less accurate than bullion weights
- Not suitable for laboratory or fine-scale calibration
Bullion (Stainless Steel or Brass) Calibration Weights
Overview
Bullion weights, also known as stainless-steel or brass calibration weights, are used in analytical and laboratory applications where precision and environmental stability are critical.
They are highly polished, corrosion-resistant, and designed for very tight tolerances.
Applications
- Analytical balances and microbalances
- Laboratory precision scales
- Pharmaceutical, chemical, and research industries
- Calibration laboratories and metrology institutes
Features
- Made from non-magnetic stainless steel or brass
- High density and corrosion resistance
- Comes in box sets (1 mg – 20 kg)
- Usually supplied with calibration certificates
Accuracy Class
Available in OIML E1, E2, F1 or F2 classes, providing high to ultra-high precision.
Advantages
- Highest accuracy and stability
- Resistant to oxidation and corrosion
- Suitable for clean-room and lab use
- Long calibration intervals
Limitations
- More expensive than cast iron
- Not economical for large capacities (> 50 kg)
- Must be handled carefully to avoid surface damage
Cast Iron vs Bullion Weights – Quick Comparison
| Feature | Cast Iron Weights | Bullion (SS/Brass) Weights |
|---|---|---|
| Material | High-grade cast iron | Stainless steel / brass |
| Accuracy Class | M1 / M2 | E1 – F2 |
| Surface Finish | Painted / powder-coated | Polished, mirror finish |
| Capacity Range | 1 kg – 500 kg + | 1 mg – 20 kg |
| Precision Level | Moderate | Very high |
| Corrosion Resistance | Low–moderate | High |
| Cost | Economical | Expensive |
| Best For | Factories, weighbridges, platform scales | Labs, pharma, analytical balances |
How to Select the Right Calibration Weights
Selecting the right calibration weights depends on your weighing scale system, accuracy requirements, and operating environment.
Step 1. Identify Your Scale Type
- Analytical balance → use E1/E2 bullion weights
- Precision balance → use F1/F2 weights
- Industrial platform scale → use M1 cast iron weights
- Weighbridge → use M1/M2 cast iron weights
Step 2. Determine Required Accuracy
Check your scale’s manufacturer specifications it will specify the tolerance class.
Your calibration weights must be one class higher than the scale’s accuracy.
Step 3. Check Environmental Conditions
- If humidity or chemical exposure is high → choose stainless-steel weights.
- If outdoor or dusty environment → choose cast iron with anti-corrosion coating.
Step 4. Verify Certification & Traceability
Buy weights with:
- OIML / NABL / ISO 17025 certificate
- Individual serial numbers and adjustment cavity seals
- Calibration reports from accredited labs
Step 5. Plan Maintenance and Re-Calibration
Even the best weights drift over time due to dust, corrosion, or handling.
Re-calibrate at least once per year to maintain accuracy.
Common Mistakes to Avoid
- Buying cheap, uncertified weights with no tolerance class.
- Using cast iron weights for precision lab scales.
- Ignoring surface corrosion or chipped coating on cast iron weights.
- Mixing weight classes (e.g., using M2 and F1 together).
- Not storing weights properly in protective boxes.
Each of these mistakes can lead to significant measurement errors and production inconsistencies.
Real-World Example
A steel fabrication plant used basic cast-iron test weights for both platform scales and lab balances.
While suitable for production use, the lab results often varied due to weight tolerance mismatch.
After switching to certified F1 stainless-steel weights for lab calibration and M1 cast iron weights for floor scales, they achieved:
- 40 % reduction in measurement errors
- Faster compliance audits
- Longer calibration intervals
The result: better consistency across all production and quality-control checkpoints.
Final Thoughts
Calibration is the backbone of weighing accuracy.
Cast iron weights are ideal for heavy-duty industrial applications strong, affordable, and reliable.
Bullion (stainless-steel or brass) weights are essential for laboratories and precision environments where even a milligram matters.
Choosing the right calibration weight type ensures:
- consistent product quality
- compliance with OIML standards
- lower maintenance costs
- trust in every measurement
If you’re investing in weighing accuracy, remember the right calibration weight is not an accessory, it’s insurance for your scale’s credibility.
Frequently Asked Questions
1. What are the main differences between cast iron and bullion calibration weights?
Cast iron weights are designed for heavy-duty industrial applications, while bullion (stainless-steel/brass) weights are used for high-precision laboratory applications.
2. Which OIML class should I choose for my scale?
Generally, use one class higher than your scale’s tolerance e.g., F1 for precision balances, M1 for platform scales, E2 for analytical balances.
3. Can cast iron weights rust?
Yes. They should be re-coated periodically and stored in dry conditions to maintain accuracy.
4. Are stainless-steel weights magnetic?
High-quality stainless-steel calibration weights are non-magnetic and corrosion-resistant.
5. How often should calibration weights be verified?
At least once a year, or sooner if exposed to harsh conditions.