When purchasing weights, attention usually goes to the nominal value and the class. The material seems a side issue. Yet the material determines which classes are achievable, how sensitive a weight is to moisture and dirt, how much maintenance it needs and in which environment you can use it.
In our laboratory we see the difference every day: stainless steel weights that are still virtually unchanged after years, and cast iron block weights whose paint is damaged and whose rust has spread. In this article we set the materials side by side. How industrial weights are calibrated and maintained is explained under calibrating M1, M2 and M3 weights.
The common materials at a glance
| Material | Typical form | Usual classes | Density, indicative | Magnetic |
|---|---|---|---|---|
| Stainless steel, polished | Knob weights, compact cylinders, weight sets | E1 to F2 | approx. 7,900 to 8,000 kg/m³ | Hardly |
| Stainless steel, fine-turned | Knob weights, block weights | F and M classes | approx. 7,900 to 8,000 kg/m³ | Hardly |
| Chrome-plated brass | Knob weights | M classes | approx. 8,400 to 8,700 kg/m³ | No |
| Chrome-plated steel | Knob and cylindrical weights | M classes | approx. 7,850 kg/m³ | Yes |
| Cast iron, painted | Block weights, mass pieces | M1 to M3 | approx. 7,000 to 7,300 kg/m³ | Yes |
The densities are indicative and differ per alloy. Stainless steel here means austenitic stainless steel, the type commonly used for weights, which is virtually non-magnetic.
Why cast iron does not fit the F and E classes
A cast iron weight can have a small deviation and still never be assessed as an F2 weight. This is because OIML R111 sets requirements per class not only for the deviation, but also for material, surface, density and magnetic properties. Cast iron falls short on several points for the fine classes:
- Surface. Cast iron is porous and must be painted against corrosion. A fine class requires a smooth, dense surface that remains stable and is easy to clean.
- Density. Cast iron lies further from the reference density of 8,000 kg/m³ than stainless steel. As a result the weight is more sensitive to fluctuations in air density.
- Magnetism. Cast iron is magnetic, whereas the fine classes set strict limits on magnetic properties.
- Stability. Paint can be damaged and rust can develop. For a weight that has to remain accurate to tenths of a milligram, that is too great a risk.
For the M classes those requirements are less strict, and there cast iron is a robust and practical material. It is often also cheaper to buy, certainly for large masses.
Magnetism and density in outline
Many precision balances work with electromagnetic force compensation, in which a magnet forms part of the weighing system. A weight that is itself magnetic or can become magnetic can exert a small additional force on it. On the balance the weight then appears heavier or lighter than it is. In the fine classes that is noticeable. That is why E and F weights are generally made of stainless steel with a very low magnetic susceptibility. For industrial scales with load cells and M-class weights, this effect plays hardly any role in practice.
Density counts because of air buoyancy. A weight with a lower density has a larger volume and displaces more air. Conventional mass takes this into account through a fixed convention, but the further the density lies from 8,000 kg/m³, the greater the influence of the weather and of the room in which you measure. A detailed explanation with a worked example can be found in Conventional mass and air buoyancy.
Maintenance and service life per material
Stainless steel
Stainless steel has no coating that can be damaged and resists moisture well. With careful use, a stainless steel weight remains stable for many years. The points of attention are fingerprints, scratches and contact with strong magnets. Stainless steel is also the logical choice in hygienic environments, because it is easy to clean and no paint can flake off.
Chrome-plated brass and steel
The chrome layer protects the underlying metal and gives a smooth surface. As long as that layer is intact, the weight behaves well. After a fall or a scratch, the layer can come loose. Brass then discolours and steel starts to rust. Re-chroming changes the mass and is rarely worthwhile; in the case of significant damage, replacement is usually the better option.
Cast iron
Cast iron is strong under compressive load, but brittle. A fall onto a concrete floor can knock off a corner. The paint layer needs attention: every bit of damage is the start of rust. With dry storage and timely repair of the paint, a cast iron weight lasts a long time. Derusting, repainting and adjusting via the adjustment cavity are perfectly possible, but always require a new calibration. Practical tips can be found in Storing and handling calibration weights.
Selection guide per application
The class follows from the scale you check: the error of the weight is at most one third of the permissible error of the scale at that load. The material follows from the class and the environment.
| Application | Usual class | Suitable material |
|---|---|---|
| Analytical balance | E2 | Stainless steel, polished |
| Precision balance in laboratory or pharmacy | F1 or F2 | Stainless steel |
| Bench scale in food or pharmaceutical production | M1 or F2 | Stainless steel, for cleaning and hygiene |
| Retail or bench scale in a dry environment | M1 | Stainless steel or chrome-plated brass |
| Floor and pallet scales | M1 | Cast iron block weights; stainless steel in wet or hygienic zones |
| Weighbridge, crane scale, proof load testing | M1 to M3 | Cast iron or steel mass pieces |
If you are undecided between two classes, choose the finer one. A finer class remains usable if you later purchase a more accurate scale. Bear in mind that a cast iron weight can never be assessed higher than M1. More about choosing the class in Which calibration weight do I need? Separate points of attention apply to heavy weights, see Transporting, lifting and storing heavy test weights safely.
How MASSLAB helps you
MASSLAB calibrates weights of stainless steel, chrome-plated brass and steel, and cast iron in classes F1, F2, M1, M2 and M3. We clean, derust, paint and adjust cast iron weights in-house before we calibrate them. If you are going to purchase new weights, you will find block weights and knob weights in various materials in our webshop, and we are happy to advise you on the choice. More about our working method under calibrating M1, M2 and M3 weights.