A weight comes back from the laboratory with a neat certificate. A year later it turns out to be outside its class. In most of the cases we see, something happened to the weight during that year: it was picked up with bare hands, placed on a wet floor, left rolling around in a drawer or rubbed clean with a cloth. The mass of a weight rarely changes by itself, but it does change through the way it is handled.
This article is about what you can control yourself between two calibrations. How a laboratory determines the mass and, where necessary, cleans and adjusts your weights is described under weight calibration.
What makes a weight heavier or lighter
It helps to know which processes affect the mass. Broadly speaking, there are two directions.
A weight becomes heavier through anything that adheres to its surface. Think of skin oil and salt from fingers, dust, oil, a thin film of moisture or residues of cleaning agents. Rust also makes a weight heavier at first, because the iron binds oxygen and moisture.
A weight becomes lighter through loss of material. Scratches, sliding across a workbench, knocking against other weights, and flaking rust or paint each remove a small part of the mass. For small weights that is relatively a lot: a 1 g F1 weight may deviate by no more than 0.10 mg.
In addition, a stainless steel weight can become slightly magnetic if it comes close to strong magnets, such as magnetic stirrers or magnetic holders. On a sensitive balance that produces an apparent deviation, even if the mass itself has not changed.
Storage: a fixed place for every weight
Weight sets and F-class weights
Always keep precision weights in their case or box, each in its own compartment. That way the weights do not touch each other and stay free of dust. Keep the case closed, even if the weights will be used again the same day. Place it in a dry room with a stable temperature, away from windows, heating, ventilation grilles and chemical vapours. Never leave weights lying on the scale.
Individual M-class weights
Knob weights and smaller block weights belong in a closed cabinet or in a case, not loose in a drawer or on an open shelf in the production hall. Stand knob weights on their base, not on the knob. Do not stack weights that are not designed for it: the bottom one gets damaged and the top one can fall.
Heavy weights
Place block weights and mass pieces on a pallet, a plastic sheet or a rubber mat, not on a bare concrete floor where moisture can penetrate. More about storing and moving large weights can be found in Transporting, lifting and storing heavy test weights safely.
Handling: never with bare hands
A fingerprint weighs little, but not nothing. In the fine classes it is one of the most common causes of deviations. Therefore follow these rules:
- Small weights and sheet weights are picked up with tweezers with plastic or coated tips. Bare metal tweezers can scratch. Sheet weights are picked up by the raised edge.
- Larger F-class weights are handled with clean, lint-free gloves or with a weight fork. Touch the weight as little as possible.
- M-class weights may be handled with clean work gloves. Lift them and set them down; do not slide them across the scale or the floor.
- Temperature: allow a weight that comes from a cold or warm room to reach ambient temperature before measuring with it. A warm weight causes an upward air current and therefore appears lighter on a sensitive balance.
Put the tweezers and gloves back in the case so that they stay clean themselves. Replace worn gloves and tweezers that no longer close properly.
Cleaning: what you do yourself and what the laboratory does
Cleaning seems harmless, but incorrect cleaning damages more weights than dirt does. A cleaning agent leaves residues, a cloth with dust particles acts like sandpaper, and a solvent temporarily changes the surface layer. After cleaning with a solvent, a fine weight also has to stabilise first before it can be measured reliably.
| Type | What you can do yourself | Leave to the laboratory |
|---|---|---|
| F1 and F2 weights, weight sets | Remove loose dust with a soft, clean brush or a small bellows | Any cleaning with liquid, solvent or cloth |
| Stainless steel and chrome-plated M-class weights | Wipe with a clean, dry, lint-free cloth | Stubborn dirt, grease residues, discolouration |
| Cast iron weights | Brush off loose dirt and let the weight dry | Rust removal, sanding and repainting |
Never sand or repaint a cast iron weight yourself. A new coat of paint adds mass, and material that has been sanded off is lost. After such treatment the weight must always be recalibrated. If at the next calibration you want to know what the weight weighed before cleaning, for example to assess earlier checks, say so when you register it.
Transport without damage
In transit, weights are most at risk of knocks and temperature changes. Transport precision weights in their closed case, upright and secured, so that the case cannot slide or fall. Check beforehand that each weight sits properly in its compartment. Loose weights in a bag or box knock against each other.
Do not leave weights in a car in heat or frost. If they arrive cold in a warm, humid room, keep the case closed until everything has reached ambient temperature. Otherwise moisture condenses on the surface. If you ship weights, use the original case with extra padding in a sturdy box.
What damages weights, by material
| Material | Usual classes | Main risks |
|---|---|---|
| Stainless steel, polished | Mainly E and F classes | Fingerprints, scratches, magnetisation, incorrect cleaning |
| Stainless steel, fine-turned | F and M classes | Dirt in grooves, dents from falls |
| Chrome-plated brass or steel | M classes | Damaged chrome layer, allowing the underlying metal to corrode |
| Cast iron, painted | M1 to M3 | Paint damage, rust, moisture, caked-on dirt |
| Sheet and wire weights below 1 g | E1 to M1 | Bending, getting lost, static charge |
The choice of material therefore partly determines how much care a weight needs. More on this in Cast iron or stainless steel weights. Good care pays for itself in stable results and a longer calibration interval.
How MASSLAB helps you
At every calibration we assess the condition of your weights and clean them according to the requirements for their class. We derust and repaint cast iron weights in-house and adjust them where necessary. If we see recurring damage, we point it out so that you can address the cause. If you are missing tweezers, a pair of gloves or a proper case, you will find them under cases and accessories. More about our working method under weight calibration.