Capacity vs Readability: Choosing the Right Weighing Range

Capacity and readability are the two numbers that define what a scale can weigh and how finely it can show the result. Choosing them well is a balance: a scale with too little capacity is overloaded by the heaviest boxes and the impacts of a moving deck, while a scale with too much capacity shows small loads in coarse steps. This article explains the terms, gives a rule of thumb for minimum load, and maps common marine weighing jobs to WPL weighing ranges.

Key terms: capacity, division and minimum load

The international vocabulary for scales comes from OIML R 76-1, the recommendation for non-automatic weighing instruments. The terms that matter for selection are:

Term	Symbol	Meaning in one sentence
Maximum capacity	Max	The largest load the scale is designed to weigh, not counting additive tare.
Actual scale interval (division, readability)	d	The step between two consecutive displayed values, for example 5 g.
Verification scale interval	e	The interval used to classify and verify an instrument; it may equal d.
Number of verification scale intervals	n	How many verification steps the weighing range is divided into: the maximum capacity divided by the verification scale interval.
Minimum capacity	Min	The load below which results may have an excessive relative error.

Readability is not the same as accuracy. A display that shows 1 g steps can still be several grams wrong if the scale is poorly adjusted, badly mounted or disturbed by motion. Readability sets the finest possible step; accuracy is how closely the result agrees with the true mass.

WPL specifications give readability as a range, for example 15 kg (2–5 g). The finer figure is the smaller step; which step applies to a given scale depends on its configuration and application, so confirm the value for your use when ordering.

The minimum load rule of thumb

A practical rule: do not weigh loads smaller than 20 divisions, and for good relative resolution keep typical loads above about 200 divisions. The first part comes from legal metrology. OIML R 76-1 sets the minimum capacity of a class III (medium accuracy) instrument at 20 verification scale intervals, of a class IIII instrument at 10 intervals, and of a class II instrument with an interval of 0.1 g or more at 50 intervals. The US requirements for at-sea catch scales in 50 CFR 679, Appendix A use the same convention: a minimum load of 20 scale divisions for class III and 10 divisions for class IIII.

The second part follows from simple arithmetic. One division is a fixed number of grams, so its share of the load shrinks as the load grows:

Load expressed in divisions	One division as share of the load	Example on a 5 g division
20 divisions	5%	100 g load
50 divisions	2%	250 g load
200 divisions	0.5%	1 kg load
1,000 divisions	0.1%	5 kg load
3,000 divisions	0.033%	15 kg load

At sea there is a further reason to respect the minimum load: residual motion affects small loads in the same percentage as large ones, but the stability criterion works in divisions. A small load close to the minimum is more likely to fluctuate by several divisions and take longer to settle.

Choosing capacity: plan for gross load and dynamic peaks

Choose the capacity from the heaviest gross load you will put on the scale, then add a margin for motion and handling. The gross load is the product plus container, ice and water.

1. Heaviest net load. For example, a 40 kg fish box filled to its nominal fill weight.
2. Add the tare. Container, lid, ice and retained water. Weigh your own wet containers; do not rely on catalogue weights of dry boxes.
3. Add a dynamic margin. On a deck accelerating upwards at 1.5 m/s², a 55 kg gross load presses on the load cells like a 63.4 kg static load, about 15% more than at rest. Boxes that are dropped rather than placed create short impact peaks well above their weight.
4. Pick the next capacity up that still gives the readability you need at your smallest regular load.

Overload protection matters for the same reason. The US at-sea requirements state that a platform scale must be designed so that an overload of 150% or more of capacity does not affect its metrological characteristics.

WPL weighing ranges and their minimum loads

The table lists WPL marine weighing ranges with the load equal to 20 divisions, as an illustration of the minimum-load rule. It is not a statement of legally verified minimum capacity for any model.

Series	Capacity	Readability	Load of 20 divisions	Platforms
M3	300 g	0.1–0.2 g	2–4 g	120 × 120 mm
M3	600 g	0.2–0.5 g	4–10 g	120 × 120 mm
M3	1,500 g	0.5–1 g	10–20 g	120 × 120, 270 × 270 mm
M3	3,000 g	1–2 g	20–40 g	120 × 120, 270 × 270 mm
M3	6,000 g	2–5 g	40–100 g	120 × 120, 270 × 270 mm
M2	6 kg	1–2 g	20–40 g	S
M2	15 kg	2–5 g	40–100 g	S, M, L
M2	30 kg	5–10 g	100–200 g	M, L, XL
M2	60 kg	10–20 g	200–400 g	M, L, XL, XXL
M2	100 kg	20–50 g	0.4–1 kg	M, L, XL, XXL
M2	200 kg	50–100 g	1–2 kg	XL, XXL
M2	400 kg	100–200 g	2–4 kg	XL, XXL
M5 / M6	600 kg	200–500 g	4–10 kg	S, M, L
M5 / M6	1,500 kg	500–1,000 g	10–20 kg	S, M, L
M5 / M6	3,000 kg	1,000–5,000 g	20–100 kg	S, M, L

M2 platforms: S 300 × 250, M 400 × 300, L 500 × 400, XL 600 × 400 and XXL 800 × 600 mm. M5 and M6 platforms: S 1000 × 1000, M 1200 × 1200 and L 1500 × 1500 mm. Higher capacities for M5 are available on request. Platform choice is covered in Choosing the Platform Size for Fish Boxes, Baskets and Samples.

Matching use cases to weighing ranges

Start from what you weigh most often, then check the lightest and heaviest regular loads against the ranges above. Items lighter than a few grams, such as many otoliths, fall below 20 divisions even at 0.1 g, so decide whether they need to be weighed at sea or can be weighed ashore.

Use case	Typical load	Suitable WPL range	Why
Organs, stomach contents, very small fish	5 g to a few hundred grams	M3 300 g or 600 g	Sub-gram readability; wind cover available on 120 × 120 mm
Individual fish for length–weight sampling	10 g to 5 kg	M3 1,500 g to 6,000 g	Fine steps across a wide size range; 270 × 270 mm for larger fish
Fillet packs, small trays	0.2 to 5 kg	M2 6 kg	1–2 g steps keep packs well above minimum load
Fillet boxes of 10 to 15 kg	10 to 17 kg gross	M2 30 kg on XL	15 kg is exceeded once the box and ice are added; XL matches the 600 × 400 mm footprint
Euro fish boxes of 25 to 40 kg	25 to 45 kg gross	M2 60 kg on XL or XXL	30 kg is too small once tare and ice are added; 800 mm boxes need XXL
Stacked boxes, tubs, catch subsamples	50 to 350 kg	M2 100, 200 or 400 kg	Headroom for gross load and impacts
Pallets, big tubs, bags, equipment	0.1 to 3 tonnes	M5 or M6	4 load cells; fold-up M5 where deck space is critical

Worked example: one vessel, two scales

Consider a hypothetical vessel that lands fish in 40 kg boxes measuring 800 × 450 mm and also takes weighed samples of 50 g to 3 kg for its catch records.

1. Gross box load: 40 kg fish plus an assumed 2.5 kg wet box and 2.5 kg ice gives 45 kg.
2. With a dynamic margin, the 30 kg range is clearly too small and 60 kg is the smallest suitable capacity.
3. The 800 mm box needs the M2-XXL (800 × 600 mm) platform, which offers 60 kg with 10–20 g readability.
4. On that scale, 20 divisions is 200–400 g. A 50 g sample is far below the minimum, and a 1 kg sample is only 50 to 100 divisions.
5. Samples therefore go on a second scale: an M3 3,000 g (1–2 g) puts a 50 g sample at 25 to 50 divisions and a 3 kg sample at 1,500 to 3,000 divisions.

Using one large scale for everything is a common and costly compromise. Small weighings end up with coarse steps and large relative errors, which also shows up in the total cost of ownership.

How WPL approaches this

WPL builds its marine scales in fixed capacity and platform combinations, so the range can be matched to the job instead of forcing one scale to do everything. Settings such as capacity and rounding are configured with the R10 Configuration Panel. For the full selection process, see the buyer's guide; for why readings move at sea in the first place, see Weighing at Sea. To discuss a specific application, use the quote request.