How to choose an analytical or semi-micro balance: readability, capacity, repeatability

Buying a laboratory balance often starts with one question: how many decimal places does it show? That is only one part of the story. A balance that displays 0.01 mg is not automatically able to weigh a 0.01 mg sample reliably. This guide explains the three numbers that matter most - readability, capacity and repeatability - and how they connect to the smallest sample you can weigh with confidence.

Know the balance categories

Sartorius groups its laboratory balances by readability, which it defines as the smallest difference in mass that can still be read on the display. Its current range, as listed on its laboratory balances page, runs roughly like this [1]:

Other sources quote slightly different typical ranges for each class, so always read the datasheet of the specific model [3]. The general pattern is the same: the finer the readability, the lower the capacity.

Readability: choose what your work needs

Start from the smallest mass difference you must read. A 0.1 mg analytical balance suits most routine analytical weighing, while a semi-micro balance (0.01 mg) is worth considering when your samples are small or your tolerance is tight. Paying for more readability than your method needs adds cost and makes the balance more sensitive to draughts, vibration and static, without improving your results [3].

Capacity: match your heaviest load

Capacity is the maximum load the balance can carry, and it includes the container as well as the sample. Sartorius advises matching capacity to your heaviest load [1]. If you weigh very small amounts of substance in large, heavy vessels, a micro-class balance is the better fit; for larger samples, a precision balance is usually enough [1]. Remember that the tare vessel counts towards the maximum, so check that the vessel plus sample stays comfortably within the range.

Repeatability: the number behind "how small can I weigh"

Repeatability is the spread (standard deviation) of repeated weighings of the same load under unchanged conditions [3]. It matters because any weighing carries some uncertainty, and that uncertainty weighs more heavily on a small sample. Sartorius illustrates this with an analytical balance: an uncertainty of 0.1 mg on a 100 g sample is only 0.0001 % of the result, but the same 0.1 mg on a 1 mg sample is 10 % [2].

That is why a minimum sample weight is defined. In the pharmacopoeia approach described by Sartorius, a repeatability test of at least 10 readings is done, and the result is acceptable when twice the standard deviation, divided by the smallest net weight you intend to weigh, is not more than 0.10 % [2]. The theoretical minimum weight is then about 2000 times the standard deviation, with a lower limit of 820 times the readability so that a very lucky test does not give a minimum of zero [2].

A simpler rule of thumb found in general lab guidance is to keep the sample at least 1000 times the readability, which keeps the relative uncertainty below about 0.1 % [3]. For a 0.1 mg balance that is around 100 mg, and for a 0.01 mg balance around 10 mg [3]. Treat this as a quick screening rule only. For regulated work, use the measured minimum weight of your own balance, in your own room.

Net weight, not tare difference

Sartorius points out that minimum sample weights apply to the net amount. You cannot, for example, weigh 0.5 g of substance in a 50 g vessel on a balance whose minimum is 1 g, and treat the difference between the full and empty vessel as a valid result [2]. Plan the smallest net amount you will really weigh, then choose a balance whose minimum weight sits comfortably below it. Sartorius also advises against simply resetting your smallest desired weight to whatever the latest repeatability test produced, because that result can change from test to test [2].

Conditions in your laboratory

• Vibration, air currents, temperature changes and static can all shift readings, and static can matter more at higher resolution [3].

• Let cold or warm samples reach room temperature before weighing, to avoid convection effects [3].

• Sartorius states that its balances can be used between 5 and 40 degrees, though a constant temperature is preferred [1].

• Balances with the isoCAL function recalibrate automatically when conditions change, but occasional external calibration with certified weights is still recommended [1].

• Repeatability published for a balance may not be reached on difficult samples such as fine powders. Ask for data measured under conditions close to your own [3].

Quick checklist before you choose

• Write down the smallest net sample you will weigh and the tolerance your method needs.

• Choose the readability class from that tolerance, not from the longest display.

• Add the weight of your largest vessel to the largest sample, and check that it fits within capacity.

• Check the repeatability and minimum weight figures in the datasheet, and confirm them on site.

• Check the bench: stable, away from air-conditioning vents, doors and vibrating equipment.

• Plan calibration: internal where available, plus periodic external checks with certified weights.

• If you work under a pharmacopoeia or an accreditation scheme, check what that scheme requires for routine tests.

Conclusion

Readability tells you what the display can show, capacity tells you what the balance can carry, and repeatability tells you how far to trust the last digits. Decide your smallest real sample first, pick the balance class whose repeatability supports it, and then confirm performance in your own laboratory. This article is general guidance only; always rely on the manufacturer's datasheet for the exact specification of any model.