Analytical balances: how an analytical balance, a laboratory analytical balance and other laboratory balances are specified, sited and calibrated, what a laboratory balance owes in balance calibration services reporting, and what the quoted readability does not tell you

Specifying and installing one

1. Readability against repeatability. Readability is the smallest increment the display shows. Repeatability is the spread you actually get weighing the same object repeatedly in your own room. Ask the supplier for the repeatability specification and treat the readability as a display characteristic rather than a measurement capability.
2. Capacity, and why bigger is worse. A balance with a larger capacity at the same readability is a compromise, and semi-micro and micro balances trade capacity for resolution deliberately. Size the capacity to the heaviest thing you will weigh including its container, and no further.
3. Internal or external calibration. An internal motorised weight lets the balance adjust itself on a temperature change or a schedule, which matters in a room that is not tightly controlled. External calibration with certified weights is cheaper to buy and depends on somebody actually doing it.
4. Siting decides the result. Put the balance on a heavy, isolated bench away from doors, walkways, air conditioning outlets and anything that vibrates, and let it stabilise after moving. A fine balance in a poor location performs worse than a coarse balance in a good one.
5. Static and draughts. Static on plastic weighing vessels and low humidity together produce drifting readings that look like instrument faults. An ioniser, antistatic vessels or a controlled humidity range fix them, and none of that appears in the purchase specification.

Calibration, verification and the paper trail

Adjustment sets the balance against a known mass; verification checks it and records the result. Both are needed, and the verification record is what an auditor reads. Establish who performs each, how often, and with what class of weights before the instrument arrives.

Check weights are themselves certified and drift, so they carry their own recalibration interval. A lab that verifies diligently against uncertified weights has a record that proves nothing.

What to ask the supplier

Ask for the repeatability figure, the minimum sample weight the instrument can support for your required uncertainty, the warm-up time from cold, and the service network's response time. Minimum sample weight in particular is what stops a lab weighing tiny quantities on an instrument that cannot support the claim.

Ask what the draught shield is made of and how it opens, because a shield that is awkward with gloved hands will be left open and the specification will not be met.

Common questions

What is the difference between an analytical and a precision balance?

An analytical balance reads to a finer division and is enclosed in a draught shield; a precision balance offers more capacity at a coarser division and usually no shield. The choice follows the smallest mass you must weigh reliably.

How often should an analytical balance be calibrated?

Adjust and verify on a schedule set by use and by what depends on the result, and verify with certified weights before critical work. Many labs verify daily with a single check weight and have the instrument serviced and adjusted externally on a fixed cycle.

Why does my balance drift?

Most often the environment rather than the instrument: draughts, vibration, a temperature change, static on the vessel, or an object not in thermal equilibrium with the room. Rule those out before calling a service engineer.

What is minimum sample weight?

The smallest mass the balance can weigh while still meeting a stated measurement uncertainty. Weighing below it produces a number on the display that does not support the accuracy the method assumes.

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