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Analytical balance · Calibration procedure

Minimum weight

Determines the smallest net sample mass that can be weighed on the balance while meeting the 0.10% accuracy requirement.

Photo of a Analytical balance
Photo: Karelj, Public domain (cropped), via Wikimedia Commons
PURPOSE

Why this is checked

Below a certain mass, the random scatter of the balance becomes too large compared with the sample. The minimum weight tells analysts the smallest amount they may weigh. It must be determined at the place of use, because draughts and vibration there affect it.

Frequency

Typically every 6 or 12 months together with repeatability, and whenever the balance location or environment changes, as defined in your SOP.

Learn how the Analytical balance works
YOU WILL NEED

Standards & tools

  • Small calibrated test weight, e.g. 100 mg (OIML class E2), placed on a typical tare container if your SOP asks
BEFORE YOU START

Prerequisites

  • Balance in its normal place of use, level and warmed up
  • Same tare container type that analysts normally use, if required
PROCEDURE

Step by step

  1. 1Zero the balance (with the tare container on the pan if used, then tare).
  2. 2Weigh the small test weight 10 times, removing it and re-zeroing between readings.
  3. 3Calculate the standard deviation (s). If s is less than 0.41 × d, use 0.41 × d.
  4. 4Calculate the minimum weight: m(min) = 2000 × s.
  5. 5Apply the safety factor set in your SOP (commonly 2 or 3) to set the working minimum weight.
  6. 6Label the balance with its minimum weight and update the balance register.
RECORD

Example observation table

ReadingObserved (g)
10.1000
20.1001
30.1000
40.1000
50.0999
60.1000
70.1001
80.1000
90.1000
100.1000
CALCULATION

How to calculate the result

m(min) = 2000 × s (from 2 × s ÷ m = 0.10%); working minimum = m(min) × safety factor
  1. s = 0.0000568 g = 0.0568 mg (larger than 0.41 × d = 0.041 mg, so use s)
  2. m(min) = 2000 × 0.0568 mg = 113.6 mg
  3. With a safety factor of 2: working minimum weight = 227 mg
ACCEPTANCE CRITERIA

Pass if

  • Minimum weight determined and displayed on the balance.
  • Every routine net weighing must be at or above the working minimum weight.
IF IT FAILS

What to do

  • If the minimum weight is higher than the smallest samples the lab needs to weigh, improve the environment, use a more sensitive balance (semi-micro or micro) or increase sample size.
  • Review recent weighings below the new minimum weight.
MORE

Other Analytical balance calibrations

In LimsPulse · Calibration Management

LimsPulse schedules this calibration, records the result against limits, and blocks the instrument everywhere if it fails or goes overdue.

See how