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Microbalance · Calibration procedure

Repeatability

Weighs one small test weight ten times and calculates the standard deviation, then checks it against the smallest sample the lab weighs.

PURPOSE

Why this is checked

On a microbalance the random scatter of readings is the main source of error for small samples. USP <41> requires that twice the standard deviation, divided by the smallest net weight, is not more than 0.10%.

Frequency

Typically every 6 or 12 months, and after repair, relocation or a failed daily check, as defined in your calibration SOP.

Learn how the Microbalance works
YOU WILL NEED

Standards & tools

  • One calibrated test weight, e.g. 1 g OIML class E1 or E2. The exact value is not critical for repeatability.
BEFORE YOU START

Prerequisites

  • Balance level, clean and warmed up
  • Room stable; no draughts or vibration
  • Weight at room temperature, handled with forceps
PROCEDURE

Step by step

  1. 1Zero the balance with the doors closed.
  2. 2Place the weight in the centre of the pan, close the doors and record the stable reading.
  3. 3Remove the weight and let the reading return to zero (re-zero if needed).
  4. 4Repeat until you have at least 10 readings.
  5. 5Calculate the mean and the standard deviation (s).
  6. 6If s is smaller than 0.41 × d, use 0.41 × d instead (USP <41>). For d = 1 µg this is 0.41 µg.
  7. 7Check 2 × s ÷ m against 0.10%, where m is the smallest net weight the lab plans to weigh.
RECORD

Example observation table

ReadingObserved (g)
11.000002
21.000001
31.000001
41.000000
51.000002
61.000001
71.000001
81.000002
91.000001
101.000000
CALCULATION

How to calculate the result

Repeatability passes when 2 × s ÷ m × 100 ≤ 0.10%, where s = standard deviation and m = smallest net weight to be weighed
  1. Mean = 1.0000011 g; s = 0.000000738 g = 0.738 µg
  2. d = 1 µg, so 0.41 × d = 0.41 µg. s is larger, so use s = 0.738 µg
  3. Smallest net weight weighed on this balance: m = 2 mg = 2000 µg
  4. 2 × 0.738 ÷ 2000 × 100 = 0.074% (≤ 0.10%, passes)
ACCEPTANCE CRITERIA

Pass if

  • 2 × s ÷ m not more than 0.10% for the smallest net weight to be weighed (USP <41>).
IF IT FAILS

What to do

  • Check the environment (draughts, vibration, static, temperature) and repeat.
  • If it still fails, raise the working minimum weight or remove the balance from use and call service.
  • Assess weighings made below the new minimum weight since the last passing check.
MORE

Other Microbalance 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