LimsPulse
Microbalance · Calibration procedure

Minimum weight

Finds the smallest net mass that can be weighed on the microbalance while meeting the 0.10% requirement, using a small weight at the place of use.

PURPOSE

Why this is checked

Analysts often weigh only 2–10 mg on a microbalance. The minimum weight tells them the smallest amount they may weigh. Because draughts, vibration and static at the bench all affect it, it must be measured where the balance is used.

Frequency

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

Learn how the Microbalance works
YOU WILL NEED

Standards & tools

  • Small calibrated test weight, e.g. 10 mg (OIML class E1 or E2), used on the typical tare container if your SOP asks
BEFORE YOU START

Prerequisites

  • Balance at its normal place of use, level and warmed up
  • The same container type analysts use (e.g. a small weighing boat), if required
PROCEDURE

Step by step

  1. 1Place the tare container on the pan (if used) and tare.
  2. 2Weigh the 10 mg weight 10 times, removing it and re-zeroing between readings.
  3. 3Calculate s. If s is less than 0.41 × d, use 0.41 × d.
  4. 4Calculate the minimum weight: m(min) = 2000 × s.
  5. 5Multiply by the safety factor in your SOP (often 2 or 3) to get the working minimum weight.
  6. 6Label the balance and update the balance register.
RECORD

Example observation table

ReadingObserved (mg)
110.001
210.000
310.001
410.002
510.000
610.001
710.001
810.000
910.002
1010.001
CALCULATION

How to calculate the result

m(min) = 2000 × s; working minimum = m(min) × safety factor
  1. Mean = 10.0009 mg; s = 0.000738 mg = 0.738 µg (larger than 0.41 × d = 0.41 µg, so use s)
  2. m(min) = 2000 × 0.738 µg = 1476 µg ≈ 1.48 mg
  3. With a safety factor of 2: working minimum weight ≈ 2.95 mg
ACCEPTANCE CRITERIA

Pass if

  • Minimum weight determined, recorded 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 needed, improve the environment, use an ioniser, or increase the sample size.
  • Review recent weighings below the new minimum weight.
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