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Analytical balance · Calibration procedure

Eccentricity

Places the same weight in the centre and at four corners of the pan and checks the readings agree.

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

Why this is checked

Analysts do not always place samples exactly in the centre. The eccentricity test shows how much the reading changes with position.

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 Analytical balance works
YOU WILL NEED

Standards & tools

  • Calibrated test weight of about one third to one half of capacity, e.g. 100 g for a 220 g balance (as defined in your SOP)
BEFORE YOU START

Prerequisites

  • Balance level, clean and warmed up
  • Mark the positions on a diagram: centre, front-left, front-right, back-left, back-right
PROCEDURE

Step by step

  1. 1Zero the balance.
  2. 2Place the weight in the centre and record the reading.
  3. 3Move the weight to each corner position in turn (about halfway between centre and edge, or as the manufacturer describes) and record each reading.
  4. 4Return the weight to the centre and confirm the reading.
  5. 5Calculate the difference between each corner reading and the centre reading.
RECORD

Example observation table

PositionObserved (g)Difference from centre (g)
Centre100.00000.0000
Front-left100.0002+0.0002
Front-right99.9999−0.0001
Back-left100.0001+0.0001
Back-right100.0002+0.0002
CALCULATION

How to calculate the result

Eccentricity error = Corner reading − Centre reading (report the largest)
  1. Largest difference = 100.0002 − 100.0000 = 0.0002 g
  2. As a percentage: 0.0002 ÷ 100 × 100 = 0.0002%
ACCEPTANCE CRITERIA

Pass if

  • Largest difference within the manufacturer's eccentricity specification or the limit in your SOP (for example not more than 0.0005 g at this load).
IF IT FAILS

What to do

  • Check the pan is seated correctly and not touching the draught shield.
  • Repeat the test. If it still fails, call service to adjust the weighing cell.
  • Until repaired, the balance must not be used.
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