SOP: Balance Calibration and Use

Every quantitative result in the lab divides by a weight, so the balance is the most leveraged instrument in the building. Daily checks with certified weights catch drift before it corrupts a batch of results.

General laboratory procedures form the foundation every analytical result stands on. Balances, pipettes, glassware, and waste handling rarely make headlines, but errors here propagate silently into every test the lab performs. Treating these routines with the same rigor as the analyses themselves is what separates reliable laboratories from the rest.

Purpose

To ensure analytical and top-loading balances weigh accurately through daily verification and scheduled calibration.

Scope

Applies to all balances used for food analysis sample and reagent weighing.

Equipment and Materials

• Certified calibration weights (covering the working range)
• Weight handling forceps and gloves
• Calibration logbook
• Level indicator (built into most analytical balances)
• Draft shield

Procedure

1. Level the balance using its bubble indicator before each day’s use.
2. Allow the balance to warm up per the manufacturer (usually 30 minutes for analytical models).
3. Zero the empty pan. Place a certified weight near the typical working load and record the reading.
4. Acceptance: within the stated tolerance (for example plus or minus 0.5 mg at 100 g for a 0.1 mg balance). If out, do not use; tag out of service and notify supervision.
5. Weigh samples in appropriate vessels; never place chemicals directly on the pan.
6. Close the draft shield for analytical weighing and wait for stability before recording.
7. Clean spills immediately with a soft brush; never blow into the chamber.
8. Schedule external calibration annually (or per quality manual) with accredited providers; keep certificates on file.

Recording Results

Log daily checks: date, balance ID, weight used, reading, pass or fail, and initials. File external calibration certificates with due dates tracked.

Quality Control

Document every activity in real time in bound logbooks or the validated electronic system; never backdate entries. Review logbooks weekly for completeness and sign off. Train all personnel on this procedure before independent work and reassess competence annually. Track calibration and maintenance due dates on a master schedule with alerts. Audit compliance with this SOP during internal audits and record findings with corrective actions. Retain all records per the document retention policy.

Troubleshooting

• Drifting readings: drafts, vibration, or temperature changes. Relocate or shield the balance; let it stabilize.
• Static on plastic vessels: use an ionizer or metal vessels; static can shift readings by milligrams.
• Non-repeatability: the pan may be dirty or the balance overloaded previously. Clean and re-verify; service if it persists.
• Missed calibration due dates: implement automated reminders 2 weeks ahead; tag equipment with visible due-date labels.
• Incomplete logbook entries: review logs at shift handover; the next user should be able to reconstruct the work.

Interferences and Limitations

These procedures control the laboratory environment, not the analysis itself; their value depends on consistent execution by trained personnel. Human error remains the largest source of variation: rushing, skipping verification steps, or working while fatigued undermines the entire quality system. Equipment drifts between calibrations, so the scheduled verification frequencies are minimums, not targets. Review and update this procedure whenever equipment changes or audits reveal gaps.

Safety Notes

• No significant hazards. Handle certified weights with forceps; skin oils change their mass.