Micro balances: how to get 1 µg readings you can trust
Micro balances deliver 1 µg readability for ultra-low mass measurements used in advanced R&D, pharmaceutical QC, filter weighing, and micro-dosing. This guide explains when you need a micro balance, how to prepare the environment, which calibration weights to use, and the good weighing practices that turn specification sheets into reliable data.
What is a micro balance (vs. semi-micro and analytical)?
- Micro balances: 1–10 µg (0.001–0.01 mg) readability; typical capacity 2–52 g (some up to ~220 g).
- Semi-micro balances: 0.01 mg (10 µg) readability; typical capacity 80–220 g.
- Analytical balances: 0.1–1 mg readability; typical capacity 80–520 g.
Choose based on the smallest net sample you must weigh and the maximum capacity you need.
When do you need a micro balance?
- Gravimetric reference work and high-accuracy standard preparation.
- Filter weighing (PM, aerosol, microplastics) and low-mass residue studies.
- Micro-dosing of potent actives, catalysts, or isotopic tracers.
- Method development where uncertainty at sub-mg level governs results.
Key specs that matter
- Readability (d): the smallest increment displayed (e.g., 0.001 mg).
- Repeatability: short-term scatter on repeated weighings (e.g., ±0.002 mg).
- Linearity: deviation across the range (e.g., ±0.005 mg).
- Stabilization time: how fast the reading becomes stable.
- Shielding & mechanics: draft-shield design, pan diameter, anti-vibration provisions.
Rule of thumb for minimum sample weight: aim for net samples ≳ 2–10 mg with 1 µg readability, then confirm by a simple repeatability test in your lab. Formal minimum sample weight should be established per your quality system (e.g., repeatability-based criteria in pharmacopeias).
Weighing room & installation
- Location: low-traffic area, away from doors/vents/sunlight; anti-vibration table.
- Draft control: use the balance draft shield correctly; avoid opening multiple doors at once.
- Temperature & humidity: keep stable; many labs target ~20–23 °C and 45–60% RH to reduce static.
- Leveling & warm-up: level the balance and allow warm-up after power-on or relocation.
- Static & magnetism: use an ionizer for plastics/filters; avoid magnetic tools and containers.
Daily routine & good weighing practice (GWP)
- Visual check: pan clean, shield glass clean, doors glide freely.
- Level & self-test: verify level bubble; run internal test/calibration if available.
- External check: verify with certified weights at working points (see classes below).
- Handling: use forceps or gloves; pre-condition containers to room conditions.
- Technique: tare the container; minimize time with doors open; wait for stability mark.
- Documentation: print/export readings with date/time, ID, and calibration status.
Calibration weights: which class?
Match the weight class to the balance resolution and your quality requirements:
| Balance type | Typical readability | Recommended weights | Notes |
|---|---|---|---|
| Micro | 1–10 µg | OIML E2 (or better) 1 mg–200 g | Handle with forceps; store in clean, closed box; acclimate before use. |
| Semi-micro | 0.01 mg | OIML E2/F1 1 mg–200 g | Choose E2 if you need stricter uncertainty budgets. |
| Analytical | 0.1–1 mg | OIML F1/F2 | Span checks at working points (e.g., 100 mg, 10 g, 100 g). |
Common pitfalls (and quick fixes)
- Static drift: ionize the vessel and sample; increase RH; avoid rubbing plastics.
- Convection: warm lids, hot samples, and large temperature gradients cause drift—wait to equilibrate.
- Air currents & vibration: re-site or shield; use an anti-vibration table.
- Buoyancy effects: for highest accuracy at different air densities, apply buoyancy correction or keep conditions constant.
Takeaway
Great micro-balance results come from a capable instrument and disciplined weighing practice. Control the environment, verify performance with the right weights, document your workflow, and your 1 µg numbers will stand up to any review.
