Microbalance Specifications

1. Weighing performance: the key item

• Readability of 0.1 microgram, one part in ten million, or 1 microgram, one part in one hundred thousand

• Capacity by model, around 5 g, 21 g or 52 g

• Readability and capacity trade against each other, no balance gives 0.1 microgram at 50 g

• Define the smallest sample and the largest capacity first, then choose readability

2. Repeatability and linearity

• Repeatability as standard deviation within 0.15 microgram at 0.1 microgram readability and 5 percent load

• Linearity deviation within 0.2 microgram

• Off-centre loading error within 0.3 microgram

• Repeatability reflects real working accuracy and matters more than readability

• Note: published repeatability is measured in stable, draught-free conditions and is worse in practice

3. Calibration

• Internal motorised weight for automatic calibration, triggered by temperature change or time

• External calibration with E1 or E2 class weights

• Internal calibration is preferable, since a microbalance is sensitive to environment and manual routines lapse

• Calibration records must be traceable for GLP and GMP

4. Draft shield and pan

• Fully enclosed draft shield opening on three sides to keep air movement out

• Small pan, typically 20 to 30 mm on a microbalance, with the sample placed centrally

• Anti-static device, usually an ioniser, to remove static effects on trace weighing

• Built-in or external ioniser selected by ambient humidity

5. Settling time and response

• Settling time of 10 seconds or less, typically 3 to 6

• Longer settling means slower work, and speed should not be over-valued for trace samples

• Weighing modes including standard, piece counting, percent, formulation and density

6. Interfaces and data

• RS-232 or USB for printer or LIMS connection

• Ethernet and wireless on some models

• Export with time stamp and operator identity

• Barcode scanning for sample traceability

7. Environmental requirements: a key item

• Operating temperature 15 to 30 degrees Celsius, drift within 2 degrees per hour

• Relative humidity 45 to 60 percent, low humidity causing static and high humidity causing moisture uptake

• Away from air conditioning outlets, doors and walkways

• Marble anti-vibration bench recommended, in a separate or stable area

• Note: nine out of ten microbalance complaints trace to the environment, not the instrument

8. Compliance functions

• Tiered user access, audit trail and electronic signature under 21 CFR Part 11 as options

• Calibration certificates and traceable weights

• IQ OQ PQ validation packages

• Expected in pharmaceutical, food and precious metal work

9. Where procurement goes wrong

• Readability alone is compared without repeatability, and a 0.1 microgram model performs five times worse in a real room

• No anti-static device, so readings drift in dry seasons and repeatability never settles

• The balance sits in draught or traffic, where no instrument can weigh accurately and no service visit can help