Microbalance
A very sensitive balance that reads to 0.001 mg (1 µg) or better. It is used when the sample is only a few milligrams, for example a potent reference standard or a DSC sample.
- Pharmaceutical
- Biotech
- Chemicals
- Environmental testing
- Academic research
- Materials science
How Microbalance works
Like an analytical balance, a microbalance uses electromagnetic force restoration (EMFR). The load moves a lever very slightly; a position sensor detects this and a coil in a magnetic field pushes the pan back to zero. The coil current is proportional to the mass. Because the forces are tiny, the cell is mechanically isolated, temperature-compensated and shielded from air movement.
The path of a sample
- 01
Load
A small container with the sample is placed on the pan inside the close-fitting draught shield.
- 02
Displacement
The pan and lever move by a tiny amount under the extra mass.
- 03
Position sensing
An optical sensor detects the movement away from the null position.
- 04
Force restoration
The control circuit raises the coil current until the magnetic force returns the pan to the null position.
- 05
Filtering
The signal is averaged and filtered to remove small disturbances, then converted to mass using the stored adjustment.
- 06
Record
Once stable, the reading is printed or sent to the LIMS with the balance ID, date and time.
Main parts and what they do
Weighing cell
The EMFR lever, coil, magnet and position sensor. Often a monolithic, temperature-compensated block.
Small draught shield
A small glass chamber around the pan that cuts air movement. Often motorised so doors can open without touching the balance.
Weighing pan or grid
A small pan, or a grid pan that reduces the effect of air currents. Special holders exist for filters and DSC pans.
Separate electronics unit
Holds the display and electronics away from the weighing cell so their warmth does not cause drift.
Internal adjustment weights
Built-in weights that the balance loads to adjust its sensitivity automatically after temperature changes.
Ioniser / anti-static device
Neutralises electrostatic charge on containers and powders before or during weighing.
Anti-vibration weighing table
A heavy stone table, separated from the bench, that isolates the balance from building vibration.
Data interface
USB, Ethernet or RS-232 link to a printer or LIMS for direct, attributable data capture.
Terms every analyst should know
Readability (d)
The smallest display step, 0.001 mg for a typical microbalance. It is not the same as accuracy or repeatability.
Repeatability
The standard deviation of repeated weighings of one object. At the microgram level it is the main limit on how little you can weigh.
Minimum weight
The smallest net mass that still meets the 0.10% requirement of USP <41>, calculated as 2000 × s. It must be determined at the place of use.
Electrostatic forces
A charged container or powder is pulled towards or pushed away from the draught shield. This shows up as drift of many micrograms. Ionisers remove the charge.
Thermal effects
A sample even slightly warmer than the air sets up rising air currents, so it reads low. Samples and containers must be at room temperature.
Buoyancy
Air pushes up on every object. It is usually ignored in routine work but matters when calibrating weights or weighing very low-density materials.
Keep every Microbalance in control: usage logs, calibration and maintenance schedules, and automatic blocking of overdue instruments.