Analytical balances, which are accurate to at least 0.0001 g or 0.1 mg, represent the highest precision weighing standard for general use. Analytical balances also excel at quantifying changes in mass.
Software built into balances is a huge time-saver, says Tom Delano, business development manager at Adam Equipment (Danbury, CT). Software that performs calculations such as density or mass differences save operators many hours each week.
Because their mechanisms are delicate, analytical balances are sensitive to almost any mechanical or environmental disruption. Some of the more common sources of sample-associated error include condensation on cold objects, evaporation of volatile fluids, chemical reactivity, and convection from hot/ cold objects. Systemic or external errors arise from drafts, temperature changes, vibration/ movement of the instrument or its environment, and static electricity.
Depending on usage, balance calibration may be carried out every few weeks or as often as twice during a shift. Lab workers should consider recalibration after noting fluctuations of more than about half a degree, moving the instrument, or disconnecting it from the wall socket.
Analysts should take special precautions to ensure weighing accuracy. Users are urged not to handle samples with their bare hands, lest oils and moisture from the skin add to the sample’s weight; leaning on the bench during weighing is another taboo, since vibrations can cause shifts in the balance’s mechanism.
A single-cell, or “uni-block,” weighing mechanism is becoming standard for high-end balances, particularly for labs that make repeated measurements that demand high accuracy. In a single-cell mechanism, the weighing pan consists of a single block that provides constant temperature, shorter stabilization times, and shockproofing. Single-cell weighing is accurate even when the sample is placed at the edge of the weighing pan.
“Single-cell is an advanced technology with fewer moving parts than conventional balances,” says Rachel Kohn, Ph.D., a partner at Tovatech (South Orange, NJ). “This means the balance lasts longer [and] has shorter stabilization times, and corner-load performance is improved because response is uniform over the surface of the weighing pan.”
Another feature, drift compensation, is useful for labs that perform weighing experiments over very long time periods or perform kinetics experiments involving the gain or loss of mass or whose sample masses change regularly over time. Automated drift compensation will change the baseline constantly and linearly, and only record changes that occur suddenly. Users can also program the balance to adjust for planned additions or withdrawals from the sample.
Other factors to consider before purchase, according to Dr. Kohn:
• Absolute capacity and resolution: how much are you weighing, and to what degree of accuracy?
• Do you need internal or external calibration?
• For very accurate work, buyers should consider a semi-micro analytical balance, with a resolution of 0.00001 g, or 0.01 mg.
• Availability of accessory sets for determining densities of solids or liquids.
• Compatibility with GLP/GMP or ISO record-keeping through a standard computer/printer interface.
• Automated calibration, which initiates at preset times or in the event of a potential interference such as rapid temperature change. Make sure the balance is “smart” enough not to begin calibrating in the middle of a weighing.
- Includes large backlit digital display, color-coded keypad and a removable stainless steel pan
- Offers protection in case of accidental overloading
- Security feature allows the use of a standard Kensington™-type lock and cable to prevent theft.
A&D Weighing GH Series
- Anti-static glass draft shield
- 5 models to choose from – 0.1mg and 0.01mg resolution
- Rs232C and WinCT software included as standard
- 5-year warranty
- GLP/GMP/ISO compliant
A & D
Mettler Toledo XS205
- Innovative SmartGrid weigh pan for reduced stabilization times
- Green Ergoclip accessories to secure any tare container and allow for direct dosing without weigh paper
- MonoBloc weighing cell provides outstanding measurement precision
- Rugged design with overload protection as well as easy clean surfaces
- Ranges in capacity from 82 grams to 220 grams
- Features UniBloc one-piece aluminum alloy mass sensor technology, which ensures stable temperature characteristics, excellent response time, and stable corner-load performance
- ATX model offers automatic calibration at the touch of a button