Laboratory workers will immediately recognize the two basic types of balances: analytical balances and top-loaders. Analytical balances use enclosed weigh pans and come in two basic varieties: microbalances, accurate to 1 μg, and semi-microbalances (10 μg). As their name implies, top-loading balances accept samples on a pan that sits atop the instrument, directly above the weigh cell. Top-loader capacities range from the low hundreds of grams to several kilograms, with specified accuracy of either 1 mg or 0.1 mg. Top-loading balances are fast, easy to operate and suitable for all but the most demanding analytical applications. But because they are usually not enclosed, top-loaders are susceptible to errors due to drafts.
Modern balances come equipped with built-in applications for piece counting, density calculations, statistical analyses, and other straightforward calculations. Additional features include color and/or touch screens, faster microprocessors and stability times, better repeatability, hands-free operation, multiple interface options for open-architecture connectivity, and regulatory compliance (e.g., for pharmaceuticals). Automatic doors (on analytical balances) offer convenience while minimizing human contact with the instrument. Many balances are networked to a lab’s computer system, automatically uploading or printing data as it is generated.
Like many other instrument types, balances have come to rely heavily on electronics. Compensating physical weights were replaced long ago by strain gauges and frequency-modulated force measurement in low-end balances, and by electromagnetic force compensation in higher-end analytical instruments.
High schools and colleges use a mix of analytical and top-loading balances, generally with few applications. Industrial users, particularly in pharmaceutical and environmental fields, specify higher- end instruments with more sophisticated applications and communication options. According to Ryan Titmas, VP at Sartorius Mechatronics America (Bohemia, NY), the price of a balance is a function of its capacity (maximum weight handled) to readability (resolution) ratio, plus number of features. Users can expect to pay anywhere from a few hundred to a few thousand dollars for a top-loading balance, and $2,000 to $8,000 for analytical balances.
Digital features, in particular those related to data handling, have become “ubiquitous” and are driving the evolution of balance interfaces, says Steve Wildberger, product coordinator at Shimadzu Scientific Instruments (Columbia, MD). Data capabilities emerged in response to the high error rates associated with manual data transcription. “But customers with unique weighing needs often find that preloaded programs fall short. Users always want an application to do one thing more than it can.”
The problem, Wildberger says, results from balances’ limited memory and data processing capabilities. This has, in turn, led to the emergence of “wedge” software packages that interface the instrument to the lab’s information backbone to provide automated data entry and other functions. These packages, Wildberger argues, add an additional level of complexity for users, and a higher validation burden for labs operating in regulated industries. The future, he says, belongs to balances (and other instruments) that act as sensors or input devices, directing data into familiar computing environments and applications, for example Microsoft Windows spreadsheets. “Microsoft Excel has tremendous capability for handling weighing operations, including statistical analysis and checkweighing [for determining that a piece falls within a specified mass range].”
Some experts believe the choice of analytical balance is relatively straightforward. For Steve Wildberger, the decision tree reduces to instrument capacity and resolution (“the first criteria”), followed by calibration capabilities (internal or external) and interface/data features. And in the experience of Ryan Titmas of Sartorius, “Most users take the middle road and choose a solid analytical balance with accuracy, speed, and a few other features.”
But Ian Ciesniewski, technical director at Mettler Toledo (Columbus, OH), believes purchasers should be more methodical. “Choosing the right balance is critical to reducing measurement risk in the laboratory.” Ciesniewski recommends that users first generate a “design qualification” that accounts for the acceptable weighing uncertainty, and from this figure define the required precision (repeatability). “Remember that repeatability is adversely affected by changes in the laboratory environment, which can cause both acute and long-term problems. We recommend selecting a balance that is better than required by a safety factor of two or three. This will minimize out-of-tolerances and avoid the dreaded ‘do not use’ notice.”
Other factors to consider, according to Ciesniewski, are “ergonomic and productivity” features such as the ability to enter data on sample identity, batch or users; touch-screen operation; color screens (for fewer reading errors); built-in applications; communication and software capabilities; and maintenance/ calibration requirements.
“It is a common error for specifiers to determine a budget for a balance before actually calculating the required accuracy. Too many people believe that readability equals accuracy, when it is measurement uncertainty that should be dictating their needs.”
Angelo DePalma holds a Ph.D. in organic chemistry and has worked in the pharmaceutical industry. You can reach him at firstname.lastname@example.org.
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