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A balance is a crucial and fundamental piece of equipment for any laboratory. The first and foremost consideration in shopping for a Laboratory Balance is clarity of thought on use and application. The purpose of the balance, the type of materials, and the amount of materials all factor into the choice of instrument. For solid, inert materials of a kilogram or more, an industrial balance would be the right choice. For measuring microgram quantities of pharmaceutical compounds, a precision Analytical Balance would be necessary.
Similarly, consider the laboratory and the overall environment. For a dusty environment, such as a factory floor, look for balances that are sealed against outside materials. If there are drafts, vibrations, or temperature fluctuations, a sensitive analytic balance may not function well. Some balances come with their own enclosures, to keep contaminants out.
If the work being done requires certification, or involves toxic or radioactive materials, regulatory and personnel training requirements may need to be satisfied. In addition, the frequency of use will influence the choice. A balance that is used all day, every day, needs to be very durable and reliable.
According to Mettler Toledo (Columbus, OH), a manufacturer of analytical balances, customers frequently underestimate the level of accuracy required by their processes. Once the application has been clarified, it can be determined what is expected in terms of accuracy according to the relevant international norms and guidelines.
High readability does not necessarily equate to high accuracy, according to a company spokesperson. For example, a customer may choose a balance with 0.1 mg readability (4 decimal places). The accuracy of the balance relates to the measurement uncertainty of each reading, i.e., the ± tolerance in the result. A fast, stable weighing result is not necessarily an indication of accuracy. A balance that is inaccurate cannot reliably produce repeatable results.
Different types of balances are selected according to the function they must perform. Laboratory balance types include toploading balances, portable balances, analytical balances, semimicrobalances, and microbalances. There are some overlaps in terms of readability and accuracy. In these cases, the user’s application and weighing environment will generally determine the type of balance required.
Balances may also be defined according to their weighing sensor. Some balances use a strain gauge, which is essentially a metallic object that deforms as strain is applied, resulting in a change in electrical resistance that can be correlated to the magnitude of the strain induced by the weight. Strain gauges are frequently found in shop scales and other larger instruments.
In scales of higher accuracy, a sensor based on electromagnetic force compensation may be used. In that type of sensor, the weight applied to the scale is compensated for by an electromagnetic force. Vertical position changes in the coil trigger a current that returns the position to zero. Since the current is proportional to the weight, a weight value can be calculated from the current.
“Although it can’t be seen, [the weighing sensor] is the heart of every balance and it is a major factor influencing its accuracy and its price,” according to a spokesperson at Mettler-Toledo. The weighing sensor should provide the balance with the ability to weigh rapidly and reproducibly, be stable to thermal fluctuations, and be reasonably repairable.
Toploading balances are appropriate when the upper margin of error on a weight is 0.05 g or larger, as opposed to analytical balances, which have a margin of error of 0.5 mg. The toploading balance does not offer protection from air disturbances, and generally can weigh up to the low-kilogram range.
A Portable Balance may be chosen for field work or simply for convenience. Durability becomes a very important feature of a portable balance, since it is likely to be heavily used and possibly take some abuse. Many appreciate a quick response and user-friendly design in a portable balance, especially if there is a large number of samples to be weighed.
Although portable balances are not as fussy as their analytical balance cousins, they do offer many of the same features, such as piece counting, dynamic weighing, and unit conversion. Sealed keypads and stainless steel platforms are useful for cleanup.
Most life science laboratories need an analytical balance for precision weighing. Analytical balances should only be used by trained personnel, and require regular calibration and maintenance. Because of this, they are generally set up in a separate, interior room of the laboratory, out of the way of general foot traffic, chemical spills, flying objects, vibrations, and other disturbances. Additionally, locking the room can keep well-intentioned, but untrained, visitors from using them.
For very sensitive balances, an antistatic mat will help to improve accuracy. ISO 9000 and GLP regulations require documentation of the accuracy and calibration of an analytical balance. Some analytical scales have a feature that allows automatic recalibration at specific intervals or under certain circumstances, such as temperature fluctuation. Other balances must be calibrated by hand. Calibration can be internally motor-driven, or external.
While a balance need not be set up in a cleanroom, it should not be subjected to dusty and dirty environments. When dust cannot be avoided, the user should look for a scale with an ingress protection (IP) rating. Scales with high IP numbers are resistant to particulate and liquid contaminants. The two-digit rating gives the resistance of the scale to solid and liquid matter, respectively.
Weighing capacities of an analytical balance range from tens or hundreds of grams to as low as 0.1 or 0.01 mg. Practical features include a clock, mass unit conversion, piece counting, and data record-keeping.
Semimicrobalances can weigh to onehundredth of a milligram. They are generally used for extremely specialized applications, such as differential weighing of a sample before and after incineration, measurement of coatings, or weighing chemically sensitive samples inside a glove box.
Even greater accuracy is possible with the use of a quartz crystal microbalance, an instrument that falls into the gap between analytical balance and mass spectrometer
Conclusion
The crucial requirement to fulfill when purchasing a balance is to meet the accuracy requirements of the laboratory. After that, it is up to the user to decide what other factors are important, such as built-in applications, protection against external influences, corrosion protection, user safety, cross-contamination risk, balance longevity, spare parts warranty, and service. Scientists on a budget may choose to compromise in certain areas, but the price/performance ratio and the return-on-investment should also be considered.
Ms. Shaffer is a Consulting Editor, American Laboratory/Labcompare; e-mail: [email protected].
Please check out our Laboratory Balances section for more information or to find manufacturers that sell these products.