The simplest balance that I ever used in a lab required a set of weights be placed on one tray to counterbalance another that held a sample. Anything more advanced than that needs regular attention to ensure accurate measurements, but only some of the maintenance can be performed by lab personnel.
To get started, Kevin Olsen, instrumentation specialist in the chemistry and biochemistry department at New Jersey’s Montclair State University, encourages readers to distinguish a balance “check” from a “calibration.” “Many people use these words interchangeably,” Olsen says, but there’s a distinction. In a check, he says, a scientist puts “a known mass on the pan, and if the balance reads correctly, then they record the result and are done.” A calibration, on the other hand, involves adjusting the balance mechanism or electronics so that it reads the correct mass of a known weight. “Calibrations are usually done by a trained service engineer, but anyone in the laboratory is qualified to do a check,” he explains.
Timing the tweaks
The required calibrations of a balance depend on the lab. According to information on calibration from Mettler Toledo1 (Columbus, OH) , “Ideally, calibration intervals are defined following a risk-based methodology; for example, what is the probability of something going wrong and how high is the impact?” A high impact could be, for instance, a medical mistake resulting from an inaccurate measurement. The higher the risk, according to the Mettler Toledo guide, the shorter the calibration interval should be.
In many cases, calibration schedules must be followed for a lab’s balances. “If the laboratory is governed by any kind of government regulation, there is usually a published schedule of calibrations,” Olsen explains. “For example, many pharmaceutical laboratories that are regulated by the FDA put their balances on either a quarterly or semiannual calibration schedule.”
In a balance check, a technician uses calibrated weights that are traceable to the National Institute of Standards and Technology and certified by the manufacturer as meeting certain quality criteria. “If the check is off,” Olsen says, “they usually call in a service engineer for a full recalibration or repair.”
It’s not just federal agencies that place requirements on balances. The New Jersey Department of Environmental Protection, for example, requires balances to be checked monthly with class S calibration weights. “If the balances read the correct mass, we make a record of it and are finished,” Olsen says. “If the balances do not measure the correct mass of the certified weights, we usually call in a service engineer.” In addition, the balances must be calibrated annually by a factory-trained service engineer. “Most states have similar requirements,” he notes.
The information in the Mettler Toledo guide agrees with the benefits of routine checks. As the guide states:
Some scientists might rely on the internal calibration mechanism that many electronic balances include, but that’s not a complete solution. “I would suggest speaking to a balance manufacturer about what this mechanism can and cannot do,” Olsen encourages.
It pays to always track a balance’s performance. “If anyone is ever not sure if a balance is working correctly, they should put a nickel on the pan—a nickel is about 5 grams,” Olsen says. “This will give them a rough idea [regarding] whether the balance is working correctly.” So here, five cents can go a long way.
For additional resources on balances, including useful articles and a list of manufacturers, visit www.labmanager.com/balances