Weighing In: Getting the Most From Laboratory Balances
At the simple end, cleaning balances requires little more than common sense. Taring vessels, usually stainless steel or glass, can go into the dishwasher or a dedicated cleaning tub. No special care need be taken, as even material losses due to scratches or pitting disappear after taring.
“Worrying about taring vessels is more a question of aesthetics than function,” observes Gilbert Vial, product manager for physical measurement at Shimadzu Scientific Instruments (Columbia, MD).
Almost any noncorrosive cleaner will do for the main balance structure and weighing chamber; a paper towel wetted with ordinary household surface or glass cleaner usually does the trick. But Vial cautions against sprays that might get into crevices or external weighing mechanisms. “If you remove the pan to clean it or the weighing area, make sure to protect the now-uncovered area,” Vial says. “Clean a balance the way you would clean a fine watch.”
Location, location, location
Vendors advise locating balances in quiet, temperature- controlled, draft-free locations, but real-world laboratories cannot always afford this luxury. “We very often find balances in clean rooms with laminar flow,” observes Dirk Ahlbrecht, marketing manager for high performance balances and mass comparators at Sartorius (Göttingen, Germany). “Vendors must come up with products that serve non-ideal conditions.”
Drafts are a serious issue for both top-loaders and analytical balances with enclosed weighing compartments. The former are generally unprotected from blasts of air, while enclosures on analytical balances may fail to fully protect due to the instruments’ sensitivity.
The higher the required resolution or accuracy, the more care must be taken with environmental conditions. Top-loading balances expected to weigh ten noncritical milligrams work almost anywhere. But pharmaceutical quality assurance labs whose assay standards rely on five decimal point readability should consider better-controlled conditions for weighing.
Ann Crowley, product manager at Rice Lake Weighing Systems (Rice Lake, WI), suggests positioning all balances away from drafts on a dedicated balance table or marble slab, or at the very least on a surface that does not bend, even imperceptibly. Most wooden tables have some degree of flexure and are therefore unsuitable for serious weighing.
Ahlbrecht goes one step further: “A weighing table should be decoupled from the environment.” Stone surfaces are fine, but only with an additional layer of shock-absorbing material such as rubber or cork. “A large mass, for example marble, will thereby be less sensitive to shock and vibrations.”
For ultrasensitive work, managers must consider every conceivable physical perturbation that might affect weighing results. For example, the middle of a floor bends more than areas closer to outside walls, and upper floors sway; even slight misalignment along the vertical to the center of the earth can introduce error.
“Unless balances are specifically designed to handle those circumstances, an R&D or QC balance on the twentieth floor will experience problems with vibration and movement,” Ahlbrecht tells Lab Manager. In that situation, a weighing table near an outside wall is a necessity. Managers might also consider purchasing balances that self-correct for or filter out external mechanical or gravitational influences.
Changes in temperature, humidity, and air pressure all affect balances but temperature changes are by far the most serious because they occur everywhere. Temperature effects are mostly insignificant for top-loaders but a difference of 1.5° C can cause sensitivity and zero point to shift in analytical balances. Zero point drift is trivial, thanks to automated taring, but sensitivity drift will introduce systematic error. Sensitivity drift is normally specified for a particular balance, depending on the sample weight and temperature difference. For example, a difference of 5°C will affect the observed weight of a 100 g sample by up to 1 mg.
Operators should be wary of warming or cooling of tare vessels during removal from the balance. Julian Stafford, sales trainer at Mettler Toledo (Zurich, Switzerland), demonstrates this effect during training sessions by removing a beaker from the weighing pan with his bare hands. Most trainees believe that the mass contributed by oils and sweat from Stafford’s fingers would cause the weight to rise. In reality, updrafts caused by warming the beaker by a few degrees more than compensate for the added mass. Similarly, downdrafts caused by cooling (say, placing the beaker directly onto a cold bench) can cause significant apparent weight gains. Temperature difference effects apply to balance and sample, and particularly to glassware that has recently been removed from a dishwasher.
“Most people don’t realize that air from drafts or updrafts has mass,” Stafford says. “Because we live in it and can’t see it, we tend to ignore it.” He suggests allowing sample, tare vessel, and balance to reach mutual temperature equilibrium whenever temperature differences are suspected. “For very high-precision work, users should introduce the tare container and sample into the balance chamber up to one hour before weighing. Accurate weighing takes patience and diligence.”
Humidity effects are indirect: low humidity conditions tend to promote static buildup in samples. In addition to causing annoying physical dispersion of powders, static adversely interacts with metal components, resulting in serious drifts. “You may never achieve stability for that sample, no matter how long you wait,” Ahlbrecht says. Air conditioning, which removes moisture from air, is one culprit.
Mettler Toledo’s Stafford observes that labs in tropical climates with higher humidity rarely experience serious static problems. “It’s more of a problem in locations that have four distinct seasons,” he says.
Glass or plastic vessels tend to promote static; Teflon is the worst offender, according to Ahlbrecht. Even expensive conductive glass may not satisfactorily overcome serious static issues. Balance vendors and third parties sell accessories to eliminate static, including charge dissipation devices and systems that deionize air around the balance. Several companies, Sartorius included, incorporate static dissipation in some of their balances.
To move or not to move?
Because transporting can throw analytical balance instruments out of calibration and balance, managers should think twice about moving them.
Some vendors take a conservative position in spite of increased automation of calibration and leveling. “I do not recommend moving a balance unless it’s absolutely necessary,” says Andrew Hurdle, market manager at Ohaus (Parsippany, NJ).
What about cleaning up spills? Many balances have space between the bottom and the table or benchtop. Hurdle suggests using a cloth or towel that fits into that space while disturbing the balance as little as possible. “And you can always blow away dust and dirt with compressed lab air.”
But when moving is unavoidable, Hurdle recommends having an SOP in place that covers powering down, removing and securing the weighing pan, and handling the instrument as gingerly as possible. “Pick it up from underneath and relocate to where it will not need to be moved again. Then recalibrate and level.”
Many balances prominently feature a level indicator, similar to a carpenter’s level, and options for correcting level through either two or four adjustable feet. Achieving true level is easier with four-foot leveling.
More than leveling, the need to recalibrate makes moving balances unattractive to lab workers. Calibration options include do-it-yourself, or calling an in-house engineering department, the vendor, or a third-party organization. Calibration should ideally follow usage: more weighing means higher frequency. In practice, labs follow SOPs that designate intervals between calibrations, or follow regulatory guidance (e.g., GLP, GMP) regardless of usage.
When moving is unavoidable, users should first lock the balance (if it has such a feature), turn the power off, disconnect the electrical cord, and cautiously relocate the balance. Most users will wish to recalibrate after the balance has reached its final destination.
Moving becomes practically a nonissue with certain balances that employ high-precision electric discharge processing (HPEDP), which places many of the components of a conventional electromagnetic balance into a monolithic metal structure. Such balances are practically impervious to shock.
Users must still verify calibration with HPEDP balances, but often need not perform a formal calibration provided they allow the balance to equilibrate for 15 minutes after powering up. Users may recalibrate using standard low, medium, and high weight points, or by applying automated recalibration based on an internal dead weight.
Vial notes that the worst effects of moving result from damage to the spring or load cell on balances equipped with these components. “Load cells are classic strain gauges that easily become overloaded,” he says. Locking disengages springs and load cells from forces related to movement.
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