Lab balance calibration is a critically important process in manufacturing settings, including pharmaceutical and medical products. Not only must daily calibration be performed to ensure accurate functioning, annual calibration is also recommended — and often required — in order to ensure compliance with regulatory needs.
In this article, we will examine testing and calibration processes and steps and learn why they are so important.
What Are Eccentricity and Corner Load Tests?
Eccentricity tests, also known as corner load tests, are intended to ensure that a balance will produce the same result (or results within an acceptable tolerance range) no matter where on the weighing platform the item is placed.
This test is important because of the nature of how scales and weighing equipment operate, typically, through load-detecting cells under the weighing platform. If equipment is not calibrated properly, if one or more cells are damaged, or if debris has built up under one or more cells, weighing equipment may return materially different results depending on where an item is placed. Corner load tests are the primary method through which this scenario can be avoided. Corner load tests are carried out by placing a weight (typically, in the U.S., 50% of the maximum capacity of the scale) in different, predefined sections of the weighing equipment and recording the weight measure returned. If measurements are outside of an acceptable tolerance range, the scale must be taken out of service and repaired, cleaned, or recalibrated.
How to Calibrate Balances Via Internal Calibration and External Calibration
Internal and external calibration are important processes in maintaining the accuracy and reliability of a weighing device.
• Internal calibration is conducted using a process built into the device itself. Following the instructions for the equipment in use, this process usually involves a “Calibrate” or “Auto-Calibrate” button or control. The process will usually involve a “zero weight” calibration and a calibration involving an internal test weight or load.
• External calibration uses a set of NIST-traceable standard test weights to test and calibrate the equipment. In the external calibration process, the tester will enter the weight of the testing medium and will then place it on the weighing platform. The unit being tested defines the weight (or weights) to be used for the external calibration.
Balance Level and Cleaning: When and How
Balance leveling and cleaning should be conducted regularly, with baseline processes occurring daily or even multiple times per day (such as in between samples). More involved processes may occur less frequently, depending on manufacturer recommendations, customer requirements, and the application at hand. Cleaning and leveling steps include:
• Remove any powder and dust from the balance using a tissue or cloth, taking care not to accidentally spread debris to other parts of the mechanism.
• If any sticky substances are present, remove them using a damp, lint-free cloth. Again, be sure to not accidentally spread the substance or other material to other parts of the mechanism.
• Remove any other easily removeable parts (i.e., those that can be separated without tools) and clean them following the above steps.
• Be sure to always use away from air ducts.
• After reassembling the balance, level and calibrate it.
Why Perform Annual Calibration?
Daily calibration serves to ensure that weighing equipment remains clean and free of dust or debris that may affect accuracy and repeatability for standard use. Moreso, annual calibration of weighing equipment serves an even more important purpose in the equipment’s compliance to regulatory guidelines. Calibration at these intervals is required by regulatory bodies, such as the FDA or USP, and are far more detailed than the daily requirements.
Annual vendor calibrations help ensure that equipment is kept up to date with manufacturer recommendations as well as any changing vendor expectations and regulations, and quality remains at the expected level. A detailed vendor lab scale or balance calibration should include multiple weight standards checked throughout the usable range of the weighing device, eccentricity testing, and in the case of USP 1251, the repeatability of the weighed readings.