If you are in an industry that is heavily regulated and relies on accurate weighing, then you have heard of the Food Drug Administration (FDA) and United States Pharmacopeia (USP). The FDA is a federal agency of the United States Department of Health and Human Services that oversees the USP. The USP is a scientific, nongovernmental, organization that determines public quality standards. These standards and regulations are used in more than 140 countries to ensure the quality, safety and benefits of medicine, food, dietary supplements and medical devices. USP 41 and 1251 are followed to meet quality standards when it comes to weighing.
What is USP 41?
USP 41, titled “Balances”, defines the minimum quality standard to guarantee accurate weighing. It states the balance must be calibrated over the operating range and meet the defined requirements for repeatability and accuracy when substances need to be accurately weighed. Before you can calibrate the balance, you must determine the operating range. The operating range is determined by repeatability and accuracy. Repeatability is the starting point of the operating range.
Repeatability
- Step 1 – Set the balance display to 0 by pressing Tare
- Step 2 – Position the test weight on the center of the pan
- Step 3 – Record the final value displayed
- Step 4 – Repeat 9 more times, with the same test weight, for a total of 10 measurements
The test weight used does not need to be calibrated; however, it is often recommended to use a calibrated test weight that is approximately half maximum capacity of the balance to determine accuracy simultaneously
- Step 5 – Once the 10 measurements are recorded, calculate the standard deviation
If the standard deviation is < 0.41d, where d is the scale interval, then give it the value of 0.41d.
- Step 6 – Now calculate 2 times the standard deviation to get X
- Step 7 – Divide X by the smallest weight planned to be used on the balance
Repeatability is satisfactory if this value is ≤ 0.10%.
- Step 8 – Calculate X times 2 times 1000 to get the starting point of the operating range
Accuracy
- Step 1 – Take a measurement using a calibrated test weight with a mass between 5-100% of the balance’s max capacity
- Step 2 – Get the difference of the measured value and the test weight value
The measured value should be ≤ 0.10% of the test weight value. The test weight must be calibrated to prove the uncertainty of that weight. The calibration uncertainty of the test weight must be ≤ 1/3 of 0.10%. This informs us that it isn’t necessary to use high class weights.
USP 1251
USP 1251, titled “Weighing on an Analytical Balance”, addresses the weighing procedure. When using a balance for any analytical procedure, one should proceed with a risk-based approach. Proper planning and preparation of the equipment and chemicals used should be done first. The balance should be checked, calibrated and in a controlled environment; and when weighing the material, follow the correct procedure. It is recommended to only weigh net samples that weigh more than the minimum weight to account for varying environmental influences that can affect the balance’s performance.
We Can Help
There are many ways to calibrate a balance and perform accurate weighing. In order to get the most accurate results, the USP recommends using the globally established ISO/IEC 17025 standard. Grand Rapids Metrology is ISO/IEC 17025 accredited and performs balance calibrations to the highest quality standards. If you need help calibrating your instruments, or are unsure if you are following USP standards and regulations, give us a call!
