Are You Using the Best Balance to Weigh Your Samples?
OHAUS Explorer Semi-Micro Balances
Semi-micro balances — those with a readability of 0.01mg — are frequently regarded as overkill or recommended only for extremely sensitive weighing applications. But are they, in fact, the right instrument for the job?
While the readability of a balance does not ensure its accuracy, a balance with a smaller readability will typically offer greater accuracy than one with a larger readability. For example, a balance capable of reading to 0.1mg will provide a more precise reading than a balance that only measures to 1mg.
So let’s say that you wish to accurately measure the samples that weigh 0.05g (50mg) — which balance would you choose? A balance with 1mg readability? Or one that reads to 0.1mg?
The correct answer first requires you to define your maximum uncertainty tolerance, or how much of a difference between the measurement and the absolute truth will be acceptable. Do you require a maximum uncertainty of one percent of the sample weight? Or perhaps 0.1 percent?
Many users, including the United States Pharmacopeia (USP), choose balances based on the minimum weight convention; this provides a relative uncertainty that is less than the published value. Adopting a minimum weight requirement can help to ensure that your samples are being weighed according to your acceptable criteria for accuracy. You can ensure a maximum measurement uncertainty by choosing a minimum weight for your sample size and any heavier samples (within the range for the balance).
To determine the minimum weight, divide the standard deviation of the balance’s repeatability (Rstd) by the desired accuracy. In some cases, a buffer factor is applied (USP uses 2 x Rstd, just to be safe).
For a sample weighing approximately 50mg, one might assume that a balance readability of 0.1mg is sufficient, but is this correct? In this example, a maximum uncertainty tolerance of 0.1 percent with an Rstd of 0.1mg [(2 x 0.1mg) / 0.001] suggests a minimum weight of 200mg. So a balance with 0.1mg readability would not meet the stated accuracy requirements — the balance would need to deliver readability of 0.01mg.
That readability level requires a semi-micro balance such as the Explorer 5 Series from OHAUS. In fact, OHAUS Explorer semi-micro balances are guaranteed to weigh samples as small as 20mg (and, with proper qualification, down to 10mg) while maintaining compliance with USP41 minimum weight standards.
The OHAUS Explorer semi-micro balances also incorporate anti-static devices to help ensure maximum weighing accuracy — because static buildup can cause substantial error when weighing small samples. In addition, the Explorer Series is available with automatic draft shield doors, a large color touchscreen display, an intuitive user interface, and other features that support GMP/GLP as well as regulatory compliance and traceability.
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