The 10:1 Rule
When calibrating an instrument, we’re told to always use standards, masters or reference materials whose accuracy is 10 times as great as that of the unit under test.
Anyone who has learned about calibration and traceability in the last 100 years has been taught the 10:1 principle. This ratio, 10 to one, is usually referred to as the test accuracy ratio (TAR) or test uncertainty ratio (TUR), and conventionally describes the ratio of the accuracy of the unit being calibrated to the accuracy of the standard used for the calibration.
The Other 10:1 Rule
There is another 10:1 rule which says that when one is trying, for example, to weigh 1 mg (or 0.001 g), they need to move the decimal over one place and weigh 1 mg on a 0.0001 g analytical balance, not a milligram balance. Hence the 10:1 principle.
Do note that all balances have errors, and that it is unsafe practice to weigh at the bottom of the readability range. The recommended minimum load for a model LS 320 C with a readability of 0.001 g (see data sheet) is 0.01 g. The repeatability and linearity errors on an LC 320 C are +/- 0.001 g and 0.0015 g respectively. Weighing one milligram on this balance could mean an error of 100%!
The accepted method of recommending a balance readability is the 10 to 1 rule. If you want to weigh 0.1 g you need to be using a 0.01 g balance. If you want to weigh a milligram (0.001 g) you need to move to a 4 place balance (0.0001 g).
We recommend that if a customer wishes to weigh 0.1 g then he should move to a balance with 0.01 g readability. For instance, we would recommend the model PBW-3200 as our lowest cost 0.01 g balance. Many of our customers do not, at first inquiry, understand this message.
Please note: the above recommendations do not only apply to Intelligent Weighing Technology products, but apply to all balances, whatever the manufacturer.