Q.When assessing a product, it must read accurately down to the 0.01 g digit. In this case, is it OK to use an electronic balance with a minimum display of 0.01 g?
A : Generally the accuracy of the lowermost digit past the decimal point on a balance is considered to be the accuracy rounded to the nearest integer for a worst-case scenario with respective errors factored in, though this depends on the accuracy of that balance. The main factors that indicate the performance of a balance are:
- Repeatability (or reproducibility)
- Linearity
However, let us, for example, take a look at a balance having the following specifications (specifications of a typical balance on the market).
- Weighing capacity: 300 g, Minimum display: 0.01 g
- Repeatability: σ = 0.01 g
- Linearity: ±0.02 g
If "1. Repeatability (or reproducibility)" is considered to be a width of deviation, there exists 2σ, which is ±0.02 g (0.01 g × 2 = 0.02 g). Next,
"2. Linearity" of ±0.02 g means that there exists a deviation of ±0.02 g in balance indication. If these factors were added together, a width of deviation of ±0.04 g might exist in the worst case. Moreover, if other factors, such as the environment at installation and mechanical changes over time, are factored in, then there is no other option but to consider the lowermost digit having a precision rounded to the nearest integer. So, if precision of the 0.01-g digit is needed, a balance having a minimum display of 0.001 g, one digit further past the decimal point, must be used.
Note) The above is a typical example. However, both repeatability and linearity are superior on the UX Series, etc. incorporating the Shimadzu new-generation mass sensor "UniBloc®". (Comparison with older Shimadzu models) (In the case of the UX420S with a weighing capacity of 420 g and minimum display of 0.01 g, repeatability of σ ≤ 0.008 g and linearity of ±0.01 g are achieved.)