When discussing laboratory balances, accuracy and readability are common terms that are used to understand the readings they give. Weighing uncertainty is an important factor, however it is less known and not as discussed. Uncertainty is a variable that can be measured. It is essential, especially for calibration laboratories. It can be useful for the comparison of results from experiments carried out by different people.
The definition of weighing uncertainty is the range of values that are assumed within reason to be the exact value of a measurement. This definition may sound broad and very similar to other terminology about measurements. Here we hope to help expand on this definition and resolve any confusion.
Why is it important?
First it is important to understand why uncertainty matters. Good uncertainty controls are needed for laboratory accreditation. It is a key factor for:
- Instrument calibration results
- Sensitivity
- Readability
- Repeatability
- Standard deviation
- and more.
With calculated uncertainty, you can ensure high standard repeatable results.
Readability, accuracy or uncertainty?
Terminology within data collection and analysis can often sound very similar, even when describing different things. Uncertainty can often get confused with readability and accuracy. When discussing readability, this is understood to be influenced by the number of decimal places that the balance reading goes to. A balance that can read to 0.0001g has a higher readability than a balance that reads to 0.001g.
Accuracy can often get confused with uncertainty. Both terms refer to how correct the reading on the balance is to the real sample weight. However, accuracy is more about how close the measured compared to the real weight are, while uncertainty refers to how reliable the results are.
Error is also commonly mistaken to be the same as uncertainty. Errors are mistakes that can cause a balance to not provide the correct results. They are a factor that can influence and increase uncertainty, however they are not the same thing.
Calculating uncertainty
When uncertainty is calculated, it can be done for many factors influencing a value. Theses include (but not exclusive to):
- Readability
- Repeatability
- Calibration
- Density
- Reference mass
- Environmental influences
These are then combined based on importance. It can get a bit confusing and long winded, which is why uncertainty is usually tested and calculated by calibration laboratories.
Standard deviation is a factor with a correlation to uncertainty. They have a relationship where if the standard deviation is higher, the uncertainty will also be higher.
Conclusion
Overall, uncertainty can be difficult to wrap your head around. It is a factor that is unavoidable due to there always being a chance of something causing a deviance between the true weight, and the weight recorded. However, it is important to remember that it is always based around the reliability of the produced weight value compared to the true weight value. It is impossible to avoid outside factors that can influence a measurement, so uncertainty must always be considered.
