Selecting the correct analytical balance for precise weighing is not an easy task. We’ve covered several key points in previous posts but it’s always a good idea to provide a refresher course – especially as it relates to terminology that is sometimes confusing yet important. In this case it’s the 3 Rs: readout, repeatability and reproducibility plus linearity.
We also provide hints on applying these criteria to selecting a lab scale.
R #1: What’s readout? Also called readability it’s what you can read on the scale’s display panel. Simple enough but in terms of specification you’ll have to decide on what you require in terms of the smallest difference displayed when weighing. Usually its 0.1 milligram but in the case of a semi-micro balance it may be 0.01 mg.
R #2: Repeatability is the ability of a balance to display the same result when an object is repeatedly placed on the weighing pan and removed. In general the difference between the largest and smallest result is used to specify repeatability.
R #3: A scale’s ability to deliver the same results at different locations or when operated by different technicians at the same location determines its reproducibility. The difference is expressed in milligrams such as 0.1 or 0.2 mg. Note that reproducibility is sometimes confused with repeatability and that determining reproducibility requires comparing scales of the same make and model.
Linearity is the maximum positive or negative deviation of the readout (see above) from the actual load of a calibrated balance throughout its weighing range. For example a 10-gram test weight on the balance should show 10 grams. A 20-gram test weight should read 20 grams. Both on the balance should display 30 grams. But nothing is perfect. High-end analytical balances with 0.1 mg readout generally have a linearity of ±0.2 or ±0.3 mg.
More Useful Info
Analytical balance spec sheets will provide the maximum weighing range, usually in grams and may also indicate the minimum load. Taking into account the deviations described above helps you specify the correct balance for your applications. For example, if you need to know the weight of a sample within 0.1 mg accuracy a model with 0.1 mg readout and 0.3 mg linearity should not be used. You’ll need a semi-micro balance with a linearity of at least 0.1 mg and 0.01 mg readability.
All things being equal accuracy is improved as sample weight increases because of the diminished effect of linearity and repeatability deviations. Stated another way the impact of a ±0.2 mg linearity and ± 0.2 mg repeatability deviation is more pronounced on a 15 mg sample than on a 120 g sample.
For additional information on this topic see our post on precision scale terminology.
The scientists at Tovatech are ready to answer your questions on selecting and using precision scales and analytical balances in your lab based on your measuring and weighing requirements.