How to Judge Scale AccuracyFebruary 27th, 2012
At Tovatech we frequently find that customers are confused about weighing terms associated with analytical scales and precision balances, especially as it relates to scale accuracy. This comes as no surprise to us because, in fact, the terminology is confusing. Different manufacturers of analytical balances will use different weighing terms, some of them incorrect, by the way, to describe the performance and accuracy of their products. When you evaluate electronic scales you should compare the performance of a specific manufacturer’s products against each other rather than with those of different manufacturers.
Why Weighing Terms are Important
The importance of scale accuracy applies to many weighing operations across the weighing spectrum from moisture analyzers to counting scales. Some applications are more important than others. For example, you might tolerate a small degree of inaccuracy when weighing out a quantity of bolts, but such is not the case when researchers calculate active pharmaceutical ingredients and pharmaceutical excipients going into a medicine. Here any inaccuracy puts you at risk of running afoul of the FDA and other regulatory authorities, not to mention potential risk to patients.
Let’s start with the difference between accuracy and precision. Many folks think they mean the same thing, but such is not the case. An accurate scale is always right in that it shows the correct weight. This is further assured when the scale is calibrated (see below). Precision means the result is always the same. Unfortunately a scale with excellent precision could also be wrong, and therefore inaccurate.
Common Weighing Terms used to describe Scale Accuracy and Precision
Linearity: The positive or negative deviation of the readout from the actual load of a calibrated analytical balance throughout its weighing range. For example, using rubber-tipped tweezers place a 10-gram test weight on the balance and allow it to stabilize (see below). The scale should show 10 grams. Then place a 20-gram test weight on the balance, which should now read 20 grams. Then place them both on the balance, which should display 30 grams. But nothing is perfect. High-end analytical balances generally have a linearity of ±0.2 or ±0.3 mg.
Readout (a.k.a. Readability): For precision scales and analytical balances this shows smallest difference in weight that can be read and displayed by the unit. For an analytical balance it is typically 0.1 milligram.
Repeatability: This is the ability of an analytical balance to display the same result when an object is repeatedly placed on the weighing pan and removed. One would think that this is a given, but not necessarily so when dealing in milligrams. In general the difference between the largest and smallest result is used to specify repeatability.
Take-away No. 1: If the readability of a balance is identical to the repeatability, repeated sequential weighing of the identical item should display exactly the same results.
Take-away No. 2: What weighing resolution do you require? If you need to know the weight of a sample within 0.1 mg accuracy, an analytical balance with 0.1 mg readout and 0.3 mg linearity will not be adequate. In such a case the balance would need a linearity of at least 0.1 mg. This means you need a balance with the 0.01 mg readability available on a semi-micro balance.
Reproducibility: Frequently confused with repeatability but it signifies something different. As one example, will a technician at the West Coast lab get the same result as one at the East Coast lab using the same make and model of analytical scale and following the same procedure? Or, in the same lab will the results be the same if done by different people at different times but on the same scale?
Other Weighing Terms for Digital Weighing Scales
Calibration: A subject onto itself. Please refer to our blog post on internal and external calibration.
IP: Ingress protection. IP is the ability of certain industrial scales to withstand the ingress of dirt and moisture that would affect performance. IP is usually presented as two numbers, the first for protection against solid objects (0 to 6) and the second (0 to 8) against liquids. Higher is better.
Stabilization Time (a.k.a. Response Time): The time it takes, usually about 2 seconds, for a scale to display the weight of products placed on the weighing platform of, for example, a counting scale. You can slowly add or remove bolts to get the desired quantity but you have to wait for the weight to stabilize before taking the reading.
Taring: Eliminating the weight of the bolt bin to get an accurate bolt count or weight. Most digital scales have a tare button to accomplish this.
Validation: A documented act of demonstrating that a procedure, process, and activity will consistently lead to the expected results. It often includes the qualification of systems and equipment (such as scale validation) and is a requirement for good manufacturing practices and other regulatory requirements.
We hope this little tutorial is helpful as you compare the performance of precision scales and analytical balances. We at Tovatech welcome your inquiry and will be delighted to help you select weighing instruments of any type for you particular operations.