Accuracy is one of the most important specifications for a laser-based diameter measurement system and is sometimes the most confusing. Other terms that are used in context with the word accuracy include resolution and repeatability. Despite the frequent use of these terms, they are often misunderstood. Since some of these specifications play a key role in the selection process of a precision gauge, it’s important that engineers and technical managers fully understand this measurement terminology. As you know, the key reason for purchasing a precision gauge is to ensure process control. Misunderstanding the terminology can cause you to pay more than necessary for an over-specified gauge. Or, it may cause you to inadvertently select a measurement system that lacks the critical performance to meet your unique application requirements.
What is Accuracy, Repeatability, and Resolution?
Defines the maximum deviation of a measurement from a known standard or true value. In other words, it is the difference between a measurement reading and the true value of that measurement. Using the analogy of shooting an arrow at a target, accuracy describes the closeness of the arrow to the bullseye. Understand that there is no universal test method for evaluating diameter measurement accuracy. A ± sign is typically used before the accuracy specification, meaning the measurement error can be added or subtracted from the true value. The following illustrates how the term accuracy is used.
A diameter gauge can measure the outer diameter (OD) of a product up to 50.00 mm. It has an accuracy of ±0.003 mm. A manufacturer produces a cable product with an outer diameter of 40 mm and wants to tightly control this dimension. The diameter gauge can measure the outer diameter of this product with a minimum accuracy measurement of 39.997 mm and maximum accuracy measurement of 40.003 mm. The following illustrates the accuracy measurement and deviation from the target value:
39.997 mm <---------------- 40 mm OD -----------------> 40.003 mm
(-0.003 mm) (+0.003 mm)
Refers to the degree of consistency between independent measurements under the same conditions. This also refers to how stable the measurement will be over time. The measurement of consistency does not however indicate the measuring values are true, accurate, or reliable. Using the arrow bullseye analogy again, repeatability is the ability to closely group the cluster of arrows on the target.
Let’s use the manufacturing example illustrated in the previous example: the production of a cable product with an outer diameter of 40 mm. This diameter gauge has a repeatability of ±0.64 μm. The diameter gauge can measure the outer diameter of this product with a minimum repeatability of 39.99936 mm and maximum repeatability of 40.00064 mm.
The smallest increment of measurement that a diameter gauge can calculate and display. This measurement doesn’t refer to repeatability or accuracy. For example, a gauge may be able to calculate and display a measurement down to 0.000001 mm, but the accuracy may be 100x this value. Many times manufacturers publish this measurement for marketing purposes to create a perceived value of high accuracy. In reality, if the gauge has an ample amount of noise in the measurement, the system will have to average a large number of those readings to get a repeatable or accurate value.
A diameter gauge has a resolution of 0.00001 mm. This means the gauge can display the measurement to five decimal places. So, if the measurement of the outer diameter of a product is 40 mm, the gauge will display a measurement value of 40.00001 mm.
Keep in Mind
It’s important to understand the terms accuracy, repeatability, and resolution in order to properly select the best laser diameter gauge for your unique measurement application. There are many factors that affect the overall accuracy and repeatability of measurements. It’s important to closely evaluate the gauge specifications, product quality, and vendor’s reputation in the industry for solutions delivery, service, and support. The end goal for every manufacturer is to achieve the maximum accuracy and repeatability measurements with minimal amount of error to optimize process control, increase productivity, and realize bottom-line savings.