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A guide to the five parameters of gauging probe measurement

The five parameters of gauging probe measurement are accuracy, repeatability, precision, resolution and linearity.

When choosing gauging probe systems for your applications, it is important to have a clear understanding of these parameters and your requirements in relation to them.

To help provide a clear picture of the parameters and to help you make the right decision when it comes to your measurement system, we have put together this short guide to the five parameters of gauging probe measurement.

Defining accuracy, repeatability and precision in gauging

Defining accuracy and repeatability is essential to understanding your measurement system requirements. They can be defined as follows:

  • The accuracy of a measurement is how close the measured quantity is compared to the quantity’s true value.

     

  • The repeatability of a measurement is how well repeated measurements made under the same conditions match each other. This is also a measure of how precise the measurements are. Therefore, repeatability is equivalent to precision.

A measurement system can provide a combination of these parameters: accurate but not repeatable, repeatable but not accurate or accurate and repeatable. Accurate and repeatable is the gold standard and is recommended.

Reference masters and the role of quality gauging probes

In a lot of gauging systems, two reference masters are used to calibrate the measurement: a lower limit master and an upper limit master. If the gauge probe is calibrated against these masters and the measured parts are within these limits, a repeatable system will give good results if you are interested in a pass/fail result.

However, if you wish to know how good the manufacturing process is, then choosing gauge probes with good accuracy is important. Repeatable measurements can be corrected for accuracy within the gauge, but an accurate gauge probe removes the need to do this.

To benefit from this functionality, it is recommended that you choose digital gauging probes with excellent repeatability of better than 0.1µm that are calibrated for accuracy against a laser interferometer to an accuracy of 1µm per millimetre of displacement.

Defining resolution in relation to measurement systems

Resolution is often quoted in relation to linear measurement transducers. In some cases, with sub-micron numbers, this is only useful if the sensor is sufficiently repeatable and accurate to make use of the resolution.

Non-contact sensors, in particular lasers, often claim resolutions in nanometers as a key feature. However, it is important to carefully read the accompanying datasheet to see how this is achieved because it may not be possible in relation to a real factory application.

Defining linearity in relation to measurement systems

Linearity is often misunderstood as accuracy. A sensor can be very linear but not accurate. Linearity is usually the difference between the sensor output and the best-fit straight line across its working range. It is not an indication of accuracy. A sensor that is only specified with a linearity figure will always need a low and a high master to calibrate it.

It is recommended to use digital gauging probes that have a good accuracy specification that includes any linearity errors that can be used with a nominal mid-point master to accurately and repeatably gauge parts, providing data that can feed into a statistical process control system. Using a mid-point master is cost efficient because only a single master needs to be made and maintained.

Choose your precision measurement system today

Solartron Metrology provides a wide range of market-leading precision measurement systems.

To find out more about what Solartron Metrology can do for your business, contact us on +44 (0)1243 833333 or sales.solartronmetrology@ametek.com.