In materials testing systems, it is vital to understand a measurement system and its accuracy. One method of assuring a system is accurate is to calibrate the machine by utilizing a verified and traceable “master”, for instance, a known mass or load cell.
While accredited calibrations are a vital part of any quality system, more steps may be taken to understand the total measurement uncertainty of a measurement system. Measurement uncertainty is the result of any input into a measuring system that could alter or skew a result.
A measurement system may be affected by multiple factors. These include the machine operator, the specimen holding device, the properties of the specimen, or environmental conditions.
A gage repeatability and reproducibility study is a helpful tool for calculating the amount of variation in a measurement system that is accredited to the measuring device and the operators carrying out the measurement.
Image Credit: ZwickRoell GmbH Co. KG
Gage R&R addresses both the reproducibility and repeatability of measured values. Repeatability of an instrument is the ability of a measurement system to supply similar indications for a repeated application of the same measurement in the same conditions.
Reproducibility on the other hand, is the closeness of the agreement between measured results from the same specimen under different conditions. Gage R&R studies are also part of lean manufacturing and six sigma as they may supply insights into the variations observed in a manufacturing process.
A gage R&R study can statistically verify that a calibrated measurement’s repeatability is good enough to determine production or process decisions. Cg and Cgk are the critical values which result from these studies.
Cgk measures the deviation of measured values between a master and the specimen, or reproducibility. Cg is a measurement of the deviation of values across a study, or repeatability. If the Cg and Cgk are both ≥1.33 a measurement system is thought to be “capable”.
ZwickRoell is now providing gage R&R studies on a number of measurement systems and calibrated instruments. For these studies, the following calibrated instruments are available:
- Both contact and non-contact extensometers
- Load cells
- Swing duration or friction on impact testers
- Crosshead displacement
This information has been sourced, reviewed and adapted from materials provided by ZwickRoell.
For more information on this source, please visit ZwickRoell GmbH Co. KG