Contributed by James Hood, Team Leader Calibration Services
The 4:1 Test Accuracy Ratio is a guideline to minimize the tolerance errors.
Occasionally here at Duncan Calibration Services we need to inform our aircraft tool calibration customers their tool’s Test Accuracy Ratio (TAR) is less than 4:1 and ask if they are willing to accept a calibration with this decreased test accuracy ratio. The most common response we hear is, “What is a test accuracy ratio?”
Test Accuracy Ratio
TAR is a ratio of the accuracy of a tool, or Unit Under Test (UUT), and the reference standard used to calibrate the UUT. Metrology labs strive for a minimum 4:1 TAR. Simply put, this means that the standard is 4 times more accurate that the tool being calibrated. A test accuracy ratio of 1:1 indicates the UUT and the standard have the same tolerances. A TAR of less than 1:1 (0.5:1 for example) indicates the UUT is more accurate than the standard.
For the sake of simplicity, if the accuracy of a pressure gauge at 100 psi is +/- 4 psi. The standard pressure gauge needed to test the UUT would have to have an accuracy of +/- 1 psi to maintain a 4:1 TAR. If exactly 100 psi of pressure is applied to both gauges the UUT can read anywhere from 96 psi to 104 psi and still be in tolerance. The standard pressure gauge would have to always read from 99 - 101 psi to be considered in tolerance.
The 4:1 TAR is a guideline to minimize the tolerance errors a reference standard may have, such as being near the edge of its specified limits. The higher the TAR the more errors from the standard are reduced. Most of the time, this is not a problem. Under normal circumstances our standards are able to achieve a 10:1 or sometimes a 1000:1 TAR.
The test accuracy ratio or TAR can be a simple calculation to verify if the standard used to perform a calibration is accurate enough to calibrate the UUT.
Duncan Aviation's Calibration Services has test and calibration capabilities for a multitude of aircraft tools and instruments that routinely require calibration. Contact us for all your instrument and tool calibration needs. Save money on your instrument calibrations by bringing your tools along on your next Duncan Aviation maintenance event.
James Hood is the Team Leader for Duncan Aviation Calibration Services. He specializes in keeping all the tools utilized by Duncan Aviation’s aviation technicians and customers calibrated and ready for service. His aviation career began in 1999.