In most wind tunnels, standard models are tested during the commissioning and calibration of the facility. This sometimes has an unfortunate effect that the test results are not as good as they can be, because the wind tunnel and its measurement system have not yet been optimally tuned at the time of the test. However, some laboratories have adopted the practice of periodical testing of a standard model every couple of years in order to provide a continued confidence in the reliability of measurements in their wind tunnels .