Reading accuracy is sometimes a problem with goniometers. Calibration balls can be used to calibrate contact angle goniometers, CNC machines and similar equipment. The British admiralty used goniometers (radio compasses) to track the German fleet. These goniometers come in different forms that some argue increase reliability. Many goniometers also provide a VU or PPM as a secondary function. In fact, good digital and software goniometers provide artificial and even user-adjustable persistence. In crystallography, goniometers are used for measuring angles between crystal faces.