Compressibility error becomes significant at altitudes above 10,000 feet (3,000 m) and at airspeeds greater than 200 knots (370 km/h). Hysteresis is an error that is caused by mechanical properties of the aneroid capsules located within the instruments. These capsules, used to determine pressure differences, have physical properties that resist change by retaining a given shape, even though the external forces may have changed.