In metrology, the error of rapidity is that metrological quality of a measuring instrument that expresses the ability to follow the (dynamic) variations in the time of the measurand; it is essential because it allows evaluating the limits within which a measuring instrument can be suitable for measuring variable quantities over time (dynamic quantities).
Another practical definition of the error of rapidity establishes that: it is the smaller, the faster the index of the measuring instrument in changing its position on the graduated scale of the instrument. The speed, limited by the inertia of the moving parts of the instrument and by the damping to which they are subjected, is characterized differently depending on the type of temporal variations of the size.
- In the case in which the quantity to be measured is constant, the speed of the instrument is characterized by the response time. This is defined by the time required, for the index, to reach the final position once put in contact with the measurand.
- In the case in which the quantity to be measured is slowly variable over time, the speed is characterized by the delay with which the index of the instrument follows it. This delay is constant if the variation of the magnitude under examination is, and is higher in proportion to the said variation. If instead, the variation of the quantity is periodic, the index of the instrument provides a measurement whose maximum value is less than the maximum value of the quantity: the delay depends on the frequency of this variation.
- Finally, in the case of rapidly variable quantities over time, speed is defined by the behavior of the mobile parts of the instrument when the size varies sinusoidally. In general, the ratio between the indication provided by the instrument and the value of the input quantity decreases when the frequency increases.