Dynamic Performance of Characteristics

The dynamic behavior of an instrument can be determined by applying some form of known arid predetermined input to its primary element and then study the output, i.e. movement of the pointer. Generally the behavior is judged for three types of inputs:


(i) Step Change
In this case the input having changed, remains constant, i.e. measured medium is changed by instantaneous arid finite value.
(ii) Linear Change
In this case, the input changes linearly with time.
(iii) Sinusoidal Change
In this case, the magnitude of the input changes in accordance with a sinusoidal function of constant amplitude.

The dynamic characteristics of any instrument is defined and evaluated by the following four terms.

a) Speed of Response
It is the rapidity with which an instrument responds to changes in the measured quantity.

b) Lag
It is the retardation or delay in the response of an instrument to changes in the measured quantity. The measuring lag can be either of the retardation type in which case the response of the instrument begins immediately on a change in measured variable, or of the time delay type called dead time, in which case the response of the instrument is simply shifted along the time scale.

Generally the dead time lag is very small (of the order of fraction of a second) Instruments having appreciable dead time are not satisfactory for measuring a variable that fluctuates rapidly. The dead time can also be caused by a finite dead zone in the instrument as a result of friction. In such cases, the instrument does not respond for a certain time delay and acts only when the measured variable has changed sufficiently to overcome the starting friction. The dead time due to this cause depends on how fast the measured variable is changing and on the extent of the instrument dead zone.

Showing how dead time lag depends upon dead zone and the rate at which measured variable changes.

c) Fidelity
It is determined by the fact that how closely the instrument reading follows the measured variable, i.e. it is the degree to which an instrument indicates the changes in measured variable without dynamic error.

d) Dynamic Error
It is the difference between the true value of a quantity changing with time, i.e. measured variable and the instrument reading if no static error is assumed.