It is very important to choose a suitable instrument for a specific measurement. Therefore, for this, we need the performance characteristics of an instrument, these are of the following two types.Â
(1) Static Characteristics.
(2) Dynamic Characteristics.
Types of Characteristics of Instruments
Static Characteristics
Static Characteristics of Instruments refer to the properties of a measuring instrument that remain constant over time and are independent of the quantity being measured. These characteristics include accuracy, precision, range, resolution, and sensitivity. Static characteristics refer to the inherent properties of an instrument that do not change over time.
These characteristics are determined by the design and construction of the instrument and include:
(i) Accuracy:- Accuracy can be defined as an instrument's capability or the exact value of a variable measured by an instrument under given conditions.Â
In fact, the accuracy of an instrument is expressed in terms of its limit of error.
(ii) Reproducibility:- It can be defined in such a way that the accuracy of the number of times an instrument can give the same value under the same conditions in a given time interval. It can be specified in terms of units for a given time interval.
(iii) Sensitivity:- By changing the amount measured by a measuring instrument, the discontinuity obtained on the pail of the instrument is called sensitivity, that is, if there is a slight change in the input of a quantity, then there should be a greater change in the output, in this case, the instrument is more called sensitivity.
(iv) Static Error:- The difference between the true value of the constant with time and the value reported by the instrument is called the static error of the instrument, static error is represented by ± x unit.
Static error = Instrument reading - True value
Note:- For ± Ve error Instrument reading is high and for -Ve error Instrument reading is low.
(v) Drift:- Drift is an unreliable characteristic of industrial equipment because it is difficult to detect and it adversely affects the careful safety of the equipment due to continuous inspection and maintenance.
(vi) Dead Zone:- This is the large area of ​​the measured variable values ​​in which the instrument does not give any reading, sometimes it is also called a dead spot. It is produced by friction in normal indicating or recording. It is also found in instruments that make only micro changes.
Dynamic CharacteristicsÂ
Dynamic characteristics measure any variable quantity in any device, so the behavior of dynamic characteristics with the quantity measured from any process can be represented in the following way.
(i) Stop Change:- In this, the primary element causes a definite and momentary change in the measured quantity.
(ii) Linear Change:- In this the primary element gives a linear change in the measured variable over time.
(iii) Sinusoidal Change:-Â The primary element simulates the measured variable whose result changes according to a sinusoidal function for a fixed amplitude.
Dynamic Characteristics can be divided into two parts. A. Desirable, 1. Speed of Response, 2. Fidelity B. Undesirable, 1. Leg,2. Dynamic Error.Â
(A) Desirable
1. Speed of Response:- The speed with which a device gives a change in the value of a measured variable is called speed of response.
2. Fidelity:- Fidelity is the degree to which an instrument communicates a change in the value of a measured variable in the absence of dynamic error.
(B)Â Undesirable
1. Leg:- The delay in response to a measured quantity by an instrument is called a leg.
2. Dynamic Error:- The difference between the true value of a quantity varying with time and the value measured by the instrument is called dynamic error. While static error in instruments is considered negligible. When the dynamic error is equal to the reading of the instrument, it is called pure error.
