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Performance Characteristics
The characteristics of measurement instruments which are helpful to know the performance of instrument and help in measuring any quantity or parameter, are known as Performance Characteristics.
Types of Performance Characteristics
Performance characteristics of instruments can be classified into the following two types.
- Static Characteristics
- Dynamic Characteristics
Now, let us discuss about these two types of characteristics one by one.
Static Characteristics
The characteristics of quantities or parameters measuring instruments that do not vary with respect to time are called static characteristics. Sometimes, these quantities or parameters may vary slowly with respect to time. Following are the list of static characteristics.
- Accuracy
- Precision
- Sensitivity
- Resolution
- Static Error
Now, let us discuss about these static characteristics one by one.
Accuracy
The algebraic difference between the indicated value of an instrument, $A_{i}$ and the true value, $A_{t}$ is known as accuracy. Mathematically, it can be represented as −
$$Accuracy = A_{i}- A_{t}$$
The term, accuracy signifies how much the indicated value of an instrument, $A_{i}$ is closer to the true value, $A_{t}$.
Static Error
The difference between the true value, $A_{t}$ of the quantity that does not vary with respect to time and the indicated value of an instrument, $A_{i}$ is known as static error, $e_{s}$. Mathematically, it can be represented as −
$$e_{s}= A_{t}- A_{i}$$
The term, static error signifies the inaccuracy of the instrument. If the static error is represented in terms of percentage, then it is called percentage of static error. Mathematically, it can be represented as −
$$\% e_{s}=\frac{e_{s}}{A_{t}}\times 100$$
Substitute, the value of $e_{s}$ in the right hand side of above equation −
$$\% e_{s}=\frac{A_{t}- A_{i}}{A_{t}}\times 100$$
Where,
$\% e_{s}$ is the percentage of static error.
Precision
If an instrument indicates the same value repeatedly when it is used to measure the same quantity under same circumstances for any number of times, then we can say that the instrument has high precision.
Sensitivity
The ratio of change in output, $\Delta A_{out}$ of an instrument for a given change in the input, $\Delta A_{in}$ that is to be measured is called sensitivity, S. Mathematically it can be represented as −
$$S=\frac{\Delta A_{out}}{\Delta A_{in}}$$
The term sensitivity signifies the smallest change in the measurable input that is required for an instrument to respond.
If the calibration curve is linear, then the sensitivity of the instrument will be a constant and it is equal to slope of the calibration curve.
If the calibration curve is non-linear, then the sensitivity of the instrument will not be a constant and it will vary with respect to the input.
Resolution
If the output of an instrument will change only when there is a specific increment of the input, then that increment of the input is called Resolution. That means, the instrument is capable of measuring the input effectively, when there is a resolution of the input.
Dynamic Characteristics
The characteristics of the instruments, which are used to measure the quantities or parameters that vary very quickly with respect to time are called dynamic characteristics. Following are the list of dynamic characteristics.
- Speed of Response
- Dynamic Error
- Fidelity
- Lag
Now, let us discuss about these dynamic characteristics one by one.
Speed of Response
The speed at which the instrument responds whenever there is any change in the quantity to be measured is called speed of response. It indicates how fast the instrument is.
Lag
The amount of delay present in the response of an instrument whenever there is a change in the quantity to be measured is called measuring lag. It is also simply called lag.
Dynamic Error
The difference between the true value, $A_{t}$ of the quantity that varies with respect to time and the indicated value of an instrument, $A_{i}$ is known as dynamic error, $e_{d}$.
Fidelity
The degree to which an instrument indicates changes in the measured quantity without any dynamic error is known as Fidelity