Measuring instruments are devices that represent the measured quantity in the form of the widely displayed information. The characteristics of measuring instruments help to know the performance of an instrument and measure any quantity or parameter.
A measuring instrument may show the measured value directly or it may show a value equal to the known measurement value of the same quantity. In order to perform correctly, an instrument has to have some quality such as a non-existent error in the instrument, accurate measurement of the quantity or parameter measured, no lag, etc.
Types of performance characteristics
The performance characteristics of measuring instruments can be classified into two types.
- Static Characteristics
- Dynamic Characteristics
Static Characteristics of Measuring Instruments
The characteristics of instruments used to measure quantities or parameters that do not change with time are called static characteristics. The static characteristic is most stable over time. Sometimes, these quantities or parameters may vary gradually over time. Static characteristics are:
- Accuracy: The proximity of the instrument reading to the true value of the measured variable.
- Dead Zone: For the largest changes in the measured variables, the instrument may not respond.
- Drift: Slow variation in instrument calibration, This may be due to variations in the properties of the metal due to chemical changes. For example, the manufacture of coatings on RTD elements and thermocouples. Due to continuous exposure. RTD, thermocouple metals can have changes in atomic structure.
- Precision: The degree of accuracy for which the instrument is designed.
- Repeatability: Instruments are calibrated at different times, so it has to be shown at different times.
- Reproducibility: The degree of proximity with which a given value is repeatedly measured. The equipment has to show the same value for a longer period of time under the same operating conditions.
- Resolution: The instrument is effectively able to measure the input when the input has a resolution.
- Sensitivity: To the smallest change in the measured variable, the instrument can react.
- Static Error: The difference between the actual value of the quantity and the value shown by the instrument does not change over time.
Accuracy of Measuring Instruments
The algebraic difference between the indicated value, Ai, and the actual value At of an instrument is known as accuracy. it can be represented as
The term, accuracy, refers to how close an instrument is to the indicated value, but the Ai is nigh to the actual value At.
Static Error of Measuring Instruments
The difference between the actual value At of a quantity, which does not change over time, and the indicated value of an instrument, Ai, is known as the static error es. it can be represented as
The term static error refers to the inaccuracy of the instrument. If the static error is represented as a percentage, it is called the percentage of a static error. it can be represented as
Substitute the value of es on the right-hand side of the above equation,
Where, %es is the percentage of a static error.
Precision of Measuring Instruments
An instrument indicates the same value repeatedly. When used to measure the same quantity multiple times under the same conditions, the instrument has high precision.
Sensitivity of Measuring Instruments
The ratio of change in output ΔAout, outside an instrument to change in a given input, ΔAin that is measured is called sensitivity, S. it can be represented as
The term sensitivity refers to the smallest change in measurable input that is required for an instrument to respond.
- If the calibration curve is linear, the sensitivity of the instrument will be constant and equal to the slope of the calibration curve.
- If the calibration curve is non-linear, the sensitivity of the instrument will not be constant and will vary with respect to the input.
Resolution of Measuring Instruments
If the output of instruction will change only when there is a specific increase of the input, then that increase of the input is called resolution.
Dynamic Characteristics of Measuring Instruments
The characteristics of instruments, which are used to measure quantities or parameters that change very rapidly over time, are called dynamic characteristics. Dynamic characteristics are:
- Speed of Response: The speed with which the instrument responds whenever there is a change in the quantity to be measured is called the speed of the response. It indicates how fast the instrument is.
- Dynamic Error: The difference between the actual value of a quantity and the value shown by the instrument varies with time.
- Fidelity: The degree to which the instrument responds to a change in the measured variable without causing any dynamic error.
- Lag: The delay in the response of the instrument to the measured variable is called lag.
Related Tutorial: Introduce to Measuring Instruments.