Measuring instruments are the device which indicates the measured quantity into a broadly displayed information.
A measuring instrument can directly show the measured value or it can show the equivalent value to known measure value of the same quantity.
To do the perfect performance, an instrument should have some quality, here in this article we will discuss that.
At the end of this article you will find your PDF downloadable link
In this article we broadly classified the characteristic of an instrument in two types:
- Static Characteristic
- Dynamic Characteristic
So let me give you an overview of each one of these characteristics.
So let’s start with static characteristics, this type of characteristic generally not change so much with the time, you can say this static characteristic most probably constant with the time.
Types of Static Characteristic:
These are some of the standards goes under static characteristic:
- Scale Interval
Sensitivity is defined as the displacement of indicating device of the instrument with respect o the measured quantity.
Mathematically Sensitivity is equal to Scale Spacing/Scale Division.
Magnification means the increase of the magnitude of the output signal of the measuring devices to many times to make the output reading more visible or readable.
Generally greater the magnification smaller the range of the instrument.
Scale interval is the difference between two successive scale marks in units if the measured quantity.
Due to this factor, we can say how the instrument is accurate to show the value of the measured quantity.
Readability means how we can take the measurement quite easily, this is also an important factor of the instrument.
Calibration of any instrument is necessary to measure the quantity in terms of the standard unit. It is the pre-measurement process, where we need to calibrate the particular instrument with the stranded value, however generally this is carried out by the manufacturer of that particular instrument so that the instrument should give the zero output for zero input and similarly the maximum output for the maximum input nearly in a linear scale (Y=mx).
It is the ability of the measuring instrument to show the same value or nearly about the same value for the same quantity when the measurements are carried out by the same observer, same instrument, same environmental condition, same method and same interval of time.
Precision is also associated with this.
Discrimination is the ability of an instrument to show very smaller changes in the measured quantity.
Precision is generally the repeatability of the measuring process that means when we kept constant the other variable factor if we repeat the measurement process the instrument should give us nearly about the same result continuously.
Mathematically or quantitatively the precision can be expressed as:
When the measured value is very much closer than true value then that is called accuracy, If the difference between the measured value and true value is less then we can say the instrument is much accurate but if the difference between true value and the measured value is more then we can say the instrument is less accurate.
Reproducibility is a degree of closeness within a time period.
Hysteresis can be expressed as the difference between the indication of a measuring instrument when the value of the measured quantity is reached by increasing or decreasing of that quantity.
Mathematically it can be expressed by:
Linearity is the ratio of maximum deviation from the linear characteristic as a % of full-scale output.
So these are some static characteristics of an instrument, now let see what are the dynamic characteristic of instruments?
Dynamic characteristic means that the input variable changes rapidly with time.
Types of Dynamic Characteristic:
In dynamic characteristic the factor we should consider are:
- Responses Time
- Dynamic Error
- Step Response
- Ramp Response
- Standard Signals
- Dynamic Error
It can be defined as the time which passes after a sudden change in the measured quantity until the instrument gives us different reading from the true value.
A dynamic error is a difference between the true value changing with the time to the actual value shown by the instruments over time. It is also called a measurement error.
It is time to respond to that particular instrument when we change the input. This time is called Lag.
Lags are Two- Types:
- Retardation Lags:
That means when we change the measured quantity the instrument also changes its value in the faster way.
- Time delay lag:
Time delay lag means the instrument changes its value after some time even when the measured quantity changes.
When the measuring instrument measures the one steady step value to another steady step value, the in-between response time between these two values is called Step Response.
In ramp response, the value of response time in-between two response changes very slowly with time.
It is very hard for us to know the types of signals easily as the signal is in random nature.
So for study, the dynamic characteristic of signal there some signal equation is developed and those are:
- Ramp input
- Step input
- Parabolic input
- Impulse input
In an instrument, the moving indicator also has some mass due to having mass it has some inertia too, so when we give the input to the instrument it deflects from its zero position to some sort of reading position however due to the inertia initially the pointer moves beyond of the reading. This is called overshoot.
However, the galvanometer type instrument has some minimal overshoot.
It is the difference between the train value to the indicated value given by the instrument if we don’t consider any static error.
Fidelity means it is the ability of the system to reproduce the output the same as the input.
Here is a guide by NPTEL where you can find the dynamic Characteristic in other ways like:
- Seismic Sensor
- Step response performance
- Frequency Response Performance
- Bandwidth and Natural Frequency
So feel free to check out their resource too!
Broadly the Characteristics of the Instruments are classified into two categories:
1. Static Characteristic
2. Dynamic Characteristic
The Dynamic Characteristic of Instruments are:
1. Responses Time
2. Dynamic Error
4. Step Response
5. Ramp Response
6. Standard Signals
The Static Characteristics of Instrument are:
2. Scale Interval
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