As an electrical engineer, we will measure the electrical quantity not any non electrical quantity. So the methods or devices or the calculations which are used to measure the electrical quantities is called Electrical Measurement or we can say measurement of electrical quantities is done to measure electrical parameters of a system.
Table of Contents
Elements of Measurement System
The quantity or the elements which we want to measure is first feed to the primary sensing element.
Function of Primary Sensing Element: Most of the measuring equipments are electrical. Suppose we want to measure a value of length in terms of electrical quantity or suppose we want to see the measured value in digital display, then we need to convert the length signal into voltage signal because digital display can only display in voltage or current signal. So we need some way that we can convert the length signal to electrical signal. Or in case of measurement of speed, in tachometer the speed signal is converted into electrical signal which is shown as output on the 7-segment display. This conversion is done by Primary Sensing Element. It is also called as Transducer. So transducer is nothing but it basically converts one signal to another signal.
Function of Data Conversion Element: You must know that the processing in digital domain is easier than analog domain. And the circuits that we construct in digital domain are easier to construct as compared to analog circuit. Very easily we can generate digital circuit by means of truth table. Many time we will be making measurement or we will be processing the data in digital domain. So here as data conversion element we use ADC.
Function of Data Manipulation Element: The instrument that i am using to display, can not display the voltages in terms of milivolt. Transducer is giving output in milivolt but my instrument can not sense that. So we need to step up that voltage in a certain level that my instrument is able to sense that voltage. That means we need to manipulate the original data. So here we need an amplifier as data manipulation element to amplify the signal in a desired level.
Function of Data Transmission Element: Suppose we are recording rainfall in a hill. but we can not construct the center on the hill top rather than we should construct in more suitable location but the data is being collected on the hill top. Now what we will do is we collect the data on the hill top and transmit it to base station and the processing will be done at the base station itself. we can not process the data on the hill top, we will just keep the sensor there, they will relay the data to the base station and the processing is also done at the base station. So we need something of the sort of the data transmission element, that will transmit the data from center to base station where the data will be collected, processed and the result is displayed. So this transmission element is nothing but like a channel in communication system, it can be sort of optical fiber.
Function of Data Presentation Element: Suppose we have constructed a graph of rainfall over the year. That means we need a display device that can display the graph. So after processing the data, we need to display the result on a device and that is done by data presentation element. As a data presentation element we use CRO (Cathode Ray Oscilloscope).
So this five elements are collectively formed by measurement system. This elements can differ in different instruments but essentially the block diagram or the functionality remains same.
Characteristics of Measurement
👉 Accuracy
Accuracy means the closeness of true value and measured value. Suppose there are two instruments to measure the speed of a car and the measured value is like -
True Value Measured value
Instrument A → 131km/h 135 km/hr
Instrument B → 131km/h 133 km/hr
Instrument B has more accuracy.
So, in an instrument the accuracy should be high.
👉 Precision
Precision means the degree of closeness with which individual measurement tally with each other.
Suppose we have measured a current value of 25A and it is measured by two different instrument.
Instrument A Instrument B
23.7A 24.7A
23.9A 24.3A
24.1A 24.9A
23.5A 25.5A
From the table the difference between two measured value of instrument A is 0.2 where the difference between two measured value of instrument B is more than instrument A.
So, Instrument B is more accurate but not precise where Instrument A is more precise.
👉 Sensitivity
Sensitivity means the smallest change in the signal that can be detected by an instrument.
Suppose there are two ammeters. And in a circuit the change of current is 2A then the change in deflection due to the change of current is 30⁰ for Instrument A and 1⁰ for Instrument B.
Instrument A → Δi = 2A, Δθ = 30⁰ ⇒ S₁ = 15⁰/A
Instrument B → Δi = 2A, Δθ = 1⁰ ⇒ S₁ = 0.5⁰/A
∴ Instrument A is more sensitive.
Sensitivity should be high in a measuring instrument.
👉 Resolution
Resolution means the smallest portion in the signal that can be measure by an instrument.
Suppose to measure the temperature, two celcius scales are used. Scale 1 can measured the smallest portion of 0.001ºC and scale 2 can measure the smallest portion of 0.01⁰C. So scale 1 has more resolution than the scale 2.
Or suppose in linear scale the smallest portion is 1mm whereas the vernier scale or screw gauge has smallest portion of 0.2mm or 0.5mm, then the vernier scale or the screw gauge has more resolution than the linear scale.
👉 Threshold
Threshold value means the minimum input required to get the system working.
and gate to source voltage = VGS
If VGS < Vₜ that means it is in cut off region.
⇒ IDS = 0
∴The FET is not working.
But if VGS < Vâ‚œ then IDS ≠ 0
then system will working.
Threshold is an undesirable characteristic.
👉 Drift
Drift means how the measured quantity is deviated from the true value. So drift should be as low as possible. Drift is an undesirable quantity.
There are three types of drift.
✔ Zero Drift
Zero drift means the difference between the input and output or difference between measured value and true value when the input is zero.
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| Zero Drift |
Sometimes zero drift may be permanent in the instrument due to hysteresis.
Zero Drift doesn't mean that drift is zero, means what is the drift when input is zero.
✔ Span Drift
In case of galvanometer, the deflection(θ) of the pointer -
Where N = no of turns, A = Area, B= magnetic flux density
Now suppose due to heat or old age of the magnet, the magnetic flux density(B) is changing. so that sensitivity of the instrument is also changing and the slope of the input vs output graph is also changing.
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| Span Drift |
Span Drift is also called as Sensitivity Drift as this drift is related to sensitivity of the instrument.
✔ Zonal Drift
The drift is not in the entire range of the instrument but certain zone of the instrument. It is nothing but a combination of zero drift and span drift.
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| Zonal Drift |
👉 Reproducibility
Reproducibility means whether the system or an instrument is able or not to reproduce the same result or same reading in different environmental condition. Location and condition may varied.
So Reproducibility indicates that the result or reading is independent on the environmental condition. Reproducibility is a desirable quantity.
👉 Repeatability
Repeatability is same as reproducibility but the environmental condition or location should be same. We know precision means when we are taking more than one reading then how much close is the measured value to the actual value. Similarly Repeatability is how close my readings are when we repeating the same measurement again and again whether we are able to get same result or not.
So, Repeatability is very desirable quantity.
👉 Dead Zone
Dead zone means when instrument is not able to produce the output for a certain range of input.
Suppose if we are applying the input in range of 0 to 2 Volt. But no observable output is coming. So this certain zone is called as dead zone of the instrument.
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| Dead Zone |
So always between 0 and threshold value VT is called Dead Zone or we can say the boundary of the dead zone is called as Threshold Value.
In an instrument Dead Zone should be minimum and Dead Zone is undesirable.
👉 Dead Time
Dead time means response time of the instrument. So instrument may takes certain time to respond the changes in input, that's called Dead Time.
Suppose the needle of the ammeter is at 0 and we are measuring 5A current of a system. So the time taken by the needle to move from 0A to 5A, is termed as Dead Time.
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| Dead Time |
Dead Time should be minimum of an instrument. and it is also undesirable quantity.
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