What is measurement?
Answer: Measurement is a process of comparison b/w standard and unknown resulting in knowing the magnitude of unknown in terms of standard.
Define instrument. What are the essential characteristics of instrument?
Answer: Instrument is a device that allows us to make the comparison of unknown in terms of standard.
Essential characteristics are:
- It’s operational consumption should be negligible, and this is an indicator of Accuracy.
- It should not affect the ambient/initial conditions of the circuits in which it has been introduced. This is an indicator of sensitivity.
Define Accuracy and Sensitivity of instrument.
Answer:
Accuracy– It is defined as the degree of closeness with which measured value approaches the true value. Accuracy always specified in terms of its error.
Sensitivity– it is defined as the rate of change of output with respect to input.
Define Error in Instruments. Why we do Error analysis?
Answer: Error is defined as the deviation of measured value from true value. Objective of error analysis is to find a suitable compensation methodology so that accuracy can be improved.
Types of Error in instrument. Which Error is indicator of Accuracy and which one is an indicator of Precision.
Answer: TYPES OF ERROR:
- Gross error: occurs due to human factors such as carelessness, inexperience and improper use of instrument.
- Systematic error: It include all instrument related error.
- Instrumental error: occurs due to substandard materials.
- Environmental error: due to external factors such as- stray electrostatic fields. Ambient temp. Etc
- Observational error: due to improper observation methodology.
- Random errors: error due to the unidentified sources.
- Analysis of systematic error is an indicator of Accuracy of instrument, whereas analysis of random error is an indicator of instruments Precision.
What is Limiting error? Significance of relative limiting error over absolute limiting error.
Answer: It is the deviation of measured value from true value specified by manufacturer himself. As the absolute value of error does not signify any valuable information and also it is constant irrespective of deflection, relative limiting error decreases with increase in deflection.
Relative Limiting Error εr = δA / As ….(1)
Relative Limiting Error is expressed in percentage and hence
% relative limiting error = (δA / As)x100
Now from (1),
(absolute limiting error) δA = εrAs ………(2)
Why is always advisable to take the reading of an indicating instrument nearer to the full scale?
Answer: It is always advisable to take the reading of instrument nearer the full scale because relative error decrease with an increase in deflection.
Difference between Repeatability and Reproducibility
Answer: Repeatability is the degree of closeness among measured values when same input value measure and is measured number of times under same condition while reproducibility is degree of closeness when same numbers of input is measured many number of times by one instrument with many number of observer under different condition.
Define Resolution.
Answer: A smallest change in input for which there is definite numerical change in o/p is called Resolution.
Note: we calculate Resolution only for linear scale not for non-linear scale.
What is drift error in instruments?
Answer: It is an error occur due to instability of components and it is constant throughout the range of the scale. Drift errors are caused by deviations in the performance of the measuring instrument (measurement system) that occur after calibration. These errors may be reduced by carrying out frequent calibrations as the ambient temperature changes or by maintaining a stable ambient temperature during the course of a measurement.
Advantage of angular scale over linear scale.
Answer: Angular scale has higher sensitivity than that of a linear scale.
Deflection instrument and null deflection instrument.
Answer:
Deflection type instrument:
- Only one source of input is required.
- Output reading is based on the deflection from the initial condition of instrument.
- Measured value of the quantity is based on the calibration of the instrument.
- Operational power consumption is finite, hence less accurate.
Null deflection type instrument:
- Require two input measurand and balanced input.
- Must have feedback operation that compare meaurand with standard input.
- Indicate end of measurement with a null deflection i.e. zero
- Operational power consumption is zero, hence highly accurate.
Types of instrument on the basis of methodology of measurement
Answer: On the basis of methodology of measurement instrument are as follows;
Transfer type instrument and its examples.
Answer: Transfer type instrument is an instrument which is calibrated for DC but it can be used for AC with some modification. It has same accuracy for both AC and DC.
Example- electrodynamometer.
What is an Indicating instrument? Types of forces an indicating instrument uses.
Answer: Indicating type instruments give the instantaneous value of parameter under measurement. An indicating instrument uses three forces in order to effectively indicate the value of parameter under measurement.
- Deflecting torque– it is used to deflect the pointer away from zero position and it is produced by parameter under measurement. Ex- current in case of PMMC instruments.
- Controlling torque– it has two fold utility-
- Brings the pointer to steady state position .
- Brings the pointer back to zero position when parameter under measurement is removed from the terminal of the instrument.
- This torque is produced by gravity control mechanism and spring control mechanism.
- Damping torque– require to eliminate the oscillation produced by pointer.
What are the different Damping technique used in indicating type instruments?
Answer: There are three different type of Damping technique –
- Eddy current damping– In this method Aluminum vane and permanent magnets are used. Most suitable for instrument having strong magnetic field. For example- PMMC
- Air friction damping– suitable for instrument having weak magnetic field. Ex- MI instrument.
- Fluid friction damping- for instrument having low deflecting torque. Ex- electrostatic voltmeter.
How the range of PMMC voltmeter and ammeter can be raised?
Answer: To increase the range of PMMC ammeter, a low value resistance is connected in parallel with meter and it is called shunt resistance. To increase the range of PMMC voltmeter, a high value resistance is connected in series with the meter.
Can PMMC work for AC?
Answer: PMMC type instrument uses two permanent magnets in order to create stationary magnetic field. These types of instruments are only used for measuring the DC quantities as if we apply AC current to these type of instruments the direction of current will be reversed during negative half cycle and hence the direction of torque will also be reversed which gives average value of torque zero. The pointer will not deflect due to high frequency from its mean position showing zero reading. However it can measure the direct current very accurately. If coupled with rectifier circuits, they can be used for measuring AC Voltage and current.
If ammeter is connected in parallel, it will damage but voltmeter in series don’t, Why?
Answer: If ammeter is connected in parallel with load, the supply voltage appears across the ammeter and also because of its low internal resistance, meter draws more current and meter gets damage, But voltmeter don’t get damage because of its high internal resistance.
Can MI instrument works for AC and DC both accurately?
Ans. A moving iron instrument works by means of a coil that attracts a moving iron mass that is attached to a pivoted pointer which moves across a graduated scale. The iron mass will be attracted towards the coil, regardless of the magnetic polarity of the coil. So, as an AC current continually reverses the magnetic polarity of the coil, the iron mass is always attracted towards the coil, never repelled by it. Similarly, a moving iron instrument doesn’t care in which direction a DC current is flowing. So the instrument works equally well with both AC and DC currents. Moving iron instruments, however, are not as accurate as moving coil instruments which, of course, are sensitive to the direction of current.
