Measurement & Instrumentation

Measurement & Instrumentation

Measurement fundamentals

Any measurement operation can be seen as the retrieval of particular information carried out by the input signal. 

In nature, most of the usual applications of electrical and electronics engineering,  voltages and currents in continuous form.

Different kind of signals, such as pulse/ discrete signals can also be in nature.

These signals require to be measured with different kind of instruments.

Continuous or analogue signals  are considered

AC and DC signals
Voltage and currents can be constants with time in the DC systems.
V = v(t) = Vdc I = i(t) = Idc
Sinusoidal voltage and current are periodic signal in electric power system.

V = v(t + T) = Vmsin (2πft + φ)

Direct current

Direct Current (DC) always flows in the same direction, positive (or always negative), but it may increase and decrease. 

Alternating Current (AC)

Alternating Current (AC) flows one way, then the other way, continually reversing direction. An AC voltage is continually changing between positive (+) and negative (-). 

This triangular signal is AC because it changes between positive (+) and negative (-). 

AC from a power supply This shape is called a sine wave

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Sensitivity

This is the relationship between a change in the output reading for a given change of the input. (This relationship may be linear or non-linear.)

Sensitivity is often known as scale factor or instrument magnification and an instrument with a large sensitivity (scale factor) will indicate a large movement of the indicator for a small input change.

Meter Sensitivity 

(Ohms-per-Volt Rating)

•Measured in Ω/V.

•Higher the sensitivity, more accurate is the measurement.

•If current sensitivity (CS) of a meter is known, its Ω/V rating can easily be determined.

•Consider a basic meter with CS of 100 μA. 

•If used as a voltmeter of range 1 V,

  RT = 1 V / 100 μA = 10 kΩ

•Thus, the meter sensitivity is simply 10 kΩ/V.

In analog systems, full scale may be defined by the maximum voltage available, or the maximum deflection (full scale deflection or FSD) or indication of an analog instrument such as a moving coil meter or galvanometer.

The scale range is defined as the difference between the nominal values of the measured quantities corresponding to the terminal scale marks.

   ie. Minimum scale value and maximum scale value.

•The instrument range is the total range of values which an instrument is capable of measuring.

In a single range instrument this corresponds to the scale range.

In a multi range instrument the difference is taken between the maximum scale value for the scale of highest value and the minimum scale value for the scale of lowest values, provided that adjacent ranges overlap.

•    Note that if the same meter was used for 2 V range, the required RT would be 20 kΩ.

•   Its ohms/volt rating is 20 kΩ / 2 V = 10 kΩ/V.

•   The ohms-per-volt rating does not depend on the range of the voltmeter.

•    Also, note that the range of a voltmeter (or an ammeter) is changed by switching in another resistor in the circuit.

•    Therefore, for a given range the internal resistance of the voltmeter remains the same irrespective of the deflection of the pointer.