Aug
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Quick Guide Introduction The instruments, which are used to measure any quantity are known as measuring instruments. This tutorial covers mainly the electronic instruments, which are useful for measuring either electrical quantities or parameters. Following are the most commonly used electronic instruments. Voltmeter Ammeter Ohmmeter Multimeter Now, let us discuss about these instruments briefly. Voltmeter As the name suggests, voltmeter is a measuring instrument which measures the voltage across any two points of an electric circuit. There are two types of voltmeters: DC voltmeter, and AC voltmeter. DC voltmeter measures the DC voltage across any two points of an electric circuit, whereas AC voltmeter measures the AC voltage across any two points of an electric circuit. An example of practical DC voltmeter is shown in below figure. The DC voltmeter shown in above figure is a $(0-100)V$ DC voltmeter. Hence, it can be used to measure the DC voltages from zero volts to 10 volts. Ammeter As the name suggests, ammeter is a measuring instrument which measures the current flowing through any two points of an electric circuit. There are two types of ammeters: DC ammeter, and AC ammeter. DC ammeter measures the DC current that flows through any two points of an electric circuit. Whereas, AC ammeter measures the AC current that flows through any two points of an electric circuit. An example of practical AC ammeter is shown in below figure − The AC ammeter shown in above figure is a $(0-100)A :$ AC ammeter. Hence, it can be used to measure the AC currents from zero Amperes to 100 Amperes. Ohmmeter Ohmmeter is used to measure the value of resistance between any two points of an electric circuit. It can also be used for finding the value of an unknown resistor. There are two types of ohmmeters: series ohmmeter, and shunt ohmmeter. In series type ohmmeter, the resistor whose value is unknown and to be measured should be connected in series with the ohmmeter. It is useful for measuring high values of resistances. In shunt type ohmmeter, the resistor whose value is unknown and to be measured should be connected in parallel (shunt) with the ohmmeter. It is useful for measuring low values of resistances. An example of practical shunt ohmmeter is shown in the above figure. The ohmmeter shown in above figure is a $(0-100)Omega$ shunt ohmmeter. Hence, it can be used to measure the resistance values from zero ohms to 100 ohms. Multimeter Multimeter is an electronic instrument used to measure the quantities such as voltage, current & resistance one at a time. It can be used to measure DC & AC voltages, DC & AC currents and resistances of several ranges. A practical multimeter is shown in the following figure − As shown in the figure, this multimeter can be used to measure various high resistances, low resistances, DC voltages, AC voltages, DC currents, & AC currents. Different scales and range of values for each of these quantities are marked in above figure. The instruments which we considered in this chapter are of indicating type instruments, as the pointers of these instruments deflect and point to a particular value. We will discuss about these electronic measuring instruments in detail in later chapters. Performance Characteristics The characteristics of measurement instruments which are helpful to know the performance of instrument and help in measuring any quantity or parameter, are known as Performance Characteristics. Types of Performance Characteristics Performance characteristics of instruments can be classified into the following two types. Static Characteristics Dynamic Characteristics Now, let us discuss about these two types of characteristics one by one. Static Characteristics The characteristics of quantities or parameters measuring instruments that do not vary with respect to time are called static characteristics. Sometimes, these quantities or parameters may vary slowly with respect to time. Following are the list of static characteristics. Accuracy Precision Sensitivity Resolution Static Error Now, let us discuss about these static characteristics one by one. Accuracy The algebraic difference between the indicated value of an instrument, $A_{i}$ and the true value, $A_{t}$ is known as accuracy. Mathematically, it can be represented as − $$Accuracy = A_{i}- A_{t}$$ The term, accuracy signifies how much the indicated value of an instrument, $A_{i}$ is closer to the true value, $A_{t}$. Static Error The difference between the true value, $A_{t}$ of the quantity that does not vary with respect to time and the indicated value of an instrument, $A_{i}$ is known as static error, $e_{s}$. Mathematically, it can be represented as − $$e_{s}= A_{t}- A_{i}$$ The term, static error signifies the inaccuracy of the instrument. If the static error is represented in terms of percentage, then it is called percentage of static error. Mathematically, it can be represented as − $$% e_{s}=frac{e_{s}}{A_{t}}times 100$$ Substitute, the value of $e_{s}$ in the right hand side of above equation − $$% e_{s}=frac{A_{t}- A_{i}}{A_{t}}times 100$$ Where, $% e_{s}$ is the percentage of static error. Precision If an instrument indicates the same value repeatedly when it is used to measure the same quantity under same circumstances for any number of times, then we can say that the instrument has high precision. Sensitivity The ratio of change in output, $Delta A_{out}$ of an instrument for a given change in the input, $Delta A_{in}$ that is to be measured is called sensitivity, S. Mathematically it can be represented as − $$S=frac{Delta A_{out}}{Delta A_{in}}$$ The term sensitivity signifies the smallest change in the measurable input that is required for an instrument to respond. If the calibration curve is linear, then the sensitivity of the instrument will be a constant and it is equal to slope of the calibration curve. If the calibration curve is non-linear, then the sensitivity of the instrument will not be a constant and it will vary with respect to the input. Resolution If the output of an instrument will change only when there is a specific increment of the input, then that increment of the input is called Resolution. That means, the instrument is