Classification of Measuring Instruments

Measuring Instruments: Definition, Classification, and Characteristics

A measuring instrument is a device used to quantify physical and electrical quantities. The concept of measurement fundamentally involves comparing two quantities that share the same unit. One of these quantities has an unknown magnitude, which is then compared against a predefined standard value. Through this comparison, a numerical result is obtained, representing the measured value of the unknown quantity.

Measuring instruments can be categorized into three main types:

• Electrical Instruments
• Electronic Instruments
• Mechanical Instruments

Mechanical instruments are primarily designed for measuring physical quantities. They are well - suited for applications involving static and stable conditions. However, their limitation lies in their inability to respond effectively to dynamic or rapidly changing conditions. For instance, they may not accurately capture transient events or fluctuations in physical parameters.

Electronic instruments, on the other hand, offer a significant advantage with their rapid response times. Compared to both electrical and mechanical instruments, they can quickly detect and react to changes in the measured quantity. This makes them ideal for applications that require real - time monitoring and measurement of rapidly varying phenomena.

Electrical instruments are specifically designed for measuring electrical quantities such as current, voltage, and power. Examples of electrical measuring instruments include the ammeter, voltmeter, and wattmeter. The ammeter is used to measure electric current in amperes, the voltmeter is employed to measure voltage, and the wattmeter is utilized for determining electrical power. The classification of electrical instruments often hinges on the methods used to display or represent the output reading. Different instruments may use analog dials, digital displays, or other means to convey the measured values, each with its own set of advantages and applications depending on the specific requirements of the measurement task.

Types of Electrical Instruments

In this article, we will explore the various types of electrical instruments, each with its unique characteristics and applications.

Absolute Instruments

An absolute instrument determines the value of a measured quantity based on physical constants. These physical constants can include parameters such as the angle of deflection, specific degrees, or meter constants. To obtain the value of the measured quantity from an absolute instrument, mathematical calculations are typically required to interpret the relationship between the observed physical constant and the quantity being measured.

A prime example of an absolute instrument is the tangent galvanometer. In this device, the magnitude of the current passing through its coil is calculated using the tangent of the coil's angle of deflection, along with factors such as the horizontal component of the Earth's magnetic field, the radius of the coil, and the number of wire turns. Due to their reliance on precise physical relationships and the need for detailed calculations, absolute instruments are commonly used in laboratory settings where accurate and fundamental measurements are essential.

Secondary Instruments

Secondary instruments display the magnitude of the measurable quantity through deflection. For accurate measurement, calibration of these instruments against a standard instrument is crucial. Unlike absolute instruments, secondary instruments provide a direct output, eliminating the need for complex mathematical calculations to determine the measured value. This simplicity makes them highly practical for a wide range of everyday measurement tasks.

Digital Instruments

Digital instruments present their output in a numeric format, offering several advantages over their analog counterparts. One of the key benefits is enhanced accuracy, as digital readings eliminate the potential for human error associated with reading analog scales. This makes digital instruments particularly suitable for applications where precision is of utmost importance, such as in scientific research, engineering, and industrial quality control.

Analog Instruments

Analog instruments are characterized by an output that varies continuously. They typically feature a pointer that moves along a calibrated scale to indicate the magnitude of the measurable quantity. Analog instruments can be further classified into two subtypes:

Null Type Instruments

Null type instruments use zero or null deflection to indicate the magnitude of the measured quantity. These instruments are renowned for their high accuracy and sensitivity. They operate on the principle of comparing a known quantity with an unknown quantity. When the values of the known and unknown quantities are equal, the pointer of the instrument shows a null or zero deflection. Null type instruments are commonly employed in potentiometers and galvanometers to precisely identify null points, which are essential for accurate measurements in various electrical and electronic applications.

Deflection Type Instruments

Deflection type instruments determine the value of the measuring quantity based on the deflection of a pointer. When the quantity being measured acts on the instrument, it causes the pointer of the moving system (which is mounted on a calibrated scale) to deflect. By observing the position of the pointer on the scale, the magnitude of the measured quantity can be determined.

Deflection type instruments can be further subdivided into three categories:

• Indicating Instruments: These instruments are designed to display the magnitude of the measured quantity directly. They typically feature a dial with a pointer that moves over a graduated scale. Examples of indicating instruments include voltmeters, ammeters, and power factor meters, which are widely used for monitoring and measuring electrical parameters in various circuits and systems.

• Integrating Instruments: Integrating instruments are used to measure the total energy supplied over a specific period. The total energy measured by these instruments is calculated as the product of time and the measured electrical quantity. Common examples of integrating instruments are energy meters, watt - hour meters, which are essential for accurately billing consumers for the electrical energy they consume.

• Recording Instruments: Recording instruments are used to document the circuit conditions at regular intervals. They typically have a moving system that carries a pen or stylus, which lightly touches a paper sheet or other recording medium. As the electrical quantity being measured changes, the movement of the coil is traced on the recording medium, creating a curve that illustrates the variation of the electrical quantity over time. This graphical representation is invaluable for analyzing trends, diagnosing problems, and monitoring the performance of electrical systems.

In comparison to electrical and mechanical instruments, electronic instruments generally exhibit significantly faster response times. This rapid response allows them to quickly detect and react to changes in electrical quantities, making them highly suitable for applications that require real - time monitoring and analysis, such as in high - speed data acquisition systems, control systems, and modern communication technologies.