Instrument - CHAPTER 1 BASIC CONCEPTS OF INSTRUMENTATION...

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CHAPTER 1 BASIC CONCEPTS OF INSTRUMENTATION AND MEASUREMENT 1.1 Classification of instruments Analog instrument The measured parameter value is display by the moveable pointer. The pointer will moved continuously with the variable parameter/analog signal which is measured. The reading is inaccurate because of parallax error (parallel) during the skill reading. E.g: ampere meter, voltage meter, ohm meter etc. Digital instrument The measured parameter value is display in decimal (digital) form which the reading can be read thru in numbers form. Therefore, the parallax error is not existed and terminated. The concept used for digital signal in a digital instrument is logic binary ‘0’and ‘1’. 1.2 Characteristic of instruments Figure 1.1 presents a generalized model of a simple instrument. The physical process to be measured is in the left of the figure and the measurand is represented by an observable physical variable X . Figure 1.1: Simple instrument model. 1
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For example, the mass of an object is often measured by the process of weighing, where the measurand is the mass but the physical measurement variable is the downward force the mass exerts in the Earth’s gravitational field. There are many possible physical measurement variables. The key functional element of the instrument model shown in Figure 1.1 is the sensor, which has the function of converting the physical variable input into a signal variable output. Signal variables have the property that they can be manipulated in a transmission system, such as an electrical or mechanical circuit. Because of this property, the signal variable can be transmitted to an output or recording device that can be remote from the sensor. In electrical circuits, voltage is a common signal variable. In mechanical systems, displacements or force are commonly used as signal variables. Other examples of signal variable are shown in Table 1.1. Table 1.1: Example physical variables The signal output from the sensor can be displayed, recorded, or used as an input signal to some secondary device or system. In a basic instrument, the signal is transmitted to a Display or recording device where the measurement can be read by a human observer. The observed output is the measurement M . 2
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There are many types of display devices, ranging from simple scales and dial gages to sophisticated computer display systems. The signal can also be used directly by some larger system of which the instrument is a part. Two basic characteristic of an instrument is essential for selecting the most suitable instrument for specific measuring jobs: 1. Static characteristic 2. Dynamic characteristic Static characteristic of an instrument are, in general, considered for instruments which are used to measure an unvarying process condition. Several terms of static characteristic that have discussed:
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Instrument - CHAPTER 1 BASIC CONCEPTS OF INSTRUMENTATION...

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