Measurement followed a man from the very beginning of its development. Measuring methods and measuring instruments were developed in parallel with the development of electrical engineering.However, some physical laws were derived based on measurement results, such as the Biot-Savarts law, when in 1820, the French scientists Jean-Baptiste Biot and Félix Savart established the relationship between an electric current and the magnetic field it produces. Although science and metrology(the science of measurements) are developing quickly, one should always remember that a measurement principle established more than 150 years ago can still be applicable today. There will be many such principles explained here.As the instrumentation becomes more advanced, results will only become more precise.In addressing measurement problems, it is often useful to have a conceptual model of the measurement process. This chapter presents some of the fundamental concepts of measurement in the context of a simple generalized instrument model.In abstract terms, an instrument is a device that transforms a physical variable of interest (the measurand) into a form that is suitable for recording (the measurement). In order for the measurement to have broad and consistent meaning, it is common to employ a standard system of units by which the measurement from one instrument can be compared with the measurement of another.In general, there are two functions of all engineering:
Both these functions require measurements. This is because proper and economical design,operation and measurement require a feedback of information. Measurement plays a significant role in achieving goals and objectives of engineering because of the feedback information supplied by them.
An instrumentation system is an assembly of various devices or components to form an integral whole, for observation, monitoring or control purposes.In other words,it is defined as the design, construction, and provision of instruments for measurement, control etc.
eg-home security system,weather radar,thermostat,CT-scan,MRI- machine etc
Instrumentation system can be classified into following categories:
The purpose of instrumentation system is to give the user a numerical value corresponding to the variable being measured.Due to the various reasons,the numeric value may not actually be a true value(T.V.) of the variable.There may be an error due to limited accuracy in the scale calibration or misreading of scale by the user etc. Thus ,we consider a measurement system to be a system which has an input as the true value of the variable being measured and output as the measured value(M.V.) of that variable.
A measuring system exists to provide information about the physical value of some variable being measured. General block diagram and functional elements of an instrumentation system are shown in block diagram below:
Any instrumentation system (measurement system) is a combination of various components and devices to perform specific tasks. According to the functional block diagram of instrumentation system shown above, the various components can be explained below:
It gives an output that is a function of the measurand (the input applied to it). For most but not all sensors, this function is at least approximately linear. It sometimes helps to convert the variable being measured into a suitable form of energy. Some examples of primary sensors are a liquid-in-glass thermometer, a thermocouple, and a strain gauge. In the case of the mercury-in-glass thermometer, the output reading is given in terms of the level of the mercury, and so this particular primary sensor is also a complete measurement system in itself. However, in general, the primary sensor is only part of a measurement system. The types of primary sensors available for measuring a wide range of physical quantities are presented in next chapter.
2.Variable conversion element:
Variable conversion elements are needed where the output variable of a primary transducer is in an inconvenient form and has to be converted to a more convenient form. For instance, the displacement-measuring strain gauge has an output in the form of a varying resistance. The resistance change cannot be easily measured and so it is converted to a change in voltage by a bridge circuit, which is a typical example of a variable conversion element. In some cases, the primary sensor and variable conversion element are combined, and the combination is known as a transducer. Various conversion may involve analog-to-digital conversion(ADC),digital-to-analog conversion (DAC),frequency to voltage conversion etc.
3.Signal processing element:
Signal processing elements exist to improve the quality of the output of a measurement system in some way. Its main function is to manipulate the signal presented to it preserving the original nature of the signal. A very common type of signal processing element is the electronic amplifier, which amplifies the output of the primary transducer or variable conversion element, thus improving the sensitivity and resolution of the measurement. This element of a measuring system is particularly important where the primary transducer has a low output. For example, thermocouples have a typical output of only a few millivolts. Other types of the signal processing element are those that filter out induced noise and remove mean levels etc. In some devices, signal processing is incorporated into a transducer, which is then known as a transmitter.
4.Signal transmission element:
It is that element of instrumentation system that is actually physically separated,which is used to transmit signal or data to some remote point. Signal transmission is either done through wireline or wireless transmission medias. The signal conditioning and transmission stage is the intermediate stage of the instrumentation system.
5.Signal presentation(Recording) element:
It is that element which is used to display or record the signal if it is not fed automatically into a feedback control system .In order to keep the information conveyed into the intelligible(understandable) form,this element is used.The signal presentation element may consists of Video display unit(VDU) , Recorders(XY-recorders Strip chart recorders) or the data may be transmitted to any control circuit so that it can be compared with required value.
1.S.morris.alan(2001). Measurement and Instrument principle(3 ed.).A division of Reed Educational and Professional Publishing Ltd