In this article, we will learn about Measuring instruments and their need, purpose & types. And we will also discuss about functions of digital measuring instruments and their advantages & disadvantages.
Measuring InstrumentsThe device that is used for measurement of certain physical quantity is called measuring instrument. These measuring instruments are used in our day-to-day life for the measurement of various quantities like length, weight, temperature, pressure, current, voltage etc.
The instruments indicate the value of the physical quantities, by this we get some understanding and do some activities and take decisions for the process.
The main purpose of this measuring instrument is to measure a physical variable in its units, displays the variable in its units and record the value in its units.
Need for Measuring InstrumentsIn every engineering aspect exact measurement is an absolute thing for everyone to do precise work. That is the main reason for having a best measuring instrument.
Measurement tools are used to determine various physical quantities and attributes in the physical, chemical and electrical streams.
Nowadays, there are lots of equipment’s are used to complete the task, for analyzing, monitoring, measuring and testing. From smaller workshops to larger industries in every sector measuring instruments plays a vital role in daily activities.
From scale for education to thermometer for medica to vernier caliper for manufacturing to wattmeter for electrical industries measuring instruments were doing a good and important job.
Types of InstrumentsMeasuring instruments are mainly two types – analog (analogue) and digital.
Analog InstrumentsThe analogue instruments indicate the magnitude of quantity in the form of pointer movement. We want to measure readings from such instruments since there are certain markings on the scale.
Basically, they indicate the values in whole numbers though one can get the readings up to one or two decimal places by the markings in instruments.
Even though we can measure up to two decimal places it will not accurate or entirely correct. We cannot get completely accurate value every-time in this type because some human error may occur.
Digital InstrumentsThe digital measuring instruments indicate the measured value in digital format which will be in number with its unit. It is very easy to read compared to analogue instruments.
Anyone can easily measure and note the measured value by these digital instruments because it will indicate the measured value in numerical form. They can give the readings in one or more decimal place.
There will be no human error in this type of instruments because it will indicate the value as measured from the function. So the measured value will be more accurate compared to the analogue instrument.
Functions of the Digital Measuring InstrumentsMost of the measuring instruments shows the value of parameter in the form of indicator movement, which gives us the magnitude of the quantity being measured.
Here are the some other functions performed by the instruments.
The instruments are calibrated against the standard values of the physical quantities. In this pointer movement directly indicates the magnitude of quantity which want to be measured, it can be whole numbers or also fractions.
Nowadays, the digital instruments are becoming popular, which indicate the values directly into numerical form which makes reading values very easy and also more accurate.
These measuring instruments are used as controller in many industrial applications. They do both measuring and controlling operations at a time in the applications.
For example, in an oven the ammeter will measure the current and it will cut the operations when the certain amps met as it is connected with certain controllers like PLC / PID . So in many applications this measuring instruments can measure the physical quantities and also controls the operations.
Some measuring instruments are used to store and record the data nowadays. In the age of technology storing the recorded data is very easy. There are number of instruments that are connected to the computers with the help of controllers.
As the pointer varies that will makes changes in the record also. For example, the ammeter in oven is connected with PLC for controlling the operations, PLC was interfaced with computer for analyzing.
So the current value will recorded in computer, all the accurate details will be recorded. Like time, date, amps all data will be stored and recorded.
The best use of recording is we can analyze all the historic records which will help to monitor the past operations, conditions and all trends.
These measuring instruments can also be used to transfer the data to some distant areas. In some applications the instruments are need to kept in hazard place or in place humans can’t able to withstand.
Like high temperature area and low atmospheric area, in that place’s instruments are connected by wires and their output can be taken to distant places where it is safe for user.
The signal obtained from the instruments can be used to display the measured value and also, we can control the operations even we can store the data.
Some measuring instruments can also carry out a number of calculations like addition, subtraction, multiplication, division etc., Some can also be used to find solutions to highly complex solutions by the value indicating on the instruments.
All other mathematical derivations were derived already for the required operations depends on the indicating variable the results obtained in the operation may vary.
Digital Electrical Measuring InstrumentsSome of the digital electrical measuring instruments are mentioned as follows.Instrument NamePurposeAmmeterMeasures CurrentCapacitance meterMeasures capacitance of a componentCos Phi meterMeasures the power factorPower meterMeasures the amount of energy dissipatedFrequency CounterMeasures the frequency of the currentLCR meterMeasures the inductance, capacitance, and resistance of a componentMicrowave power meterMeasures power at microwave frequenciesMultimeterGeneral Purpose instrument measures voltage, current, and resistanceNetwork AnalyzerMeasures network parameterOhmmeterMeasures resistance of a componentOscilloscopeDisplays waveform of a signalPsophometerMeasures AF signal level and noiseQ meterMeasures Q factor of the RF CircuitsSignal GeneratorGenerates signals for testing purposesSpectrum analyzerDisplay frequency spectrumTransistor TesterTests TransistorsWattmeterMeasures electrical energyVectorscopeDisplays the phase of the colors in color TVVoltmeterMeasures the potential difference between two points in a circuit
Nowadays all the sectors are using the digital instruments, the trend for the digital instruments were growing day by day to measure the physical quantities like weight, length, temperature, humidity, pressure, current, voltage & rpm.
Even for measuring the human heart beat rate, blood pressure, blood sugar level digital measuring instruments are mostly. Since these instruments has evolving there was certain advantage and disadvantage also.
Advantages of using Digital InstrumentsThe following are the advantages of digital measuring instruments.
The following are the disadvantages of digital measuring instruments.
Thus, advantage of digital instruments outdoes the disadvantages so they become highly popular nowadays. Digital instruments are found in all manufacturing field, quality field and these instruments also found in the cars, air planes, motor cycles.
The trend towards digital instruments were developing everyday because they are convenient to use and have visual appeal. In future their use is sure to increase much more.
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Digital meter measurements are displayed in discrete numbers. They are available to measure AC and DC voltages, AC and DC currents and resistance.
Most digital instruments are made around a digital voltmeter (DVM). Once a basic DVM circuit is built, then with proper transducers, a wide range of physical parameters such as current and resistance can be measured and displayed.
Construction of a digital voltmeter consist of three parts. The function of three blocks is discussed below.
The signal convertor block converts the measured function of voltage, current, resistance etc. into a form which the analog to digital convertor block can deal with. This form is typically 200 mVDc or 2 VDc.
The resistive divider chain attenuates the measured voltage value in switched steps, so that out put voltage is within the input range of analog to digital convertor. AC voltage measurement is usually done with the use of a circuit called a True RMS Convertor.
Switching between ranges, when a measurement is made usually a manual operation. but some modern digital millimeters have an auto-range facility, which electrically senses the level of the measured signal and selects the appropriate range for display.
The principle of conversion of analog signal into a digital signal is very easy. The analog signal is merely sampled, that is, a measurement of the signal value is taken at regular intervals and each sample is presented in a binary digital way. Two convertors are popular.
i. Dual Slop Convertor:
The input of the integrator is electronically switched between a reference voltage Vref and the voltage to be converted Vin. By integrating the analog voltage for a fixed period of time t1, as the integrator output voltage ramps up from zero then counting the clock pulses for the variable time t2 which the integrator takes for its output voltage to ramp back down to zero. In this way a very accurate analog to digital conversion ca be made because the time t2 is proportional to the analog voltage.
ii. Successive Approximation Convertor:
The successive approximation convertor is an example of an analog-to-digital convertor which uses a digital-to-analog convertor within itself. Digital output of the convertor is reconverted back to analog by the digital-to-analog convertor, then compared with the analog input voltage by a convertor. The voltage gets closer to that of output voltage with each comparison and approximation.
Mostly liquid crystal display (LCD) are used. They have a low current consumption which is only in microamperes. But large digital meters may use light emitting diodes (LED), vacuum fluorescent displays, etc.
A digital voltmeter (DVM) can be easily converted into a digital ohmmeter. One of the commonly used technique to measure DC resistance with a digital meter is to pass a constant current through the unknown resistance and measure the resulting voltage drop.
For example, if the basic DVM were placed on the 1-V range and a constant current surce of 1 mA were used, the meter would have a resistance range of 0 to 100V.
Current may be displayed on a digital meter by measuring the voltage which the current creates across a known internal resistance.
The digital multimeter (DMM) is a signal DVM circuit and the display shares between the three basic functions of voltmeter, ammeter and ohmmeter.
The actual circuits for measuring AC and DC voltage, AC and DC current and resistance are the same as that of digital voltmeter, ammeter and ohmmeter. They have a simple DVM section combined with suitable current-to-voltage and resistance-to-voltage transducer circuits.
1. Diode Test:
This feature is represented by a diode symbol. A good forward biased junction reads 0.7V while a good reversed-biased junction will read “OL” for overload. This is also used to check the base-emitter or base-collector junctions.
2. Touch Hold:
The feature permits the meter to capture and hold a reading so that user may remove leads from the circuit under test before reading the display.
3. Peak Hold:
This feature permits the meter to store and hold the peak measured value until cleared by the user.
4. Bar graph display:
Some digital meters also have a bar graph feature on the display giving the operator some of the benefits of the analog display.