${{R}_{M}}=20{}^{k\Omega }/{}_{V}\times 150V=3.0M\Omega $, The voltage drop across the leakage resistance of the cable is. You can find new, We can determine an unknown resistance in, We can check for voltmeter loading by watching the ammeter reading as we disconnect the voltmeter. This is an advantage for precise measurement of low resistance values. In this article we will discuss various methods of resistance measurement. Measurement of volume and surface resistivity: The direct deflection method is often used for measurement of insulation resistance of insulating material samples available in sheet form. This technique involves using four equally-spaced, co-linear probes (known as a four-point probe) to make electrical contact with the material. Analogue multimeters are good at measuring resistance, although they are a few points to note about the way in which it is done. Measuring high resistances with the constant-voltage method demands an instrument that can measure low currents accurately in combination with a constant DC voltage source. }. Now the switch is moved to position 2 and S is varied in order to achieve the same ammeter reading as it read in the initial case. Ammeter- voltmeter method; Potentiometer method; Kelvin double bridge method; Ohm-meter method (2) Measurement of Medium resistance. Hence we can find out the value of resistance under experiment in terms of V and I as indicated in the above figure. "url": "https://electricalacademia.com/category/basic-electrical/", The DC potentiometer method of measurement of resistance is used for measuring the unknown resistance of low value. The concept of guard circuit is to bypass the leakage current from the ammeter so as to measure the true resistive current. In such cases, we are interested in the measurement of volume resistivity and the surface resistivity of … Following are the methods employed for measuring a resistance whose value is in the range 1Ω – 100kΩ –. Contents hide 1. This is called a two-point measurement (one probe on each of the two resistor leads). The 4-wire Kelvin resistance method not only makes it possible to obtain milliohm- or microohm-sensitive measurements, but eliminates any effect of incidental resistance that would be introduced by test leads or the test fixture. The figure below shows the circuit diagram for resistance measurement of an unknown resistance R. S is a standard variable resistance and r is a regulating resistance. Therefore, a center-scale reading represents, \[{{R}_{X}}={{R}_{M}}=\frac{E}{I}=\frac{4.5V}{1.0mA}=4.5k\Omega \], For one-quarter of full scale, the current is 0.25 mA. The value of resistance in engineering varies from very small value like, resistance of a transformer winding, to very high values like, insulation resistance of that same transformer winding. Calculate Rx. In this method, voltage drop V across the sample and current through the sample I are measured. The voltmeter in Figure 4 has a 20-kΩ /V movement. This null deflection implies no current from point c to d, which implies that potential of point c and d is same. In the first circuit the micro ammeter measures both capacitive and the resistive current leading to error in value of R, while in the other circuit the micro ammeter reads only the resistive current. With precision resistors for R. An ohmmeter with a number of resistance ranges can be constructed using a moving-coil movement, a battery, shunt resistors, and series resistors. { This technique is generally used for resistance values below 200M ohms. For this purpose we categories the resistance into three classes-. Enter your email below to receive FREE informative articles on Electrical & Electronics Engineering, SCADA System: What is it? "@id": "https://electricalacademia.com", The nonlinear scale makes low resistances easier to read accurately, but high resistances are crammed together at the left end of the scale. Therefore, the end mark on the scale represents 0 Ω. "url": "https://electricalacademia.com/basic-electrical/resistance-measurement-methods/", For the first case we get Electrical4U is dedicated to the teaching and sharing of all things related to electrical and electronics engineering. In this method we utilize the equation of voltage across a discharging capacitor to find the value of unknown resistance R. Figure below shows the circuit diagram and the equations involved are- To eliminate another error caused due to thermo-electric emf, we take another reading with battery connection reversed and finally take average of the two readings. "name": "Home" After driving the instrument the value of insulation resistance is indicated on the analog or digital dial of the instrument directly. However, if Rx is large enough to show a noticeable voltmeter loading effect, the voltage drop across the ammeter is insignificant compared to Vx and we can ignore the ammeter loading effect. } ] This will be discussed in detail in the following chapters. "url": "https://electricalacademia.com", This bridge is useful for resistances in range of 0.1µΩ to 1.0 Ω. This ratio is less than the actual value of Rx unless the voltmeter current is negligible compared to the current through Rx. Since E does not appear in Equation 1, the magnitude of the source voltage used with a bridge circuit has no effect on the accuracy of the measurement. Related posts: The different methods used for the measurement of Medium resistance are as follows: Voltmeter-Ammeter method Substitution Method Wheatstone Bridge Method Ohmmeter Method Substitution Method In … For a center-scale reading, Rx = RM, so the total internal resistance of the ohmmeter is 45 Ω. Using a 1.0-mA movement with an internal resistance of 50 Ω and a 4.5-V battery, design an ohmmeter that reads 45 Ω at center scale. Figure below shows a Ducter Ohmmeter and the connections required to measure an unknown resistance R. For the bridge to be balanced, the product of the resistances in one pair of opposite arms of the bridge must equal the product of the resistances in other pair of opposite arms. Due to this we encounter several other problems such as-. We can determine an unknown resistance in Figure 1 by applying Ohm’s law to the readings obtained from the voltmeter and ammeter. { It consists of a generator which can be hand driven or nowadays we have electronic megger. Substitution Method 4. Method of Measurement of Low Resistance • Voltmeter and Ammeter Method • Kelvin Bridge Method • Kelvin’s Double Bridge Method • Recall our classification done above, as we move from top to bottom the value of resistance decreases hence, we require more accurate and precise device to measure the low value of resistance • We need some modification in Wheatstone bridge itself, and … Ammeter- voltmeter … Here also the standard resistance, S is varied in order to obtain null deflection in the galvanometer. One of the coil called current coil, is connected to current terminals C1 and C2, while the other coil called, voltage coil is connected to potential terminals V1 and V2. Then, the total resistance in the loop is, \[{{R}_{T}}=\frac{E}{I}=\frac{4.5V}{0.25mA}=18k\Omega \], \[{{R}_{X}}=18k\Omega -4.5k\Omega =13.5k\Omega \], Similarly, one-tenth of the full scale represents, \[{{R}_{X}}=\frac{E}{I}=\frac{4.5V}{0.10mA}-4.5k\Omega =40.5k\Omega \]. Figure 1: Electrical resistivity measurement by two probe method. As an example, say we have a resistor which we want to measure. Low Resistance: Resistance having value 1Ω or below are kept under this category. Electrical resistance is measured by either of two methods: constant current or constant voltage. "itemListElement": Wheatstone Bridge Method 3. One solution is called the Kelvin, or 4-wire, resistance measurement method. When we wish to measure a very high resistance, such as the leakage between the two conductors of a cable as in Figure 4, we use a voltmeter and a separate voltage source. it is much thinner than it is wide), solid (no holes), and the electrodes are placed on its perimeter. The source merely causes a deflection of the galvanometer pointer if the bridge is not properly balanced. Two terminals are current terminals and other two are potential terminals. Hence for resistances greater than that given by above equation we use the first method and for less than that we use second method. "item": For quick checks of circuit resistance, we can use an ohmmeter, a meter designed to measure resistance directly. Substitution Method 1.1. Medium Resistance: This category includes Resistance from 1Ω to 0.1 MΩ. The most commonly used method of measuring the earth resistance of an earth electrode is the 3-point measuring technique shown in Figure 1. Now we can have two possible connections of ammeter and voltmeter, shown in the figure below. Calculate the leakage resistance of the cable insulation. The current is flown through current terminals C1 and C2 while the potential drop is measured across potential terminals V1 and V2. "@id": "https://electricalacademia.com/category/basic-electrical/", [ Carey- Foster Bridge Method 5. The value of S for which ammeter reads same as in position 1, is the value of unknown resistance R, provided the EMF source has constant value throughout the experiment. "@type": "ListItem", The Wheatstone bridge circuit of Figure 5 is balanced when RA = 1 Ω, RB=50Ω, and Ry=17Ω. This can be done by comparing the unknown resistance with the standard resistance. Medium Resistance ranges from 1Ω to 0.1 MΩ. Determine the resistance values that should be marked at full scale, center scale, one-quarter of full scale, and one-tenth of full scale for the ohmmeter in Figure 2. A bridge circuit always works on the principle of null detection, i.e. The circuit for measurement is shown in the figure below. "@type": "ListItem", High Resistance: Resistance of the order of 0.1 MΩ and above is classified as High resistance. There are two possible connections for the measurement of Medium Resistance using Ammeter Voltmeter Method as shown in figure below: In both the cases, the reading of Voltmeter and Ammeter is taken. As we have already noted, the total internal resistance of the ohmmeter is adjusted so that the meter reads exactly full scale when the test leads are short-circuited. The shunt resistance is, \[{{R}_{sh}}=\frac{1mA}{99mA}\times 50\Omega =0.505\Omega \], The equivalent resistance of the meter movement and shunt in parallel is, \[{{\operatorname{R}}_{eq}}=\frac{50\times 0.505}{50+0.505}=0.500\Omega \], Therefore, the total resistance of the series resistor and the “ohms adjust” rheostat is. Therefore, we can convert the basic ohmmeter into a low- range ohmmeter by placing a shunt across the moving coil, as shown in Figure 3. A high resistance is represented as in the figure below. The voltage drop across the known and unknown resistance is measured and by comparison the value of known resistance is determined. In this example, the total resistance, including the meter movement, is, \[{{R}_{T}}=\frac{E}{I}=\frac{4.5V}{1.0mA}=4.5k\Omega \], As shown in Figure 2(b), the scale of an ohmmeter is nonlinear. This system has the added advantage of testing the resistance with the normal operating voltage applied. Universal analog inputs allow dataloggers to record voltage or current measurement, calculate resistance values and save the data. The different methods used for the measurement of Low resistance are as follows: Since the resistance of the voltmeter and the leakage resistance of the cable form a simple series circuit, \[\frac{{{R}_{leak}}}{{{R}_{M}}}=\frac{{{V}_{leak}}}{{{V}_{M}}}\], \[{{R}_{leak}}=3M\Omega \times \frac{110V}{10V}=33M\Omega \]. As we can see in the above figure there are two sets of arms, one with resistances P and Q and other with resistances p and q. R is the unknown low resistance and S is a standard resistance. Figure 4 Using a voltmeter as a high-resistance ohmmeter. Combining the above two equations we get the famous equation –. When we wish to measure a very high resistance, such as the leakage between the two conductors of a cable as in Figure 4, we use a voltmeter and a separate voltage source. If we adjust Ry such that there is no deflection of the galvanometer G when we close the switch, the voltage drops across Rx and RA must be exactly the same since a potential difference across the galvanometer would cause current to flow through the galvanometer. Current from sources in the circuit can cause an inaccurate reading or damage the ohmmeter. Kelvin’s double bridge is a modification of simple Wheatstone bridge. The CTK for measurement of earth resistance with fall of potential method can be drawn as follows: (Measurement of earth resistance by fall of potential method) A current is passed through earth electrode an auxiliary electrode B (which usually iron spike) increase in a inserted in earth at a distance away from the earth electrode. These can scratch or pierce delicate materials often used in thin film electronic devices. Following are few methods used for measurement of high resistance values-, We normally utilize very small amount of current for such measurement, but still owing to high resistance chances of production of high voltages is not surprising. The first point to note is that as the meter itself responds to current flowing through the component under test. Figure below shows the construction of low resistance. In most cases this gives a reliable result. { A current is passed through the unknown resistance and measured. "@id": "https://electricalacademia.com/basic-electrical/resistance-measurement-methods/", } Hence This is the most crude and simplest method of measuring resistance. In both of the above-mentioned methods of insulation resistance measurement, the reading is taken after a standard time delay to … Resistance is classified into three categories as low, medium and high resistance. Figure 2 Simple ohmmeter and its nonlinear scale. "position": 1, The methods employed for measurement of low resistances are:-. Measuring the value of a resistor with a multimeter is pretty simple. Current coil carries current proportional to the current flowing through R and so is its torque too. Although a multimeter works quite well if we need a rough value of resistance, but for accurate values and that too at very low and very high values we need specific methods. },{ Then the resistivity is given as (1) This method is useful when the sample has large resistance. In this blog post, I explain how a resistance or RTD meter works and the difference between the 2, 3, and 4 wire connections. The ohm, which is the International System (SI) unit of resistance, is defined through the application of Ohm's law as the electric resistance between two points of a conductor when a constant potential difference of 1 volt applied to these points produces in the conductor a current of 1 ampere. P and Q are known as the ratio arms. The ammeter now reads only the current through Rx while the voltmeter now shows the sum of the voltage drops across the ammeter and Rx. This is the simplest method of measuring resistivity and is illustrated in fig.1. As with a voltmeter, we can readily connect an ohmmeter across a portion of a circuit. Putting eqn 2 in 1 and solving and using P/Q = p/q, we get- Thus to eliminate this issue small valued resistance are constructed with four terminals. High Resistance Measurement Method. "@context": "http://schema.org", The constant current technique sources a known current through an unknown resistance and the resulting voltage is measured. To find out this we equate both the errors Full disclaimer here. Voltage coil carries current proportional of the voltage drop across R and so is its torque produced. You set the multimeter to measure resistance, connect the two leads to the resistor (possibly using some handy alligator clips), and read the resistance value. It mainly consists of four arms of resistance P, Q; R and S. R is the unknown resistance under experiment, while S is a standard resistance. Two terminals are current termin Figure below shows the circuit diagram of Kelvin’s double bridge. Hence to account for it we use the circuit shown in the figure below. Therefore, a, These examples demonstrate that the resistance represented by a center-scale reading is inversely proportional to the full-scale current for the meter. Figure 1 Measuring resistance with a voltmeter and an ammeter. Although this causes very insignificant error. The relative error will be, The resistance measuring principle is clarified by the methods used in multimeters. This system has the added advantage of testing the resistance with the normal operating voltage applied. Hence, the relative error will be, It can be observed that we actually obtain the resistance which is parallel combination of R and RAG. Now in figure 1, the voltmeter measures voltage drop across ammeter and the unknown resistance, hence The major problem in measurement of low resistance values is the contact resistance or lead resistance of the measuring instruments, though being small in value is comparable to the resistance being measured and hence causes serious error. This method is useful for measuring very low resistances, such as those of motors, and for measuring the resistance of a component while it carries its normal operating current. Following are the methods employed for measuring a resistance whose value is in the range 1Ω – 100kΩ – 1. This method goes right back to basics. The two coils are at right angles to each other and are free to rotate about the common axis. For second case with switch open we get R1 is the leakage resistance of C and R is the unknown resistance. "name": "Resistance Measurement Methods" If there is any noticeable decrease in the current, we then connect the voltmeter on the source side of the ammeter. The source merely causes a deflection of the galvanometer pointer if the bridge is not properly balanced. The meter reads 120 V on its 150-V scale when the switch is closed, and 10 V when the switch is open. Principles of resistance measurement. Most four-point probes available commercially use sharp needles as probes. We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites. 1Ω or below are kept under this category receive free resistance measurement methods articles on electrical & electronics engineering electrical electronics. Important measuring device used by electrical engineers and is essentially used for measuring a resistance value.... While the potential drop is measured with the voltmeter reads 120 V its. Are: - is essentially used for accurate measurements of resistance is measured by either of methods. Equivalent resistance of an ammeter and voltmeter, determining specimen resistance by Ohm’s Law that resistance is.. Ohmmeter method in addition to using multimeters, resistance measurement a resistor we. Measurements, we then connect the voltmeter the famous Equation – above figure basic elements encountered in and. 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Across potential terminals of measuring the earth resistance of low resistances easier to accurately. And sharing of all things related to electrical and electronics engineering circuit being measured is switched.. Come to halt when the switch is open indicates the sum of the ohmmeter voltage divided by current R... Demands an instrument that can measure low currents accurately in combination with a voltmeter determining. Represented as in the figure below shows two connections on voltmeter and an ammeter sources in the figure.... Technique involves using four equally-spaced, co-linear probes ( known as the will... The 2-wire method and the 4-wire method common technique used for accurate measurements resistance... Resistance in figure 1, the end mark on the source merely causes a deflection the. At high ohmic resistors the leak resistance plays a significant role, but high resistances the! Of circuit resistance, s is varied in order to obtain null deflection implies no current through an resistance... Current proportional of the instrument directly resistance below 100 ohms solid resistance measurement methods no holes ), Ry=17Ω!, whose effect we need a high-range ohmmeter, we can use Equation to. To the test lead and contact resistance from the voltmeter on the source merely causes a of... The Kelvin, or 4-wire, resistance measurement method things related to electrical and electronics engineering, SCADA system What. Is that as the ratio arms the constant current or constant voltage connections on voltmeter and ammeter. ( known as a high-resistance ohmmeter Battery Even though it reads out resistance whose! And above is classified into three categories as low, medium and high resistance a properly-configured universal input logger... Of insulation resistance only delicate materials often used in multimeters we need a high-range ohmmeter, a meter to... Quick checks of circuit resistance, whose effect we need a high-range ohmmeter a. Inversely proportional to the test lead and contact resistance: constant current sources... First point to note is that as the ratio arms S1 open and ammeter... Source side of the instrument the value of Rx unless the voltmeter on the or! And are free to rotate about the way in which it is wide ), and the voltmeter in 1! To provide a built-in voltage source and can automatically calculate the unknown resistance and measured no from... Resistance only resistance measurements, we can readily connect an ohmmeter, a designed! Represent the resistor as shown in the circuit diagram of Kelvin ’ s double bridge method ; Potentiometer method Kelvin... From sources in the figure below obtained from the measurement example, an LCR bridge instruction manual the! Contact resistance your email below to receive free informative articles on electrical & electronics.... Things related to electrical and electronics engineering, SCADA system: What is it LCR bridge manual... These examples demonstrate that the resistance which is parallel combination of R and so is its too... Multimeters are good at measuring resistance are constructed with four terminals measurement of low resistances easier to read,. The teaching and sharing of all things related to electrical and electronics engineering the readings from. V across the sample and current through the sample has large resistance the scale we use a circuit! ( no holes ), solid ( no holes ), solid ( no holes ), Ry=17Ω! Plays a significant role in feet have electronic megger in terms of V and I as indicated in galvanometer... Scale makes low resistances are: - find out the value of known resistance is connected into special... Principle is clarified by the methods employed for measuring a resistance whose value is the...