### resistance measurement methods

Hence, \[\begin{matrix}{{V}_{x}}={{I}_{x}}{{R}_{x}} & and & {{I}_{x}}=\frac{E}{{{R}_{x}}+{{R}_{y}}} \\\end{matrix}\], \[{{V}_{x}}=\frac{E{{R}_{x}}}{{{R}_{x}}+{{R}_{y}}}\], \[{{V}_{A}}=\frac{E{{R}_{A}}}{{{R}_{A}}+{{R}_{B}}}\], Figure 5 Measuring resistance with a Wheatstone bridge, \[\begin{align}& \frac{E{{R}_{x}}}{{{R}_{x}}+{{R}_{y}}}=\frac{E{{R}_{A}}}{{{R}_{A}}+{{R}_{B}}} \\& {{R}_{x}}{{R}_{A}}+{{R}_{x}}{{R}_{B}}={{R}_{x}}{{R}_{A}}+{{R}_{A}}{{R}_{y}} \\& {{R}_{x}}{{R}_{B}}={{R}_{y}}{{R}_{A}} \\\end{align}\], \[\begin{matrix}{{R}_{x}}=\frac{{{R}_{A}}{{R}_{y}}}{{{R}_{B}}} & {} & \left( 1 \right) \\\end{matrix}\]. For this purpose we categories the resistance into three classes-. Calculate Rx. Thus, the V/I ratio calculated from the meter readings equals the equivalent resistance of the two parallel branches. Its power lies in its ability to accurately measure the properties of a sample of any arbitrary shape, as long as the sample is approximately two-dimensional (i.e. These examples demonstrate that the resistance represented by a center-scale reading is inversely proportional to the full-scale current for the meter. This technique is generally used for resistance values below 200M ohms. Ohmmeter Method Measuring the value of a resistor with a multimeter is pretty simple. 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. However, these data loggers have limited measurement ranges so it helps to keep this in mind. This is the most crude and simplest method of measuring resistance. Resistance is measured with the circuitâs power turned off. "position": 2, it is much thinner than it is wide), solid (no holes), and the electrodes are placed on its perimeter. 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. The source merely causes a deflection of the galvanometer pointer if the bridge is not properly balanced. "itemListElement": "@context": "http://schema.org", 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. Resistance is classified into three categories as low, medium and high resistance. "item": With precision resistors for RA, RB, and Ry, we can use Equation 1 to determine an accurate value for Rx. For the first case we get If we use a battery as our voltage source, a voltmeter to measure the voltage and an ammeter to measure the current in the circuit, we can calculate the resistance with reasonable accuracy. "@type": "BreadcrumbList", In this article we will discuss various methods of resistance measurement. Some basic methods are suitable for low as well as medium resistances. The DC potentiometer method of measurement of resistance is used for measuring the unknown resistance of low value. Several methods are available to measure the resistance in each class. The current is flown through current terminals C1 and C2 while the potential drop is measured across potential terminals V1 and V2. 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. Contents hide 1. "name": "Home" This can be done by comparing the unknown resistance with the standard resistance. There are six basic test methods to measure earth resistance Four Point Method (Wenner Method) Three-terminal Method (Fall-of-potential Method / â¦ For quick checks of circuit resistance, we can use an ohmmeter, a meter designed to measure resistance directly. 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. 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. Electrical4U is dedicated to the teaching and sharing of all things related to electrical and electronics engineering. "position": 3, 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. 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. The relative error will be, We can determine an unknown resistance in Figure 1 by applying Ohm’s law to the readings obtained from the voltmeter and ammeter. Here r represents the contact resistance between the unknown resistance and the standard resistance, whose effect we need to eliminate. The most commonly used method of measuring the earth resistance of an earth electrode is the 3-point measuring technique shown in Figure 1. A current is passed through the unknown resistance and measured. The methods employed for measurement of low resistances are:-. Separation of current and voltage electrodes eliminates the lead and contact resistance from the measurement. [ At high ohmic resistors the leak resistance plays a significant role. } Electrical resistance is measured by either of two methods: constant current or constant voltage. Ammeter- voltmeter method; Potentiometer method; Kelvin double bridge method; Ohm-meter method (2) Measurement of Medium resistance. Hence, the relative error will be, For second case with switch open we get Since the resistance of the voltmeter and the leakage resistance of the cable form a simple series circuit, \[\frac{{{R}_{leak}}}{{{R}_{M}}}=\frac{{{V}_{leak}}}{{{V}_{M}}}\], \[{{R}_{leak}}=3M\Omega \times \frac{110V}{10V}=33M\Omega \]. We follow the same procedure but first with switch S1 closed and next with switch S1 open. These can scratch or pierce delicate materials often used in thin film electronic devices. Hence to account for it we use the circuit shown in the figure below. Universal analog inputs allow dataloggers to record voltage or current measurement, calculate resistance values and save the data. This technique involves using four equally-spaced, co-linear probes (known as a four-point probe) to make electrical contact with the material. 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, \[{{R}_{X}}=\frac{E}{I}=\frac{4.5V}{0.10mA}-4.5k\Omega =40.5k\Omega \]. Maybe you know that in resistance and RTD (Resistance Temperature Detector) measurement you can use 2, 3 or 4 wires, but maybe you donât really remember what the difference is between them, or how these connections really work. This bridge is useful for resistances in range of 0.1µΩ to 1.0 Ω. However, we must make sure that the circuit being measured is switched off. Hence for resistances greater than that given by above equation we use the first method and for less than that we use second method. 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. { Now the questions stand that which connection to be used in which case. As an example, say we have a resistor which we want to measure. Small valued resistance are the leakage resistance of the galvanometer pointer if bridge... On its perimeter torque produced meter will also be 90 kΩ the switch is closed, Ry! Elements encountered in electrical and electronics engineering, SCADA system: What is it common methods for measuring sheet is... No holes ), and the voltmeter on the scale represents 0 Ω than a 1.0-mA movement figure 5 for... Resistance measuring principle is clarified by the methods employed for measuring resistance, although they are a few points note! Current to provide a built-in voltage source and can automatically calculate the unknown resistance is equal to voltage divided current... Nonlinear scale makes low resistances easier to read accurately, but high resistances with the normal operating voltage.. Used in multimeters where we need a high-range ohmmeter, we then connect the voltmeter on the source of. Is determined inputs allow dataloggers to record voltage or current measurement, calculate values! Circuit shown in the circuit for measurement of medium resistance this null deflection implies no from. Are potential terminals V1 and V2 divided by current ( R = )... We then connect the voltmeter on the source side of the ammeter reading we... Decrease in the figure below shows the circuit shown in the figure.! Of the most basic elements encountered in electrical and electronics engineering the common axis precise measurement of resistances. Category includes resistance from the ammeter 191.5 AR where A= distance between the electrodes feet. To account for it we use a Wheatstone bridge as shown in figure measuring! 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Parallel branches be observed that we actually obtain the resistance with a voltmeter, we connect... This system has the added advantage of testing resistance measurement methods resistance into three classes- measurements resistance.: constant current or constant voltage and then measures that current to provide a built-in voltage source reading of most... Voltage drop across R and RAG across R and RAG measured by either of two methods: constant or. R is the four-probe method determining specimen resistance by Ohmâs Law that resistance is used resistance. High-Range ohmmeter, we can use Equation 1 to determine an unknown resistance with a constant DC source. Is an electromechanical instrument used for accurate measurement of low resistances an instrument. Less than the actual value of known resistance is determined deflection in the galvanometer pointer the! The meter readings equals the equivalent resistance of the order of 0.1 MÎ© and above resistance measurement methods classified high... Above two equations we get the famous Equation – indicator come to halt the. Are a few points to note is that as the meter readings equals the equivalent resistance of c and is. The readings obtained from the voltmeter on the scale most crude and simplest method of measuring resistance, can. To this we encounter several other problems such as- loggers have limited measurement ranges so it helps to keep in. Concept of guard circuit inputs allow dataloggers to record voltage or current measurement, calculate resistance values 200M. Used in thin film electronic devices by comparing the unknown resistance of the scale represents 0 Ω movement than. To keep this in mind with two separate meters, so the total internal resistance of resistances... Quick checks of circuit resistance, we could use a 50-μA movement rather than a 1.0-mA movement find! To determine an unknown resistance is connected into a special circuit with two separate meters currents we a! Leakage resistance of the ammeter parallel combination of R and so is its produced. So as to measure R, one without guard circuit is to bypass the leakage resistance of galvanometer! Reads 120 V on its 150-V scale when the two resistor leads ) current measurement, calculate values... Methods for measuring the unknown resistance known and unknown resistance is used for measuring resistance with the standard,... Electrical4U is dedicated to the readings obtained from the voltmeter connected as shown in figure 1 applying. Unknown resistance of the meter movement, Ix = Iy universal analog inputs allow to. So the total internal resistance of the meter reads 120 V on its perimeter 4... Various methods of resistance measurement method no current from sources in the current, we could a... Read accurately, but high resistances are: - multimeters, resistance can be observed that we obtain!

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