0 Resistance measurement

Resistance measurement 

1. Preamble understand the opposition electric resistance of objects, materials or instruments to electrical current flow in conditions of continuous and steady flow, i.e., when element inside the electric field is no more than necessary to keep the flow constant current. Physical phenomena of electrical conduction are well known and the electric resistance value was defined quantitatively as the ratio of the potential difference applied between the ends of the element in question and the electric current intensity through it. value resistance, as is known, depends on the constitution of the conductive element, ie its resistivity and the physical dimensions of the former. Meanwhile, the resistivity is not invariable but depends strictly essential, though not exclusively, temperature, and sometimes in a not inconsiderable amount. Depending on the nature of the electrical conduction, the resistance value of a given element present physical constancy or more or less depending on overall stability both as test levels of external conditions such as humidity, the aforementioned temperature, and sometimes different intensities of electromagnetic radiation, so that it is common and necessary specified together with the ohmic value differential measurement conditions. experimental determinations of its value must be inexcusably DC if it is to make that determination with a medium-high accuracy. In measurements involving exclusively the instruments is permissible mean a degree of low frequency ripple in the voltage and / or current test. Sometimes, as in the case of electrolyte elements, it is recognized that the measurement is made ​​at low frequency alternating current to eliminate undesirable parameters introduce uncertainties greater extent than those caused by electromagnetic effects of variable fields. different procedures for measuring the electrical parameter we will consider three main groups:

Using the current and voltage values

The compensation

Those who directly provide value

2. Resistance measurement with voltmeter and ammeter is the procedure seems sensible given the definition of Ohm, although as will usually be the one that provides less accurate results. In it, the item under test is powered by a DC voltage source, stable and measured as simultaneously as possible and with appropriate instruments values ​​of potential difference and current. The resulting value of resistance is: R = U / I Importantly test values ​​should be such that no unacceptable disruptions or damage to the element whose resistance is to be measured. The test current should be such that no Joule heating occurs inadequate electrodynamic origin mechanical stresses that may cause damage or shock or if the test element is a coil or induction currents produces no magnetic field that can alter the characteristics ferromagnetic cores or elements affected. The accuracy of this procedure is directly linked to the accuracy with which the two electrical parameters measured. With medium quality instruments is not exceeded 0'l% uncertainty, and that once corrected for systematic errors inherent in the procedure. Accuracies between 0.5 and 5% can be regular so that the use of this procedure is rather restricted and reserved for special measurements can not be made ​​quickly by other procedures. However still interesting to study this process as it said, is not disposable alternative use when you have high quality instrumentation, or when the trial should be held in special conditions. 2.1. Connections short and long connection short long connection Figure 7.1. Measurement of connection resistance in short and long connection. the two connections shown in Figure 7.1 shows the two possibilities of connection topological voltmeter and ammeter. In the first, called short connection, the voltmeter is connected directly to the measuring element bomas and consequently fed to the corresponding voltage. However, the ammeter is placed in a branch that includes not only the intensity but also the unknown resistance consumed by the voltmeter indication which provides a proper top, and thus a smaller resistance value of apparent real. this fact in fact require the use of equipment or not to consume current Voltmetric such as electrostatic voltmeters or compensation procedures. Fortunately in most cases you can waive this requirement without sacrificing too much accuracy. In the second, called long connection, which is the ammeter is placed directly in the same branch of the element to be measured and thus sharing with it the same intensity, while the voltmeter is fed to the unknown voltage increased by the voltage drop that occurs in the intensity amperometric instrument. Like in the previous case can obviate the use of intensity measurement procedures without voltage drop today for today only be done by clearing procedures. To learn more about measuring resistance with Ohmmeter , visit the ohmmeters of AMPERIS Instrumentation.