This thermocouple effect will introduce errors into the measurement if steps are not taken to compensate and eliminate these thermal emfs. Cropico microhmmeters and milliohmmeters eliminate this effect by offering an automatic average mode for the measurement, sometimes called the switched DC or average method.
A measurement is made with the current flowing in the forward direction then a second measurement is made with the current in the reverse direction. The value displayed is the average of these two measurements. Any thermal emf in the measuring system will add to the first measurement and be subtracted from the second; the resulting average value displayed eliminates or cancels the thermal emf from the measurement.
This method gives the best results for resistive loads but is not suitable for inductive samples such as motor or transformer windings. In these cases the ohmmeter is likely to switch current direction before the inductance is fully saturated and the correct measured value will not be achieved.
Devices with a small mass or constructed with materials that have a high temperature coefficient, such as thin strands of copper wire, will need to be measured with the minimum current available to avoid heating.
In these cases a single pulse of current may be appropriate to cause the very minimum of heating. Should the DUT be subject to the influences of thermal emf then the switched current method described earlier is appropriate. Temperature influences It is important to be aware that the resistance of most materials will be affected by their temperature. It may be necessary, depending upon the accuracy of measurement required, to control the environment in which the measurement is made, thus keeping the ambient temperature constant.
For measurements where controlling the ambient temperature is not possible, the ATC automatic temperature compensation facility can be used. Two of the most common materials measured are copper and aluminium and their temperature coefficients are illustrated opposite.
When selecting the best instrument for your application the following should be taken into consideration Accuracy can be better described as the uncertainty of measurement, which is the closeness of the agreement between the result of a measured value and the true value. It is normally expressed in two parts i. The accuracy statement should include the temperature range applicable, plus the length of time the accuracy will remain within the indicated limits.
Warning: some manufacturers give a very high accuracy statement but this is valid only for a short period of 30 or 90 Days. All Cropico ohmmeters specify accuracy for a full 1 year. Resolution is the smallest increment that the measuring instrument will display. It should be noted that to achieve high measurement accuracy a suitably high resolution is needed, but a high resolution in itself does not indicate that the measurement has a high accuracy.
A measured value can also be displayed with a very high resolution but low accuracy i. The only meaningful digits would be 1. These fluctuations can be misleading and emphasising any instability of the DUT. A suitable resolution should be selected to ensure a comfortable reading of the display. This means that the maximum value that can be displayed is and the smallest resolution is 1 digit in You can therefore see that it is desirable to have a longer scale length than the traditional The Cropico ohmmeters offer scale lengths up to count, which would give a displayed value of 2.
Range Selection Range selection can be either manual or automatic. Whilst automatic range selection can be very useful when the value of Rx is unknown, the measurement takes longer as the instrument needs to find the correct range. For measurements on a number of similar samples, it is better to manually select the range.
In addition to this, the various instrument ranges will measure with different currents which may not be suitable for the device being tested. When measuring inductive samples, such as motors or transformers, the measured value rises as the inductance is saturated until the final value is reached. Automatic range selection should not be used in these applications, as by changing ranges the measuring current is interrupted and its magnitude may also be changed and a final steady reading is unlikely to be achieved.
Temperature coefficient The temperature coefficient of a measuring instrument is important as it can significantly affect the accuracy of the measurement. The temperature coefficient states how the measured accuracy is affected due to variations in ambient temperature. Current Magnitude and Mode Selecting an instrument with the appropriate measuring current for the application is important.
For example, if thin wires are to be measured, then a high measuring current would heat the wire and change its resistance value. Some applications, however, benefit from higher currents. The measurement current mode can also be important.
Again, when measuring thin wires, a short measurement pulse of current rather than using a continuous current, will minimise any heating effect. A switched DC measuring mode may also be appropriate to eliminate thermal emf errors, but for measuring motor windings or transformers, a current pulse or switched DC would be inappropriate.
Continuous current is required to saturate inductance giving the correct measured value. Automatic Temperature Compensation When measuring materials with a high temperature coefficient, such as copper, the resistance value will increase with temperature. This can be misleading when trying to compare the values for quality control purposes. To overcome this, some ohmmeters are provided with automatic temperature compensation ATC. Measurement speed The speed of measurement is not normally too important and most ohmmeters will measure at approximately 1 reading per second, but in automated processes such as component selection and production line testing, fast measuring speeds, up to 50 measurements per second, can be desirable.
You can also attach alligator clips to the multimeter probes to keep the terminals of the resistor or component in place while testing. Did you know you can get expert answers for this article? Unlock expert answers by supporting wikiHow. Bess Ruff, MA. Support wikiHow by unlocking this expert answer. Not Helpful 10 Helpful 5. What is the purpose of the multiplier and tolerance color bands of a carbon moulded resistor? The colored stripes define the resistance in ohms and its tolerance. With the metallic band on the right, the first and second bands define the first two digits of the resistance value, and the third band defines how many zeroes follow the second digit.
The colors and the values they represent are as follows: black 0 brown 1 red 2 orange 3 yellow 4 green 5 blue 6 violet 7 grey 8 white 9. Not Helpful 1 Helpful There are many historical units no longer in use. Ohms is just the SI unit, but everyone uses ohms for that reason. Not Helpful 5 Helpful 7. Black 1 , brown 10 , red , orange , yellow , green , blue , violet , gray , white Not Helpful 7 Helpful 3.
If your multimeter if that is what you are referring to is showing 0, turn the range down. If it is showing 1 or OL, turn the range up. Not Helpful 2 Helpful 3. Are there any analogue multimeters which use a potentiometer instead of a bank of resistance in measuring voltage?
Any ammeter, including a multimeter in a current range, has a certain resistance. Most multimeters inherently measure voltage, and pass a current to be measured through a shunt resistance, measuring the voltage developed across it.
The voltage drop is known as the burden voltage, specified in volts per ampere. Not Helpful 4 Helpful 1. Any 'component' has resistance. Variable or not, you need to know said 'component' can handle the voltage due to the current flowing through it. Refer to Ohm's law for reference. Not Helpful 4 Helpful 2.
Include your email address to get a message when this question is answered. By using this service, some information may be shared with YouTube. How accurate a multimeter is depends on the model. Low-end meters are usually accurate within 1 percent of the correct value. You can expect to pay more for a meter more accurate than this. Helpful 21 Not Helpful You can identify the level of resistance of a given resistor by the number and colors of the bands on it.
Some resistors use a 4-band system, while others use a 5-band system. One band is used to represent the level of precision. Helpful 18 Not Helpful Multimeter probe tips are often sharpened to a needle-point.
If you have to handle the probe tips, do so along the length of the tip to prevent stabbing yourself. Helpful 7 Not Helpful 2. Related wikiHows How to. How to. About This Article. Electrical resistance plays an extremely important role in the circuitry of electronic devices.
Such devices may malfunction if the resistance in their circuitry diverges from the proper level. However, electricity is not visible. A specialized measuring instrument is necessary in order to investigate whether a circuit has the proper resistance. An instrument such as a tester is necessary in order to measure resistance, but how is such measurement carried out?
This page provides a detailed introduction to how a tester or multimeter can be used to measure resistance. Resistance is measured using an instrument such as an analog multimeter or digital multimeter.
Both types of instrument can measure not only resistance, but also current, voltage, and other parameters, so they can be used in a variety of situations. Instead, resistance is calculated by measuring the current and voltage applied to the circuit.
When a current is applied to the circuit under measurement, the circuit resistance exhibits a voltage or more precisely, a voltage drop. In fact other forms of test equipment that measure resistance also use the same basic principle. The basic idea is that the multimeter places a voltage at the two probes and this will cause a current to flow in the item for which the resistance is being measured. By measuring the resistance it is possible to determine the resistance between the two probes of the multimeter, or other item of test equipment.
Analogue multimeters are good at measuring resistance, although they are a few points to note about the way in which it is done. The first point to note is that as the meter itself responds to current flowing through the component under test. A high resistance corresponds to a low current and the meter needle settles on on the left hand side of the dial, and a low resisatnce corresponds to a higher current and the meter needle deflects more so it appears on the right hand side of the dial as shown below.
It will also be noticed that the calibrations become much closer together as the resistance becomes higher, i. Another aspect of using an analogue multimeter for measuring resistance is that the meter needs to be "zero'ed" before making a measurement. This is done by connecting the two probes together so that there is a short circuit, and then using the "zero" control to give full scale deflection on the meter, i.
Each time the range is changed, the meter needs to be zero'ed as the position may change from one range to the next. The meter needs to be zero'ed because the full scale deflection will change according to aspects such as the state of the battery.
There are a few simple steps required to make a resistance measurement with an analogue multimeter:. Analogue multimeters are ideal pieces of test equipment for measuring resistance. They are relatively cheap and they offer a reasonably good level of accuracy and general performance. They normally provide a level of accuracy that is more than sufficient for most jobs.
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