Measuring Resistance
Digital Multimeter - Ohmmeter
An ohmmeter can be used to directly measure an unknown resistance R.
This is the simplest way to measure resistance. It is important to realise that an ohmmeter can not be used when the resistance is connected in a circuit and a current is flowing through it.
Typical Digital Multimeter
| Scale | Measurement Range | |
|---|---|---|
| in words | ohms(Ω) | |
| 2M | two megohm two million ohms | 0-2,000,000 |
| 200K | two hundred kilohm two hundred thousand ohms | 0-200,000 |
| 20K | twenty kilohm twenty thousand ohms | 0-20,000 |
| 2K | two kilohm two thousand ohms | 0-2,000 |
| 200 | two hundred ohm | 0-200 |
Setting up the meter
To use the multimeter as an ohmmeter turn the central selector dial to point to the ohms range. On some models (usually not on the cheaper ones) the meter will have a single ohms setting and will auto select the best range for you.
To measure resistance you must connect the test leads to the sockets labelled 'COM' and 'Ω'. These may be in different positions on your meter but the labelling should be the same.
Without anything connected between the test leads the display will give an out of range indication (-1 in this case) to show that the resistance is too large to measure. In effect, you are trying to measure the resistance of the air between the two inputs, which is very large.
Taking measurements
To check the meter is working touch the leads together and the display should read 0.0 Ω. It may read slightly higher, e.g. 0.3 Ω. This is the resistance of the test leads themselves.
Turn the selector dial to the lowest resistance range that gives a valid reading.
In this case the resistance wire has a resistance of 4.2 Ω.
To make a measurement the meter passes a small current, I, through the object being tested and measures the voltage, V. It then calculates the resistance from R=V/I.
Practical Notes
Accuracy
The overall accuracy of the reading will depend upon the multimeter model. The more expensive the meter the better the accuracy will be. For a cheap meter the accuracy is typically in the region of +/- 1% of the displayed value. Check in the manual for the accuracy of your model.
It is important to select the correct measurement range. In the above example if the 2 kΩ range is selected the display will read .004 kΩ and the decimal place will be lost. If the dial is set to 20 kΩ the display may read 0.00 kΩ! Always turn the dial down to the lowest range to get the best accuracy.
Measuring Low Resistance Values
A typical multimeter is not suitable for measuring very low resistance values. Below a few ohms the accuracy is not very good. Below one ohm the accuracy is very poor. To measure these low values it is much better to use the ammeter-voltmeter method. It is possible to get more accurate ohmmeters that will measure milliohms and even microhms but you are not likely to have these in a typical school laboratory.
External Links
Here are just a few links to web sites with good technical data about ohmmeters. You can find many more sites using a search engine and search words such as "ohmmeter", "micro-ohmmeter", etc.
Micro-ohmmeters
- Thurlby Thandar
- www.tti-test.com
- Tinsley Precision Instruments
- www.tinsley.co.uk
- Flite Electronics
- www.flite.co.uk
Multimeters
- Digital-Meters.Com - Test and Measurement Tools
- www.digital-meters.com
©8886 20-Feb-2004