Temperature measurement function in DMM is a handy feature. Normally, DMMs with this function are provided with a K-type thermocouple cable to measure temperature. Check the DMM for maximum range of temperature in the datasheet and see if it suitable for your requirement.
Another range that you need to check is the diode function range. Choose the ones with maximum range. For example, if the diode function range is only 2V, you will not be able to test an LED with more than 2V forward voltage drop.
Resolution. The resolution of a DMM is the smallest reading that can be shown on the display. Resolution of the DMM used to be described in digits (such as 4.5, 5.5, etc). But these days, the resolution is specified in counts. For example, let us look at 4.5 digits, or 4 and 1/2 digits DMM. The ‘4’ represents full digits, meaning digits that can take up values from 0 to 9 on the display. The ‘1/2’ or ‘0.5’ represents the first digit of the display and the maximum it can read. In this case, ‘1/2’ digit can represent digits 0 or 1. Therefore 4.5 digits mean that the display can show 00000 to 19999, which means 20,000 counts. To calculate the smallest reading that can be shown on the display, just divide the full-scale reading by the number of counts. For a 6000-count DMM with maximum DC voltage range as 600V, the resolution will be 600/6000 = 0.1V.
Accuracy. Though all the measurement features will have their corresponding accuracies mentioned in the datasheet, but first check basic DC voltage accuracy. Accuracy will be mentioned in percentage (such as 0.5 per cent). For regular electronics work, you do not need a DMM with very high accuracy (such as 0.05 per cent). You can do reasonably well with just 0.5 per cent accuracy. Accuracy in other measurement functions is also important, and it should be very close to basic DC voltage accuracy. Current accuracy is a different story as for current measurement a shunt resistor is used, which makes it difficult to achieve high accuracy.
Input impedance. Input impedance is an important specification. Buy the DMM that has input impedance as high as 10 mega-ohms. When you are measuring voltage across a component, you are effectively putting the input resistance of the DMM in parallel to the resistance of the component. Therefore small input impedance will change the values you are trying to measure.
True RMS. There are two types of DMMs available: ‘average-responding’ and ‘true RMS’ (root mean square). True RMS is the measurement of AC voltage or current that reflects the amount of power dissipated by a resistive load driven by the equivalent DC value. An average responding AC DMM is calibrated to read the same as a true RMS meter for sine wave inputs only. For other waveform shapes, an average-responding meter will exhibit substantial errors.
Updating speed. Display update rate is an important specification. General-purpose DMMs are available with update rate of 5 per second. Higher the update rate, the better.
Battery life. A good battery life will make your life easier with electronics. Some advanced DMMs are really power hungry devices. For a good general-purpose DMM, the battery life should be 300 to 500 hours.
Once you have understood the specifications mentioned above and finalised the specifications you need, you can go ahead and buy the DMM of your choice. The selected multimeter will provide you the measurement confidence together with safety.
The author is a technical editor at EFY
