The first thing you need to look at while buying a digital multimeter (DMM) is the environment in which you will be using it and the required safety ratings. DMMs are designed with different levels of protection, so ensure the one you have selected withstands the electrical hazards in your working environment.
Next, list down the features you really need and the ones you desire as a bonus. Think about whether you just need to take basic measurements, or you also need the more advanced troubleshooting options offered as special features. Selecting a device with more capabilities than you need means spending more money unnecessarily.
Last, you need to finalise the measurement ranges for each quantity and other technical specifications. The measurements need to be accurate for all engineering jobs as your analysis will depend entirely on the measured value. But, if due to lack of understanding of the technical specifications, you take the measurements with a wrong DMM, you can never get the correct measurements.
Safety is normally not given due consideration while selecting a DMM. Different DMMs are designed for different environments and they will not have the same level of protection against common electrical hazards. For this reason, one of the most important considerations in selecting a handheld DMM is thorough understanding of the test environment and test requirements.
Now the obvious question is: How do the measurement environments become different from one another? There are three key factors that differentiate these: voltage rating, transient voltage rating and energy capacity of the circuit.
Voltage rating. Different electrical equipment have different operating voltage ranges. For example, the supply voltage for equipment on trains in Indian railways is 110V DC but our home appliances work with 230V AC. So it is important to identify the maximum voltage at which the circuit is designed to work and choose a handheld DMM rated to measure that voltage.
Transient voltage. As distribution systems and loads become more complex, the possibility of transient over voltages increases. If you are taking measurements on electrical systems, these transients are invisible and largely unavoidable hazards. They occur regularly, can reach peak values in many thousands of volts and can last from 50 to 200 microseconds.
Transient voltages can result from natural causes, such as lightning, or can be generated by switching operations on the power distribution system.
Energy capacity. It is the most important consideration for safety in addition to the transient voltage. The energy capacity is defined by three characteristics: operating voltage, circuit impedance and the characteristics of the circuit fuse or circuit breaker.
It is important to find out the energy capacity of a particular circuit before you take any measurements. Otherwise, you might blow the DMM in your hands.
The International Electrotechnical Commission (IEC) has defined the ‘measurement categories’ for mains circuits. Categories are defined by probability of over-voltage. Higher the over-voltage, higher the category number, and the danger posed by transient voltages.
All manufacturers of handheld DMMs are required to mention the rated measurement category on their products. This marking is a convenient way for users to identify the maximum transient voltage that the device can safely withstand. Most handheld DMMs display this rating near the voltage/current input terminals. CAT III is the one you will generally find on the DMMs but, beware, some cheap Chinese multimeters write CAT III just like that, because they know what users are looking for. Table I shows the different categories and the related transient voltages.
All categories only apply to low voltage (CAT II-1000V is not safer than CAT III-600V instrument. They both provide protection up to the same level (6000V). But CAT III-1000V is safer than CAT III-600V. It is important to note here that all DMMs are only designed to measure nominal steady-state voltages continuously while also being able to withstand a momentary high transient. They cannot withstand a continuous over-voltage condition.
The DMMs have a variety of features and it is important that you first know what exactly they are. Other than the basic measurement functions, the features mentioned below can be useful but they come with additional cost.
Auto range with manual override. In manual range DMMs, you are required to set the range before taking any measurement, which is very tedious. There is also a possibility that you may not know the range every time. And if you take the measurement selecting a wrong range, you might not get the correct measurement, instead there is a high chance of the meter getting damaged.
Automatic touch hold. A very handy feature for taking measurements at places where both your hands are engaged and you cannot look at the display for the reading. With this feature, you can press the hold button and take the measurement without looking at the display. The measured value will stay there until you reset it. This feature should be automatic, which means that you do not have to press any button to freeze the reading.
Relative measurement. This is a highly useful feature to analyse relative changes in the measured value and/or compensate the losses in the probes.
Min/max mode. This is another essential feature for a DMM. You can hook up the DMM to a circuit and, using this feature, you can get the minimum and the maximum values it records during that period. This feature makes the measurement of min/max very easy as you do not need to continuously look at the display.
Duty cycle measurement. This is really not an essential feature as you will never measure duty cycle of the waveforms using a DMM. There are other equipment for such measurements such as an oscilloscope.
Voltage sense detector. The voltage sense, or the non-contact voltage detector, is an important feature for any person who is likely to come in contact with energised electrical components. For example, Vsense detects the presence of AC voltage in insulated wires, wall receptacles, fuses, junction boxes, switches and other voltage-carrying electrical systems without breaking into the power lines.
Display size, contrast and backlight. It is always better to go for bigger display for clear readings. Also, the contrast of the display should be good. Some cheap Chinese DMMs have very poor display contrast. Backlight comes as a useful feature while taking measurements in dark.
Capacitance, temperature and frequency measurement. Capacitance and temperature measurement are really useful features for a DMM. Though the capacitance measurement is not very accurate in DMMs, and you will need an LCR meter for it, but if you can get the capacitance measurement accuracy up to 2 per cent, you can get reasonably good measurement. These features are highly useful and will reduce the number of test equipment on your table. Frequency measurement is not a highly useful function as you already have oscilloscopes for that. but if you are getting it, look for the highest possible range.
Low-pass filter. The low-pass filter (LPF) function in a DMM is designed to help block unwanted voltages above 1kHz when measuring AC voltage or AC frequency. The LPF can improve measurement performance on composite sine waves that are typically generated by inverters and variable-frequency motor drives.
Data logging. This function is very good for field job where you would want to log your measurements and recall them back when needed. But buy the DMM with data logging facility only when you really need it as it will cause a big price difference.
Fast-response continuity latching. This is one of the features that you are going to use the most. It is useful for tracing tracks, finding shorts and completing hundreds of other tasks. A fast-response continuity latching is must for any DMM. No datasheet will mention it but you will need it, and the troubleshooting will be really annoying if you have a slow-response continuity feature in your DMM.
To test whether the DMM has a fast response, set the DMM in continuity mode and quickly touch the probes to each other. Next touch them very briefly, and you will see the difference between the good one and the average one. The tone should be solid and appear every time you touch the probes. With this function, you can find the shorts on a chip just by wiping the probe across the pins, which is impossible with slow response continuity DMMs.
Select the features that you need from the ones mentioned above to narrow down your choices. Then move to the third step and check various specifications in the datasheet and finalise the values suitable for you.
Range. This specification defines the minimum and maximum of the quantities that can be read without significant errors. This is the first specification that you need to look at while selecting a DMM. The minimum quantity is also critical in selecting the DMM. For example, if you want to work on the low-power circuits, you will have to read the currents in microamperes. So you would require a DMM that can read current values in microamperes too.
Do check the ranges of resistance, capacitance, frequency and temperature also. For regular electronics jobs, the resistance range should be at least 20 mega-ohms. In case of capacitance measurement, smaller the minimum range the better, as nobody will measure very high values of capacitance in electronics. Frequency measurement feature is a bit overrated as nobody will measure frequency using a DMM.
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