There are hundreds of companies around the world manufacturing digital multimeters. Each of them offers several models with slightly different specifications. If judged visually, they all look alike with similar features. It becomes almost impossible to know which one is better than the rest for your requirements. Some of the cheap Chinese products look exactly the same as high-end popular multimeters but lack the same design and safety features. Therefore careful evaluation is a must to choose the best multimeter for your application.
Broadly, there are four main criteria for assessing a digital multimeter: measurement confidence, measurement convenience, safety and construction.
For all engineering jobs like research, design and testing, measurements have to be correct because all your analysis will depend on the measured value. If a value is not measured correctly, you might end up with no or strange results. There is also a possibility that due to lack of understanding of the technical specification, you are taking these measurements with a wrong multimeter that does not support the range of measured quantity. So understanding technical specifications in the datasheet is a must to select a multimeter that will support the required range.
Technical specifications are not as complicated as they look. Here I explain these one by one:
Range. This specification defines the minimum and maximum of the quantities that can be measured without significant errors. This is the first thing that you need to check while selecting a multimeter. If you have to measure electrical lines up to 1000V but have a multimeter that can read a maximum of 600V, you will never get the results beyond 600V. If you are working on low-power circuits, you will have to read currents in microamperes. So you would require a multimeter that can read current in microamperes too.
Do check the range of other measurement functions like resistance, capacitance, frequency and temperature also. For regular electronics jobs, the resistance range should be at least 20 megaohms. In case of capacitance measurement, the smaller the minimum range, the better. As very high values of capacitance are not very relevant in electronics, focus on the minimum range here.
Frequency measurement is a little overrated feature as nobody will measure frequency using a multimeter. Also, the range of frequencies supported by a multimeter is very small (say, 100 kHz). You would need to measure much higher frequencies when working with electronic circuits and a multimeter cannot be used in that case.
Temperature measurement is a handy feature. Normally, multimeters with temperature measurement function are provided with a K-type thermocouple cable to measure temperature. Check the minimum and maximum range of temperature measurement in the datasheet and see if that meets your requirement.
Another range that you need to check is the diode function range. Choose a multimeter with a good 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. Resolution of a multimeter is the smallest reading that can be shown on the display. Earlier described in digits like 4½ and 5½, these days resolution is specified in counts.
To help you understand resolution, let me take the example of a 4.5-digit or 4½-digit multimeter. Here ‘4’ means four full digits that can take up values from ‘0’ to ‘9’ on the display. Fractional digit represents the maximum value that the first digit (most significant digit) can display. In this case, ‘1/2’ or ‘0.5’ means that the first digit can have values ‘0’ or ‘1.’ Therefore 4½-digit means that the display can show values from 00000 to 19999, which totals 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 multimeter with maximum DC voltage range of 600V, resolution will be 600/6000 = 0.1V.
Accuracy. Accuracies of all the measurement features are mentioned in the datasheet. First, check basic DC volts accuracy. Accuracy is given in percentage (like 0.5 per cent). For regular electronics work, you need not use a multimeter with a very high accuracy like 0.05 per cent. You can do reasonably well with just 0.5 per cent accuracy.
Accuracy of other measurement functions is also important and should be very close to basic DC volts accuracy. Current measurement accuracy will not be so much high as a shunt resistor is used to measure current.
Input impedance. Ensure that the multimeter you choose has an input impedance as high as 10 megaohms. When you measure voltage across a component, you are effectively putting the input resistance of the multimeter in parallel to the resistance of the component. Therefore a small input impedance will change the values you are trying to measure.
True RMS. Two types of multimeters are available: average-responding and true RMS (root mean square). True RMS is 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 multimeter is calibrated to read the same as a true-RMS meter for sinewave inputs only. For other waveform shapes, an average-responding meter will give substantial errors.
Updating speed. General-purpose multimeters are available with display update rate of five per second. The higher the update rate, the better.
Battery life. A good battery life will make your life easier with electronics. Some advanced multimeters are quite power-hungry. Buy them only if you really require their advanced features. It is recommended to have a general-purpose multimeter with a battery life of 300 to 500 hours.
Below-mentioned features can make measurements very convenient, which means a lot while struggling with electronics circuits:
Auto range with manual override. In a manual-range multimeter you have to set the range before taking a measurement, which is very tedious. Go for an auto-range multimeter if you are serious about your electronics project. Ensure manual override function too as you will need it sometimes.
Automatic touch hold. This feature is very handy when both your hands are engaged in taking the measurement and you cannot look at the display for the reading. With this feature you can press the hold button and take measurement without looking at the display. The measured value will stay there until you reset it. Make sure that this feature is automatic, which means that you don’t have to press any button to freeze the reading because you will not have a spare hand to press the hold button.
Relative measurement. You can use this feature to analyse relative changes in the measured value. You can also use it to compensate for losses in the probes: Short both the probes and keep the relative measurement button pressed until it shows ‘0.’ This is very useful in resistance and voltage measurements.
Min/max mode. Hook up the multimeter to a circuit and use min/max mode to get the minimum and maximum of the values it recorded during this time period. This feature makes measurement of minimum and maximum values very easy as you don’t have to continuously look at the reading display.
Duty-cycle measurement. This is not really an essential feature in a multimeter as you will never measure the duty cycle of waveforms using a multimeter. There are other equipment for such measurements, such as an oscilloscope.
Display size, contrast and backlight. It is always better to go for a bigger display so that the readings can be read easily from a distance. Also, the display contrast should be high. Cheap Chinese multimeters have a very poor display contrast. Backlight is useful for taking measurements in dark.
Capacitance, temperature and frequency measurements. Capacitance measurement with a multimeter is not very accurate and you will need an LCR meter for that. However, a multimeter with capacitance measurement accuracy of up to 2 per cent is reasonably good. Temperature measurement is a highly useful feature that will reduce the number of test equipment from your table. Frequency measurement is not that useful but if you are getting it, look for the highest possible range.
Low-pass filter. Low-pass filter in a multimeter helps to block unwanted voltages above 1 kHz when measuring AC voltage or AC frequency. It can improve measurement performance on composite sine waves that are typically generated by inverters and variable-frequency motor drives.
Data logging. Data logging is very useful for a field job where you would want to log your measurements and recall them when you need. However, buy a multimeter with data logging facility only if you really need it because 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. Continuity testing is useful for tracing tracks, finding shorts and hundreds of other tasks. Fast-response continuity latching is a must for any multimeter. No datasheet mentions it but you need it as troubleshooting with a multimeter having slow-response continuity will be really annoying.
To assess a multimeter for fast response, set it in continuity mode and touch its probes to each other very quickly. Now do it faster and see the response. The tone should be solid and appear every time you touch the probes.
With fast-response continuity function you can find shorts on a chip just by wiping the probe across the pins, which is impossible with slow-response continuity multimeters.
This is the most important criterion as it involves your safety. There are several norms that a multimeter should comply with in order to assure safety. Otherwise, your multimeter might blow in your hand some day.
CAT rating. All handheld multimeter manufacturers are required to mention on their products the rated measurement category (CAT II, CAT III or CAT IV). This marking is a convenient way for users to identify the maximum transient voltage that a multimeter can safely withstand. Most handheld digital multimeters have this rating marked near the voltage/current input terminals. Most multimeters are CAT III rated but beware of some cheap Chinese meters that have CAT III written on them just because users are looking for it.
CAT III. This rating applies to building circuit installations that are completely within the building, including parts of the service panel and branch circuits. It also applies to many of the building’s fixed equipment, which are connected directly to the building’s mains supply instead of being connected through cords and plugs.
Fuse rating. The datasheet of a cheap multimeter may mention that the multimeter complies with all the norms but it really may not. Check the fuse ratings for both current ranges. To be sure, open the cabinet and check the type and rating of the fuse yourself before directly measuring mains through it. It is highly dangerous. Never buy a cheap multimeter for mains measurements.
Shrouded input plug. Choose a multimeter with shrouded input plug so that you do not get a shock while doing mains measurement.
Good-quality probes. Probes should be of very good quality for high-voltage measurements. Some manufacturers do not properly insulate the probes. Measuring mains supply with such probes is very dangerous. Look for high-voltage silicon insulation.
Construction and build
This is the last evaluation criterion. It mainly depends on your work environment requirements. Though exact requirements will depend on your specific work environment, here are some basic things to look for:
Separate fuse and battery compartments. Not all multimeters have a separate fuse and battery compartment, but it will be really helpful if your multimeter features one. A separate compartment will allow easy removal for replacement of batteries and fuse. It will also mean no major damage in case the fuse blows.
Rugged input jacks. Input jacks should be rugged from inside because you will be taking out the probes and putting them back quite frequently. There is a high chance of the jack becoming loose, resulting in improper measurements.
Rugged build. For field use, it is good to have a multimeter with rugged build. It will also help the multimeter withstand any accidental fall. Normally, multimeters are provided with rubber covers that work as shock absorbers to help protect the multimeter from any damage.
The author is technical editor at EFY