Table of Contents
- Things to Consider
- Multimeter Basics
- Advanced Features
- Multimeter Comparison
- Additional Information
Handheld digital multimeters (DMMs) are basic devices for most of an electricians work. There are used for testing, troubleshooting, repairing and installation of electrical equipment. Because of the wide variety of multimeters on the market it’s hard to decide which multimeter is the right one for you. It depends on your application needs, your budget and how frequent you are going to use it. Selecting one with a lot of features that you don’t require in the end can mean you spend a lot more than you have to. On the other hand if you choose a device that’s not suited for your needs or is under-eqipped for your applications it can lead to dangerous situations and even expose you to life-threatening electrical hazards.
This guide is there to assist you in choosing the right device for your needs and applications by looking at the key attributes of digital multimeters and features targeted at specific applications.
There is also a buyers guide in video form that I can recommend to watch in addition when choosing a multimeter:
2. Things to Consider
Resolution and Digits
Resolution is defined as the smallest change in an input signal that produces a change in the output signal. The resolution of a digital multimeter is expressed by the number of digits it can display. A 3½ DMM has 3 full digits that can display values from 0 to 9, and one that can display either 0 or 1. The resolution is then ±1,999 because it can display positive and negative numbers up to 1,999. Most meters on the market are either 3½ or 4½ digit meter with a display range of ±1,999 or ±19,999. Sometimes the resolution is specified as „count“. The count is just the number that a meter can display before it has to change measurement ranges. A 4½ digit DMM can als be specified as 19,999 or 20,000 display count multimeter.
The accuracy of a multimeter is the maximum allowed error limit for readings. Usually the accuracy is expressed as ±(% of reading + number of least
significant digit (LSD). It shows the accuracy specifications as the maximum limit around the true value of the signal that is measured. The LSD represents errors due to rounding errors, offset noise and analaog digital converter (ADC) tolerances.
If you have a 4½ digit multimeter with a DC Voltage accuracy ±(0.1%+1)) while measuring a 12 V VDC output, the meter is expected to show something between 11,880 V and 12,120 V. When you take the LSD count into consideration the last digit can vary by ±1. The meter will certainly display a value between 11,879 V and 12,121 V or 12±0.121 V.
True RMS Converter
There are basically two types of multimeters available. „average-responding“ and „true RMS“ The true RMS values are not just the average of the absolute value of the curve, this only works with sinusoidal waveforms but not for more complex nonlinear waveforms. The value is proportional to the root mean square (RMS) which can be quite a complex calculation, usually only meters in the price range above $100 are coming with that feature. An average-responding AC multimeter will only for sine wave inputs display the same values. For other forms of wave shapes there will be large differences. If you want to measure non-linear loads such as found at electronic controls you want to have that feature.
Compared to the impedance of most circuits the input impedance of multimeters is very high. In this case the measurement of the multimeter will not affect the circuit’s operation and influence the readings negatively. Usually the input impedance of DMMs is larger than 1 MΩ. If you want to measure highly sensitive electronics it’s important to choose a meter with high input impedance to ensure the accuracy of the measurement.
3. Multimeter Basics
Basic multimeter can measure AC/DC voltage, AC/DC current, resistance, continuity and diodes. Some more advanced multimeters can also measure capacitance, frequency, temperature or pressure. You have to consider which kinds of measurement you need in order to choose an appropriate device.
AC/DC Voltage Measurement
It’s important to choose a multimeter with a higher main voltage rating than is used to operate the houshold and industrial electrical appliances you want to measure. Most houshold voltage ratings in the world are around 10% of 230 V. However, Japan, North America and some parts of South America use voltage ratings between 100 and 127 V.
AC/DC Current Measurement
For measuring current the same applies. This is also an important safety issue as too high current can lead to serious electrical hazards, especially when using cheap multimeters. Never use your multimeter above the range it is designed for. With cheap multimeter it’s often not even safe to use it at the high-end of the range for longer than a few seconds.
Resistance and Continuity Measurement
One of the basic features of a multimeter is to measure resistance and continuity, which is measuring the resistance between two points. Resistance is measured by passing current through the component being measured. In order to do this, the component has to be removed from the circuit. Open circuits have an infinite resistance, closed circuits will have a small resistance, however it’s very small. Most multimeter come with a beeper that indicates this small resistance measurement through an emitted tone. Some als have a visual alert, like a red light. It’s important that the tone is good to hear and doesn’t delay. You want an immediate response with a clear tone.
Most of todays digital multimeters are equipped with a diode check function that shows the forward voltage drop of the diode.
Some DMMs are able to measure capacitance. However, they use a different way to obtain this measurement than a LCR meter. The LCR meter will test the capacitor with an AC signal of a known frequency. This technique is more accurate than the one used by a handheld multimeter which observes the capacitance through computing the rate of change on the voltage accross the capacitor. However this method isn’t accurate when measuring capacitors with traits like dielectric absorbtion, leakage, dissipation factor or equivalent series resistance. This can cause substantial error readings and if accuracy is from high importance a LCR meter should be used instead.
If your device relies on a steady AC voltage and current than it’s important that it maintains the right frequency. With the frequency measurement you can monitor realt-time voltage or current within a specific frequency, duty cycle or pulse width. Not every meter comes ith this function, though.
Some multimeters have the ability to measure temperature. With a dual and differential temperature function it’s possible to measure two temperatures at the same time and calculate the difference. This is often more convenient than a single temperature measurement and if you use this feature frequently you should get one with this capabilities.
4. Advanced Features
Besides the basic measurement functions, some meters have advanced functions designed for special applications and needs of electricians and technicians.
Some more advanced multimeter come with a graphical display that can display AC measurements in real-time. Especially if you look to find irregularities this will be a very useful feature. If you often work with nonlinear AC waveforms it’s worth the extra money to invest in a graphical multimeter.
Data logging allows you to monitor and document the measured data over a longer time frame. Data logger software can collect data over an extended duration and show general trends over that duration. The data can then be used for further analysis. It’s a much needed feature for the installation and repair of systems such as heating, air-conditioning and ventilation.
Low Pass Filter
This is a feature you’ll need when troubleshooting motor drives. The output from a variable frequency drive (VFD) sends a pulse width modulated, non-sinusoidal voltage to the motor terminals. Many true RMS meters have problems showing a similar result than the drive’s controller display, because they have a wide bandwith. The Low Pass Filter is designed to block unwanted voltages above 1 kHz when measuring AC voltage or AC frequency. It will improve accuracy on composite sine waves, typically found at variable frequency motor drives and inverters.
Low Impedance Mode
The Low Impedance Mode is designed to prevent ghost or induced voltages from the measurement. This voltages can occur from capacitive coupling between energized wires. The Low Impedance Mode leads to more accurate measurements where the presence of ghost voltages are suspected.
Smart ohm or offset compensation is a feature such as the Low Impedance Mode that helps to achieve greater accuracy in certain situations. It works by testing resistance with two different test currents applied to determine if there are any offset voltages in the input circuit. In this way it removes unexpected DC voltages within the circuit displaying a corrected measurement and the bias voltage or leakage current.
In this video you can see what happens to cheap meters that are exposed to high current (20A):
Many people don’t give much thought to safety when buying a multimeter. However there are large differences in how good DMMs are protected against common electrical hazards. It’s very important to consider the environment and applications you will use in order to choose an appropriate device. In the above example if you would have done the same with a Fluke 87V the first 30 seconds no damage at all would happen and if you sustain the high current for longer the internal fuse would break. However, there would be no physical damage to the device with possible injuries or life-threatening contact to the electric energy while you hold it in your hand.
To protect yourself you should always know the energy capacity of the circuit you’re working with before you start to measure. Circuits with a high energy capacity will deliver more current and energy into faults compared to circuits with lower energy capacity. Measurements and tests performed on high-energy circuits can be a high safety-risk if not perfomed with a DMM designed for such circuits. There are four categories defined by the International Electrotechnical Commisssion (IEC) that are all manufacturers of handeld DMMs are required their products to mark with (CAT I,CAT II, CAT III and CAT IV). With this marking one can easily identify what’s the maximum transient voltage that the meter can safely withstand. Handheld Multimeters should always be fall in the category CAT III or CAT IV).
The table below lists the voltage category ratings:
|Rated Voltage||CAT IV||CAT III||CAT II||CAT I|
|150 V||4,000 V||2,500 V||1,500 V||800 V|
|300 V||6,000 V||4,000 V||2,500 V||1,500 V|
|600 V||8,000 V||6,000 V||4,000 V||2,500 V|
|1000 V||12,000 V||8,000 V||6,000 V||4,000 V|
|Resistance||2 ohms||2 ohms||12 ohms||30 ohms|
There is a large difference between the main voltage ratings for houshold and commercial electrical appliances. It is important to choose at minimum a meter that is designed to measure the maximum voltage that can be present on the circuits you want to measure. In this regard you should identify the maximum voltage the circuits you’re working with are designed to work.
Transient voltages can occur from natural sources, such as lightning outside, or they can come from switch operations of the power distribution system. Transient voltages can vary from a few hundred volts to peaks of several thousand volt peaks. They occur randomly and usually last between 50 and 250 microseconds.
Responsible manufacturers of handheld multimeters like Fluke, obtain safety certifications from independent third-party testing agencies. This safety certificates are an important sign of the safety of a specific device.
The safety marks can usually be found at the back of the meter. Multimeter probes should also be marked with a logo of a third-party safety-testing agency.
You should find some of the following marks on your multimeter:
6. Multimeter Comparison
|Name||List Price||Counts||Accuracy (VDC)||True RMS||Other Features|
|Fluke 87V||$399.95||20,000||0.05% + 1||yes||Thermometer, Lifetime warranty|
|Fluke 115||$159.95||6,000||0,5% + 2||yes||–|
|Fluke 289||$576.95||50,000||0.025%||yes||Graphical display, Data logging, Thermometer, Lifetime warranty|
|Fluke 177||$269.95||6,000||0.09% + 2||yes||Lifetime warranty|
|Fluke 113||$129.95||6,000||4.0% + 3||yes||Low Impedance Mode|
|Fluke 233||$311.95||6,000||0.25 % + 2||yes||Thermometer, Removable display|
7. Additional Information
In this DMM selection guide by Fluke you can also get a general overview of their products and which one you should get for specific jobs.