Electricity and Control July 2021

MEASUREMENT + INSTRUMENTATION

Choosing the right tool for testing and troubleshooting A Fluke information note, supplied by Comtest The difference between an oscilloscope and a digital multimeter is simply stated as: ‘pictures versus numbers’. A digital multimeter is a tool designed for precise measurement of discrete signals, enabling readings of up to eight digits of resolution for the voltage, current, or resistance of a signal. An oscilloscope is designed to depict waveforms visually, to show signal strength, wave shape, and the value of a signal. How does the difference between numbers and pictures translate to real-world testing or troubleshooting?

Digital multimeters Handheld digital multimeters typically have 3.5 to 4.5 dig- its of resolution and good accuracy. They are portable and lightweight and are used typically for frontline testing and general purpose measurements. They also contain ad- vanced functions for special-purpose testing, such as fast - Min/max - Conductance - Relative reference - Duty cycle/pulse width - Logging. High-accuracy (5 to 8 digits resolution) bench-type line-powered digital multimeters not intended for field use are also available. These digital multimeters are used in the laboratory, mostly for research and development or for production systems. An advanced bench precision digital multimeter can cost as much as a portable oscilloscope. Oscilloscopes Oscilloscopes are designed for engineering work or trou- bleshooting systems which might contain complex signals that send at speeds much faster than a digital multimeter can capture. Scopes have much faster measurement en- gines and much wider measurement bandwidths than digi- tal multimeters, but typically do not have the same accura- cy and resolution strength as a multimeter. Oscilloscopes generally have a resolution equivalent to a 3.5- to 4-digit digital multimeter. One of the advantages of an oscilloscope over a multimeter is that an oscilloscope can also display complex signals visually; a tran- sient signal that may pose a threat to a sys- tem can be displayed, measured, and isolated with an oscilloscope. The oscilloscope will also graphically show distortion and noise that may be present in the signal. Oscilloscopes can be line-powered or battery- powered and they can be big or small. Battery power and smaller size, for portability, are typically required for field use. Some oscilloscopes have built-

in multimeters, like the Fluke 120B ScopeMeter, which provides the user with numbers as well as pictures. In many cases, this kind of oscilloscope can replace a multimeter.

Multimeters versus oscilloscopes

For plant engineers and maintenance technicians, Fluke advises as follows. Keep your digital multime- ter with you for any electri-

The Fluke 87V MAX Industrial is a heavy duty multimeter.

cal work. Use it to make high-precision checks of voltage, current, resistance, frequency, and other electrical param- eters. If the multimeter raises a red flag, you may need to use an oscilloscope or a more powerful tool to diagnose the problem further. Choose an oscilloscope if you want to make both quan- titative and qualitative measurements. For general purpose maintenance or general electronic testing, a digital multim- eter is fine, but when testing or troubleshooting machine controls or other complex systems, or doing electronic de- sign work, an oscilloscope is needed. In industrial electronic applications like automation and process control, an oscilloscope with two isolated inputs and 60 MHz, 100 MHz, or 200 MHz bandwidth is the choice. In industrial machine applications measuring three- phase power electronics, or in three-axis control systems comparing and contrasting multiple signals, an oscillo- scope with four isolated input channels and 100 MHz or 200 MHz bandwidth is ideal. For industrial network applications, some oscilloscopes add industrial network physical layer analogue measure- ment algorithms to validate the network health. □

The Fluke125B Industrial ScopeMeter is a portable oscilloscope with a built-in multimeter.

For more information visit: www.comtest.co.za

16 Electricity + Control JULY 2021