Introduction to Digital Multimeters and Clampmeters

Welcome to an introduction to taking measurement with digital multimeters (DMMs) and Clampmeters

These first three sections will give you a solid foundation that underpins a wide variety of test instruments, not just DMMs.

By reviewing and understanding the concepts, you can quickly evaluate, choose, and then use your electrical/electronic tester of choice quickly and efficiently.

A quick review of these sections can help you: 

• Pick the best meter for your requirements.
• Get more out of your existing meter.
• Deep undertanding of what the meter is trying to tell you
• Be more efficient and faster at your job. 

This guide is separated into five sections.

Each section offers an overview, in some detail, of an important understanding to have about DMMs, Clampmeters, and the wider world of test equipment .

These concepts will be the basic foundation of knowledge from which we will expand into single/three phase power meters, industrial oscilloscopes, thermometers and thermal imaging, Insulation resistance testing…… and beyond.

All these tester types work off the same set of characteristics at their core. Knowing these characteristics will help you immensely.

Breaking this into five sections makes it more digestible, to avoid “death by PowerPoint”, causing fatigue and skipping over something that will be important later.

It also helps you skip around to the sections that are most important to you.

This material is a concatenation of many different “ABC’s of DMMs and Clampmeters” seminars and hand-outs conducted in industrial settings.

Visually Inspecting a Meter:

Determining what a particular meter can measure is fast if you understand the symbols on the dial or controls.

You can also compare different meters by placing them side-by-side.

Knowing what each of the functions measures is also important. 

Measurement function symbols are almost standardized across different manufacturers.

The basic Volts, Amps and Ohms are mostly the same, the main difference being the font used on the controls

Be sure the check out the back of a meter-there may be valuabe information there

Some meters have “Power On” settings that can be customized. These are activated by pressing a particular button while turn the meter on. 

That is it for this first section. The idea is for you to be able to quickly evaluate a meter or tester with a quick visual inspection.

Many more details are covered in the next four sections.

Electricity is dangerous  – always follow safe work practices.

The importance of safety and safe work practices cannot be over-emphasized in the potentially dangerous electrical work environment.

Always follow safe work practices -PPE

Be aware of the arc flash blast hazard location you are working in.

If an accident happens, you want to be protected and survive.

Your meter is part of your PPE.

There are safety standards that manufacturers must comply with to sell their products.

Don’t assume your meter meets these standards-check for embosed third-party symbols

International and local standards ensure that your meter or tester is built to recommended safety standards. 

IEC is the International Electrotek Commitee.

NFPA is the National Fire Protection Association.

The Safety Categories define locations in the electrical system, and are based on available Fault current. 

( how much current can flow if there is a short circuit)

• For example, if I short out an outlet, there is a 15 Amp circuit breaker that will interupt the fault current
• If I short out the electrical transformer feeding utility power to the building, there is no 15 amp breaker, and the fault current could exceed 1000’s of amps.
  • This can cause an Arc Flash Explosion

Safety ratings are based on hazard locations.

This diagram helps to visualize these hazard locations, and quickly determine an estimate of the protection you need in your meter.

It is very possible to have several categories present in the same plant floor or process control cabinet. Always default to the highest safety category.

The safety rating of your meter should be clearly marked.

If not clearly marked, the meter may not be safety rated.

Look for a third party certification of the safety rating. 

This helps you be confident that the meter will help protect you in case of an accident. 

Your test leads also need to be safety rated.

Like your meter, the safety rating should be clearly marked on each test lead.

• This makes them slightly more expensive, and worth it.

Test your test leads frequently-they can go bad over time

Especially if you measure :”0.000″ volts on a circuit you think is live

It is a simple procedure to ohm out the leads-takes about 30 seconds or less

Amps input jacks should be protected by fuses

It is a common to accidently measure voltage with the red lead in the Amps jack. 

• Fuse blows-you won’t even know it
• Switch the Red lead to the Volt/Ohm input jack and meter works fine on every function except Amps or milliAmps.
• Common condition when someone returns your meter after borrowing it.

​Unprotected meters can blow up if there is no fuse on the Amps jacks.

If you have an old meter, check for these fuses-they were not a standard in the “old” days

Meter fuses can be checked without having to open the case and break the calibration seal

Because the COM (common) jack is shared by all functions, internally it is the reference common to all functions, you can easily ohm out the fuses without opening the case.

• Plug a test lead into the Volt/Ohm jack
• Select the Ohms function
• poke the other end of the lead into the Amps jack
• A blown fuse will measure “OL” for overload, meaning the resistance is too high to measure (open fuse)
• A good fuse typically measures 0.1 ohms to 3 ohms in the Amps jack
• A good fuse in the miiliAmp jack may measure as high as 1000 ohms (special “slow blow” fuse)

The test lead is probing the top of the fuse, while the internal COM is acting like the black test lead internally.

No Fuses needed on the Volts functions

Old analog meters were not safety rated, and had a “ohms per volt” rating.

Digital meters have a high input resistance (DC volts) and high input impedance (AC volts).

Take the highest voltage rating for a meter and divide it by the input impedance specification to calculate the current flow through the test leads. 

1000V / 10Mohms =1000V / 10,000,000 ohms = .1mA or .0001 amps

Don’t have a manual?

• Use the same technique as on the Amps fuses, except measure between the Volt/Ohms and the COM terminal with a single test lead.

You Do need overload protection on the Ohms functions.

• The input resistance on the Volt/Ohm goes from high to LOW when the ohms function is selected.

Why? Thevenin’s Theorem

When analyzing circuits, current sources are drawn as short circuits

To measure ohms on a multimeter:

• a very small current is generated through the component (uAs or mAs)
• the resulting voltage is measured across the component
• and the resistance value is calculated by Ohms Law (R – V/I)

So, when the ohms function is selected, the meter effectively becomes a low resistance short between the test leads.

Accidentally measuring  Ohms across a live circuit is a common mistake. 

Quality meters have a thermal; or other type of protective device that opens the circuit if voltage is detected in Ohms mode.

It resets itself automaticallt when the voltage is removed

Unprotected meters can blow up.

A well-built meter should help protect you from common hazards when working with electricity.

Taking Digital Measurements in an Analog World

Physical analog parameters have infinite resolution, in that there is a smooth line of values without steps between these values.

“Digitizing” analog parameters slices them into steps, with the resolution limited by the size of the steps.

This section will help you understand the limitations of this process, and which specifications are important to evaluate how your meter handles these limitations.

Analog testers apply the analog value directly to the display.

Tik marks determine the measurement resolution, but you can always interpret values between the tik marks.

Digital Testers convert the analog value into a digital number.

This digital number is determined by the steps in the analog-to-digital converter chip, plus additional circuits inside the meter.

The “steps” are displayed as “counts” , which can be a “0” or a “1”

The number of counts specification is how many total measurements steps the meter can make.

The number of digits specification is how these steps are to be displayed a decimal , power of 10, numeric display.

The Digits verses Counts became varied over the years, so you realy have to take both into account to evaluate the measurement capabilities of a tester.

Electrical Testers may only have 1000-2000 counts, across 3 digits.

• This is fine, because speed , portability, convenience and other factors are more important than resolution on the measurement.

DMMs vary widely on digits and counts-knowing what you need and how to compare products is very important to choosing the right meter for your main uses.