The Ham’s Toolbox- Part 1: Digital Multimeters- The Hidden Difference

In this multi-part series, we’re going to discuss what tools and gadgets we think are essential for the ham’s toolbox.  For my part, I’ve been a licensed ham and an electrical engineer for enough decades to have seen my share of the excellent, the decent, and the truly ugly when it comes to tools and equipment.  One of the very foundational items every ham needs is a multimeter.

This article will discuss Digital Multimeters (DMMs).   All DMMs measure voltage, resistance, and usually current but some have additional features like measurements of capacitance, frequency, temperature, and other parameters.  You can buy a DMM for less than $10.  You can buy one for $400.  They both fit in one hand, have test leads, and display some numbers.  Sometimes the cheap meters have more features than the expensive ones.  So, what’s the difference?

Let’s ignore the features for the moment and look at basic survivability, both the meter’s and your own.  Most any meter can measure a flashlight battery or your 13.8 volt power supply.  Even a cheesy meter can be used for resistance measurements and general puttering around the test bench.  That’s usually easy and fairly safe.  Things get more serious when you’re measuring electrical utility power sources such as standard 120 volt wall outlets, 240 volt lines in breaker panels, or even the main feeders before the big breaker in your home panel.

Properly-rated DMMs have Category numbers which are displayed as Roman numerals, I through IV (1 through 4).  There is an excellent write-up on meter safety including the rating system on the Fluke website (http://content.fluke.com/promotions/promo-dmm/0518-dmm-campaign/dmm/fluke_dmm-chfr/files/safetyguidelines.pdf).  In short, it’s all about the energy that’s available at a given point in an electrical system.

Consider these word pictures:

1. Here you are, sitting in your parked car, and along comes a bicyclist traveling at 15 mph. He’s headed straight for your passenger door.  Wham!  He piles into your door and leaves a big dent.  He’s not looking too good, either.  But, all in all, it could have been worse.
2. Now let’s make it worse. Let’s change the picture to have a small car aiming straight at your door, still going 15 mph.  This time you’re really paying attention.  The car whacks yours.  Everybody’s air bags detonate.  Your car is seriously damaged and you might be suffering from whiplash and you’re wondering which lawyer to call.
3. Let’s really make it worse this time. Your car is stalled on a railroad track (!) and here comes a freight train with a pair of lumbering locomotives leading 50 loaded coal cars.  It, too, is running 15 mph.  Sadly, you and your car are totally obliterated so you won’t be calling anybody.

What changes in each case?  The speeds are the same.  The available impact energies are vastly different.  On the end of an extension cord you can get spikes up to 6kv or 200 amps during a short. At the mains it can be 10,000 volts or 10,000 amps!

DMM category numbers are based on where you are in the electrical system and the voltage you’re measuring.  A cheap meter, if it’s rated at all, might be rated Category I, 250 volts.  Category 1 means you’re hiding behind your main circuit breaker and your branch breaker and many feet of small wire so they can’t convey huge amounts of current.  If a high-energy line spike strikes at the wrong moment when measuring your AC line, your wall outlets will arc over at 6000 volts which will limit the damage to your meter.  In that position you’re probably OK when measuring up to 250 volts unless the meter is total junk.  If your meter arcs internally, it probably won’t explode because the electrical system is energy-limited.  It may smoke and make bad smells.  Then you’ll go buy a better meter.  On the other extreme is the full unprotected mains to your breaker panel.  It is fed from a huge transformer through fat wires.  If a line spike causes your meter to arc and short with that combination there is little to prevent a giant blast of energy, thousands of amps, from instantly incinerating your meter and possibly blowing up in your face.  If your cheap meter has crummy test leads, they may act like fuses and burn off first.  The mains are where Category IV meters are required.  They’re equipped with big energy-limiting fuses and other safeguards to limit the destruction.  Unless you’re an electrician you probably have no business measuring unprotected mains anyway.

What’s a ham to do?

• Avoid cheap no-name meters from the far side of the world. If it costs less than a large pizza, leave it alone!  You can have cheap or you can have good.  Not both.
• Look for a name-branded meter that is clearly marked as CAT III (or CAT IV) 600V and has been independently tested by a recognized test lab such as UL, ETL, TUV, SGS, CSA, or others on the NRTL list. ( https://www.osha.gov/dts/otpca/nrtl/nrtllist.html )  Anybody can print Category numbers on their meter but a recognized laboratory makes it mean something.
• Another good “mark” is the CE mark. It designates that the meter has been tested to European standards and is legal to sell there IF it was tested in a real lab.  BUT, beware: anybody can apply a CE sticker to anything they like.  You can buy sheets of them for pennies in Hong Kong.
• What are “name brands”?
  1. Fluke: If you can afford one, it will likely outlive you if you’re not a complete savage. It is the standard of industry and the one everybody tries to imitate, including flocks of dangerous fakes from Asia.  I’ve had a pair of them for 35 years.  They’ve been around the world and on factory floors during service calls.  I have a 10-digit high precision HP benchtop meter but the Flukes are still my “go-to” meters for most needs.
  2. For great value in a “rated” meter that is much less expensive than Fluke, try Amprobe, Digi-Sense (from Cole-Parmer), B&K Precision, or Greenlee. They’re rated and they’re lab certified.  There are probably others.  Just look for the rating and the NRTL lab.

That’s enough for this episode.  Get a good meter. You’ll be glad you did.

73,

Chuck- NA3CW