This article covers what you need to know to select a meter for DIY electronics. It focuses on the needs for audio electronics, but it’s purposely kept general in nature. Because of its focus on DIY electronics, I don’t talk about meters that are only considered “affordable” by those with the budget of a corporation or university.
Even the cheapest meter is useful for DIY electronics. There are myriad problems with cheap meters, but none of them actually prevent you from making use of the meter for DIY electronics. A cheap meter simply has limits on the questions it can answer about a circuit.
The flip side of that coin is that I’ve not yet found a price ceiling: you can spend insane amounts of money on a meter and find legitimate uses for it in DIY electronics.
No meter stays within its factory accuracy limits forever. To keep a meter within its factory accuracy limits, it must be calibrated occasionally, which costs around $50 for a handheld meter. On top of this, there’s the cost of the components to make a meter calibratable and the cost of the real-world infrastructure to do calibration. (Technicians, training programs, test equipment...) Together, all this means you don’t begin to find calibratable meters until about the $100 level.
Because of this, I think it’s foolish to buy a meter costing more than $50 but which isn’t calibratable. If you spend less than the service cost for a single calibration, you can afford to replace the meter every time it gets too inaccurate to use and still come out ahead. This only works if you can live with the cheap meter’s low accuracy to begin with, of course. It’s also a gamble. You’re betting that it will drift slowly enough that the cost of “calibration through replacement” is cheaper than the cost of a more expensive meter plus regular calibration. Unless you simply cannot afford an entry-level calibratable meter, it seems a bad gamble to me.
I have little to say on price when it comes to professional meters. (Meaning, those that can be calibrated.) It comes down to features and accuracy, the subject of the next section.
So, my simplest advice on price is to go cheap, or go pro. Stay out of the no-man’s land between these two. For a more nuanced picture, read on.
First you should decide if you want a digital meter, an analog meter, or both.
Analog meters are also called VOMs, after their measurement functions: voltmeter, ohmmeter, milliammeter. Digital meters go by the acronym DMM, for Digital Multi-Meter. The “multi” part, of course, refers to the fact that it is actually several meters in one. Way back at the dawn of electronics, the meters we now get in a single instrument were separate.
The primary advantage of a VOM over a DMM is that the analog needle readout makes it easier to see trends in a changing value. If you’ve ever driven a car with a digital speedometer, it’s the same problem: you can’t judge acceleration as easily as with a traditional speedometer. It’s worse in electronics, because unlike with the car, your physical senses usually don’t have anything else to pick up on to help interpret the digital display. Some DMMs have bar graph features that try to mimic VOM readouts graphically, but really, there’s no substitute for watching that needle swing.
There are several problems with VOMs, however.
First, a standard VOM has a far lower input impedance than any decent DMM. The impedance is a function of the selected voltage range, with the specification given as “sensitivity.” With a typical sensitivity spec of 20 kΩ per volt, choosing the 10 V range means you are effectively putting a 200 kΩ resistor across the circuit under test when measuring voltage. In many circuits, this will have no practical effect, but it’s something you have to watch out for. Most DMMs have a fixed input impedance of either 1 MΩ or 10 MΩ regardless of the selected range, so they’re effectively “invisible,” as far as most circuits are concerned. (Probing with a DMM can affect some circuits, but it is usually the lead capacitance or inductance that causes the problem, rather than the parallel resistance.)
Second, VOMs are, by definition, limited to volts, ohms, and amps readings. Most modern DMMs have additional features, covered below.
Finally, good VOMs are increasingly hard to find, and those you do find are more expensive than their feature set would otherwise indicate. This is a consequence of low sales volume, which follows from the above.
I greatly prefer DMMs. I also have an old professional VOM, which I rarely use. The VOM was given to me as a hand-me-down; if I had to go buy one, I might have bought a cheapie, given how little use it sees. By contrast, my digital meter is an expensive professional model which I use a lot.
Having got the DMM vs VOM question out of the way, you then need to decide what you want in the way of accuracy, functionality, calibration, and build quality.
Poor accuracy is not a fatal flaw in a meter for DIY electronics, but it does mean the meter can do little more than tell you whether a circuit is working or not. (And it may not even tell you that, if the circuit is malfunctioning in certain interesting ways.) If you need to discriminate between a circuit that’s working poorly from one that’s working well, you’ll need something with more accuracy than you get at the low end. For just one perspective on this, see my article Hand-Matching Resistors to Higher Tolerances: it has a good discussion about how the accuracy of a meter affects what questions you can reasonably expect the meter to answer.
If you decide that accuracy matters to you, I don’t think there’s any point in getting an uncalibratable meter. As I said above, it’s simply gambling to buy a meter based on accuracy specs if you can’t guarantee its accuracy through regular calibration.
Another reason to get a more expensive meter is to get more measurement functions. I’ve found uses for capacitance measurement, diode testing, temperature measurement, and frequency counting in DIY electronics.
Finally, there’s the matter of build quality. In a higher-quality meter, the fit and finish is better, the unit is physically more durable, the probes are insulated with silicone instead of PVC, the probe handles are bigger, the probe tips are sharper, the mode dial feels better while turning it, etc. I wouldn’t recommend buying a meter just to get these things unless you’re in the Mercedes set, but they’re certainly nice side benefits if you are after some of the other features you get in a high-quality meter.
Fluke is the leader in handheld DMMs. Period, end of sentence.
Agilent re-entered the handheld DMM market several years ago with some credible offerings, but it’s still a little early to be making decisions about longevity. Buying an Agilent product on name alone isn’t a good idea. Agilent has commissioned third parties to design and build some of its low-end products before, with mixed results.
At the low end of the professional handheld meter market, Fluke’s dominance isn’t nearly so certain. There’s plenty of competition at this level. I’ve had good experience with Wavetek/Meterman and B+K Precision in this range before. Meters by Extech and Protek sometimes get good reviews, but you rarely hear anyone actually rave about them. No other brands have any real traction at all. Many other big-name meters at this level are rebadged meters designed by other companies, usually Asian ODM houses you’ve never heard of.
In the used market, you will also find good meters from Beckman and HP, but only buy if you can get one for below market rates. (This pretty much rules out eBay and the test equipment resellers.) The problem here is that the technology gap has become too wide. HP and Beckman ceased making meters so long ago that only their very highest end meters are still competitive with a new low-end professional meter. Great bargains are possible, but unless you’re really knowledgeable about this, there’s too much chace you’ll get stuck with a poor value.
The benchtop meter market is basically the inverse of my summary of the handheld market above: Agilent is the undisputed leader, and Fluke is the new-comer. Like Ferris says, “The 34410A is so choice. If you have the means, I highly recommend picking one up.” Someone wrote to me and said he was talking about a Ferrari and not a DMM, but few know that Ferris went off to become an electrical engineer, and revised his famous quote in the way I’ve given it here.
It used to be that handheld meters were used only for field work, and that serious engineers preferred benchtop meters. With advances in technology, a high-end modern handheld meter will best older benchtop units, so for most DIYers, a handheld is the best option.
That having been said, if you are the sort who demands the very best, or simply one who has cash to burn, you should look into benchtop meters.
First, you must avoid the low end of the benchtop market. You can get better performance and reliability for less money at the top end of the handheld market. The benefits of benchtop meters begin with those costing around $800 new, and I personally would find it hard to justify the compromises in meters costing less than around $1000.
As you can see, there’s a pretty big gap in price between the most expensive handheld meters and the lowest end benchtop meters I recommend. You can split that difference by shopping in the used market. Be warned, though: it’s rare to find a used meter that has been calibrated recently, so to get full benefit from it, budget a few hundred dollars more to get it calibrated. (There’s no sense chasing the higher accuracy possible in a bench meter if you don’t know when it was last calibrated.) You may also have to replace some missing accessories. Don’t expect to pay less for a used benchtop meter than for a new handheld and still get better performance.
The best reason for an audio hobbyist to get a bench meter is to get 4-wire measurement capability. This is a superior technique for measuring resistance, because it makes the resistance of the measurement wires irrelevant. The wire and contact resistances in a regular 2-wire measurement can make some measurements impossible, particularly with lower value resistors. (See my article on resistor matching for an example where 4-wire measurement is absolutely required.)
The way 4-wire (or “Kelvin”) resistance measurement works is quite clever. Two wires are connected to each side of the resistor. The meter forces a constant current through the resistor on one pair of wires. Because the meter knows the value of this constant current (I), Ohm’s Law — R=V/I — says that the meter can calculate R once it measures V, which it does using the second pair of wires. Since a digital voltmeter has a high input impedance, the small additional wire and contact resistance is irrelevant in this pair. Any constant current source worth the name works regardless of the resistance inline with it, so the probe and contact resistance in the other pair of wires also doesn’t matter. Voilá, a resistance measurement where only the resistance in question is being measured.
(By the way, a regular 2-wire resistance measurement is made with essentially the same technique. The only difference is that the “Kelvin connection” between the CCS and the voltmeter is inside the DMM, so the wire and contact resistance now matters.)
The only new benchtop meters currently sold that have 4-wire capability that I can recommend are by Agilent and Fluke, and they start at around $1,000. Keithley and B+K Precision have meters that look better on paper than the Agilents and Flukes, but I am uneasy about the quality of these two brands. I’d feel better gambling on the quality of a used Agilent or Fluke than on a new Keithley or B+K.
You can get DMMs with a capacitance meter built in, but these generally don’t perform as well as a more specialized instrument.
Truly dedicated cap meters are rare. What’s more common are LCR meters, named after the symbols electrical engineers use for inductance, capacitance, and resistance. We have one at work that I’ve used and liked, the B+K Precision 878, which costs $275 at Stanley Supply & Services. (Part# 123-012) At home, I use the Almost All Digital Electronics L/C Meter IIB, which is available in kit form for $100, or fully assembled for $30 more. It’s cheaper because it doesn’t have nearly as many features as the B+K unit, and the build quality is lower. However, it’s still useful to me because it will measure capacitance down to below 1 pF. The cap meter in my $400 Fluke 189 only goes down to 100 pF, and it doesn’t measure inductance at all.
This article is copyright © 2001-2013 by Warren Young, all rights reserved.
|Updated Sun Nov 17 2013 05:02 MST||Go back to Electronics||Go to my home page|