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.
How Much Should I Expect to Pay for a Meter?
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.
Links
Why it’s reasonable to pay $400 for a handheld
meter: here, and
here
What Should I Look for in a Meter?
First you should decide if you want a digital or an analog meter, or
both. I prefer digital meters, but I also keep an old professional VOM
around which sees occasional 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.
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.
What Brands of Meters Are Good?
Fluke is the leader in handheld DMMs. Period, end of sentence.
Agilent recently re-entered the handheld DMM market, which holds hope
for some competition at the high end, but it’s a recent enough
development that I’m not prepared to recommend them yet. 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.
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.
What About Bench Meters?
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.
Capacitance Meters in DMMs
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.