Analog meters – The joy of movement

Editor's note: If you're like us and can't bear to part with that crusty old (but magnificent) analog meter, David Ashton offers everything you want to know about these gems in this three-part series:

Just about everything today is digital — including most meters: digital multimeters, digital panel meters, and many instruments use 1-, 2- or 4-line character displays or graphic displays as their main visual interface. So why even think about using an analog meter in this day and age?

In fact, there are many reasons, not the least nostalgia as in Max Maxfield's Vetinari Clock and Inamorata Prognostication Engine, and more. You can pick them up for next to nothing these days. Finally, the indisputable fact remains that if you're looking for a maximum or minimum reading or a certain reading like half of maximum, you can more easily see the readings on a moving needle than on a changing digital measurement. (Recognizing this, many digital meters now incorporate a bar graph display as well, but these are usually pretty low resolution compared to the needle on an analog meter). Rich Quinell’s recent article went into more detail on these aspects, so I won’t dwell on them here.

Many older readers will have owned an analog multimeter in days gone by (or, in the case of tragics like me, will still have one or two) and a lot of us will have some older test equipment around that has an analog meter. Whatever your situation, a knowledge of analog meters and how to use them can often be a very useful thing to have.

The joy of movement
The main part of an analog meter is called the movement , which is fair enough as this is the bit that actually moves. A movement consists of a needle that is attached to something that will move it across a scale to indicate a reading. (Max calls the scale a reticule , which is another word for reticle , which is another word for graticule , which is fine for oscilloscopes, but not, I reckon, for meters.)

Magnetism is usually (but not always) involved. Moving coil meters are probably the most common type, where a coil is suspended in a magnetic field, and when current passes through the coil it generates a force that is arranged to move the coil and the needle on an axis. Certainly most analog multimeters will use this type of movement. They can be made very sensitive — 50 µA or less for full scale deflection (FSD) of the needle. This is a fairly crucial point: No matter what the meter actually reads — volts, ohms, temperature — the primary driving force of an analog meter is the current passing through it.

A moving coil meter movement
In a practical meterthe components are very small and delicate. The springs are akin towatch springs, the wires are very fine, and the clearances between themagnetic pole pieces and the core are very small. Consequently, one ofthe problems with this kind of movement is that any foreign matter— especially magnetic particles — can seriously impede the movement ofthe coil in the magnetic field. This can lead to erratic operation orthe needle “sticking” in some positions. Poking around with a pair oftweezers can do more damage than it solves. Meter coil wires are usuallyvery fine and if overloaded may burn out. If any of this happens, themeter can sometimes be economically irreparable except if the meter is avaluable or irreplaceable type. As Max found recently,you may be lucky enough to find a specialist meter repair house nearyou — meter repair skills are getting very rare, and repairing a meteris expensive, but it will be worthwhile doing for a good meter, so ifyou find a good meter repair shop, don’t lose their number!  Meters areusually fairly well sealed and if they are handled and installed withcare, and steps are taken to protect them from overcurrent, they shouldgive years of good service. More about this later.

At the ends ofthe axle, above and below the springs, there are usually low-frictionbearings, and as in high-quality watches, these may be “jeweled” — madeof semi-precious stones. This can make a good meter movement expensive.But they can be very sensitive, and they have very linear scales. Thereis generally a screw adjustment on the face of the meter which adjuststhe spring above the needle to “zero” the needle in the right place.High quality meters may also have a mirror on the scale — this lets youmake sure that you are looking at the needle at exactly 90º to avoidparallax error. In this way you can get an accuracy of around 1% on agood quality meter.

There are various other names for moving coilmeters: Galvanometers (after Luigi Galvani, who was one of the firstpeople to play around with electricity) and D’Arsonval movements orWeston movements, after the scientists who invented and developed thistype of meter.

Pumping iron
Another commontype of meter is the Moving Iron meter. Here, the coil is fixed, usuallywound around a fixed piece of iron, and another a piece of iron issuspended in the magnetic field that is generated when current passesthrough the coil. The current magnetizes both pieces of iron, whichrepel each other, causing the moving one to move and deflect theindicator needle. This type of meter can be made very rugged, and theyare consequently not as delicate as the moving coil types. They are alot less sensitive, less linear (especially at low currents) and lessaccurate. Besides their ruggedness, another nice thing about this typeof meter is that they are just as happy with AC as with DC. So they arewell suited to things like battery chargers, mains voltage, currentmeters, and the like, where great accuracy is not needed and ruggednessmay be of great advantage.


Moving Iron meter movement


MovingIron 5A meter face. Note the non-linear scale, particularly at theends. Note also the AC/DC symbol underneath the “A” and the symbols tothe left of the needle. More on these later.

Thereare other types of meter — for example Taut Band movements use tautbands to suspend the coil rather than springs, giving less friction —but the above two are the types of meter that you are most likely toencounter. This is pretty universally the symbol used for a meter:

 

Apartfrom what type of meter you have, the most important thing you need toknow about a meter is its FSD — the current needed to deflect the needleto the end of the scale. [Note to Max: I wonder why they don’t call it Full Reticule Deflection? 🙂 ]On a bare meter this will usually be given as a DC current, though on ameter which has already been adapted for a specific purpose it may bein another unit — Volts DC or AC, or even something like temperature.Generally the most sensitive meter you can get is 50 μA FSD, and theyare getting rarer, though D’Arsonval made one that could detect 10 μA!As above, meters basically respond to current, but you can get them toread different parameters by using shunts, multipliers, rectifiers andthe like. This is covered further in the next article in this series. 

Going Ballistic
Meter movements have what is called ballistics ,which is how the meter responds to a step current applied to it. A bigmeter with a long needle will have a lot of inertia, the movement of theneedle may encounter air resistance, and the inductance of the coil mayhave some effect, so it will not respond very quickly and may take sometime to deflect to the correct reading. Smaller movements will rapidlymove the needle to the desired point and may even overshoot andoscillate slightly before coming to rest. Meters may have vanes or evendash pots to damp the movement if this is undesirable, but there wereballistic meters where the deflection was proportional to the totalcharge applied to it. These were used for testing telephone lines.Analog phones have a capacitor in them — usually around 2 µF — and when avoltage is applied to the line you get a “kick” of current, and thesemeters were designed to maximize the kick to indicate the health orotherwise of the line, and a skilled operator could tell how many phonesyou had connected as well! Some meters intended for broadcast audio usehave specified ballistics, so that momentary levels that wouldovermodulate a transmission can easily be seen, and there are standardsfor this too, notably from the British Broadcasting Corporation (BBC)but many other international standards organizations as well. Audiometers are usually calibrated in decibels (dB) or the closely relatedvolume units (VU) — here is a typical example:

 

Youalso get Mirror Galvanometers — where the movement moves a small, lightmirror rather than a long, heavy needle. In days of old this woulddeflect a light beam onto a scale, which could be large, giving theeffect of a long needle without the weight or inertia. These days theyare used to deflect laser beams for bar code scanners, micro marking,welding and cutting, effects in discos and the like.  Scanlabis a company that specializes in mirror galvanometers and scan systemsthat use them, and their website is instructive if these interest you.

Be sure to check back for the next article in this series where I look at shunts and multipliers and introduce additional types of meters. 

Whatare your experiences with analog meters? Do you love ‘em, hate ‘em?Have you any stories about analog meters? Please comment below.

Be sure to read the other installments in this series:


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9 thoughts on “Analog meters – The joy of movement

  1. “DavidnnGreat article, as usual. I think you have omitted a very important part of the HMI. That is when you tap on the meter when you don't believe what you are seeing. On the odd occasion it has made a difference!”

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  2. “David,nThat was a great article even down to the ballistic galvo. nIt was not until I worked in a standards laboratory that I ever thought about the current consumed by the meters in my test circuit both the current and voltage meters had to be calibrat

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  3. “Great article, David! nYou mention “mirror galvanometers”- I've mostly heard those referred to as “light beam galvanometers” to differentiate them from “mirrored scale galvanometers”, in which the mirror behind the pointer eliminates parallax error

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  4. “And my favorite place to see a real analog meter: the tachometer in a vehicle.nnThe last new motorcycle that I bought has an LCD graph tachometer and it is useless. When they refreshed the model three years later, it had a real analog tachometer in the

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  5. “Yes, I had a wonderful Italian made analogue meter that finally died in the 90's. small wonderful robust colourful thing with rounded corners. You could tell from the sound if you had a short circuit or just small resistance. Helping a PhD student last we

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  6. “@Crusty thanks….wait till you see (later in the article – they've split it into 3 parts) what current a moving iron voltmeter takes – it is frightening. I remember Weston cells and slide pots (but only just – we are seriously dating ourselves here).

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  7. “@Antedeluvian thanks… you are right, that was a serious omission. Your technique actually can work if your meter has some particles in the movement as described. I gather it also works with barometers :-)”

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  8. “@Rcurl thanks….I've mostly seen “Mirror scale” used for the mirrored scale ones, and “Mirror galvanometer” where the movement drives a mirror, but Scanlab calls them Galvanometer Scanners. So take your pick! nnhttp://www.scanlab.de/en/-/produ

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  9. “The French company Metrix used to make very good looking analogue meters, also very colourful. But I have never seen an analogue multimeter with a continuity Beeper!”

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