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Experiencing the thrill of movement in analogue meters

29 Apr 2015  | David Ashton

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Almost everything these days is digital—including most meters: digital multi-meters, 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 analogue 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. (Recognising 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 analogue meter).

Many older readers will have owned an analogue multi-meter 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 analogue meter. Whatever your situation, a knowledge of analogue meters and how to use them can often be a very useful thing to have.

The joy of movement
The main part of an analogue 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 analogue multi-meters will use this type of movement. They can be made very sensitive—50µA 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 analogue meter is the current passing through it.

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

At the ends of the axle, above and below the springs, there are usually low-friction bearings, and as in high-quality watches, these may be "jeweled"—made of semi-precious stones. This can make a good meter movement expensive. But they can be very sensitive, and they have very linear scales. There is generally a screw adjustment on the face of the meter which adjusts the 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 you make sure that you are looking at the needle at exactly 90ยบ to avoid parallax error. In this way you can get an accuracy of around 1% on a good quality meter.

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