Analog multimeters - why not full circle?

[kalel]

To start with, I don't know anything about the history or development of analog multimeters.

I am just curious, hypothetically, what would be wrong if they had a full circle, like a clock? Would reading values be more difficult? Would it take too much space?
I'm just wondering why this is an inferior method.

An oversimplified version of what I mean:

[MK14]

Not exactly what you are talking about. But vaguely similar, is ...

The mechanical (analogue) Electricity meters. Still in use in some places.
Do go full circle. But you can only see part of the circle at any time, via the front window. There are also courser dials at the front, for the higher KW/Hrs dials.

The massive circle, towards the middle of the unit, is what I am mainly talking about. You can only see a part of it, at any time, but it is actually a big circular disc. Which can rotate continually (through 360 degrees), like the seconds hand on a mechanical watch.

If you cut it in half and made the top glass. It would be similar to what you describe. I.e. with just that huge circular disc showing at the top.

[Ian.M]

Its because Hooke's law (spring law) means the suspension gives a near enough linearly increasing restoring force with deflection angle and for d'Arsonval (moving coil) meter movements, if you require a linear scale, its very difficult to maintain a uniform radial magnetic field in the arc close to the pole pieces swept by the sides of the coil over a large angle.   

Moving magnet/iron and electrostatic meter movements are limited by physics - the rate of increase of the deflecting force decreases with additional deflection past the maximum sensitivity point till in the limit further increasing the current or voltage cant produce more force to increase the deflection.

The only analog meters that do go full circle, integrate the measured quantity with respect to time and are basically carefully designed and constructed motors, i.e. no Hook's law restoring force attempting to return their indoicator to its zero position.

[joeqsmith]

Nothing wrong with it for some applications.  You could use steppers or just a graphic LCD.   

[Rerouter]

Automotive gauges use a sin / cos driver chip to translate a frequency, a pwm or a voltage in to a reading, thus removing the feature where analog meters generally dont need any power source.

There is also a reason why the scales are overblown on speed and rev, if the meter is deflected past 180 degrees on most sin cos movements without a spring return, a tiny power glitch will leave the movement stuck at the other end of the reading unless you get it to again read past 180.

[MarkS]

You need to be more clear, but if you are talking about the actual meter(the part with the needle), then I think you think a motor controls the needle. It actually uses an electromagnetic deflection mechanism that moves the needle in proportion to the voltage applied. It's construction and design necessarily precludes full 360° rotation.

[Ian.M]

Yes, but a stepper isn't itself an analog meter even if its controller makes it act as one.  Large rotation angle servos used to drive a dial also fall in the 'display device' category, though its possible for the controller to be pure analog (very unlikely nowadays).

There is one other pure analog exception - Selsyns, which way back were commonly used for transmitting angular position over a wired connection (occasionally even over radio links,) and sometimes had a circular dial type indicator on the receiving end.

[joeqsmith]

The bike clusters I show are using steppers.  My car also uses them.   They are cheap.  Article about them:

http://www.mycockpit.org/forums/content.php?r=355-An-Easy-Approach-to-an-Analog-Gauge&s=d6ae14190d1b7caa4ea74184554104d9

OP, personally I still have no use for an analog multi-meter since the last time the digital/analog meter thread. 

Maybe rather than "Analog multimeters - why not full circle? "  start with  "Analog multimeters - why?"   People still buy them so they must fill some need.

[floobydust]

It is not possible with analog meter movements.
You need a constant magnetic field and constant tension (spring), all perfectly linear. Then there's the wires, as they moving-coil.
I think 270° is the widest angle achieved.

Stepper motors would be cool, the venerable Switec X27stepper motors for car dashboards can do 315° (has internal home stop peg).

[kalel]

Maybe rather than "Analog multimeters - why not full circle? "  start with  "Analog multimeters - why?"   People still buy them so they must fill some need.

I was just wondering about their design. For that specific question, it is less important if the instruments are still used broadly.
A lot of answers have been mentioned, however.

Thanks everyone.

[ivaylo]

It's mostly how quick the needle goes where it's supposed to. The old electricity meters did use clock like rotation, but going only in one direction. Also modern digital instruments still have "analog" bargraphs for a reason.

[kalel]

It's mostly how quick the needle goes where it's supposed to. The old electricity meters did use clock like rotation, but going only in one direction. Also modern digital instruments still have "analog" bargraphs for a reason.

Yes, I'm sure that bargraphs are useful. Still not included in the low end meters (like Aneng 8002 8008  etc) but I assume they will appear at similar prices eventually. Unless there's a reason such as sampling rate limitations which would preclude it at that price range in the near future.

[Rerouter]

I went looking and there is a 1960's era speedo that get 290 degree deflection using a 90 degree movement, it uses essentially a drum line, the movement deflects and tugs on a bit of teflon fishing line wrapped around a drum behind the display needle, this means the displayed reading is not linear, so they just printed the dial non linear

[Zbig]

Adding to all the valid reasons mentioned before, I think it would be much more difficult and tedious to adjust (compensate) for the parallax effect for any kind of precise measurement on a circular dial. Many analog meters have a mirror behind the needle to help you establish a straight-on line of sight and avoid any errors resulting from the fact that the needle flies several millimeters above the printed scale. Chances are, you would look a bit ridiculous to the bystanders, trying to do that on a circular meter.

[tggzzz]

Now consider a digitally meter with a pseudo-analogue display, e.g. 360 LCD segments arranged in a circle like a clock.

That's great for a clock where the value being measured does "wrap around".
It would be bad for a DVM where segment0 = 0V and the adjacent segment359 = 1V. It would be very easy to misread 0V for 1V and vice versa.