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Date:         Fri, 4 Jul 2014 21:06:07 -0400
Reply-To:     David Beierl <dbeierl@ATTGLOBAL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@ATTGLOBAL.NET>
Subject:      Start of Draft: Analog VOMs for vans, was Re: AFM test readings
Comments: To: Rocket J Squirrel <camping.elliott@GMAIL.COM>
Content-Type: text/plain; charset="us-ascii"; format=flowed

[Post rejected for length, so splitting it in two] Dear Volks,

Here is quite a lot of info about analog meters in general for people who aren't used to them, together with my thoughts about two general classes of meter and several specific meters in each class, prices ranging from under ten to over three hundred dollars. There are a few oddballs but in general the classes are a) very small inexpensive low-sensitivity meters that are fine for the AFM but not for oxygen sensors; and b) moderately sized medium-high sensitivity meters mostly about four and a half to six inches tall that are good for both. I don't discuss any full-size bench meters or electronically driven meters.

I can't begin to describe any of them completely, but for each one I try to point out salient characteristics that may help focus your choices both among the ones I talk about and among others you may find. Many of them have very useful testing ranges for 1.5V and 9V batteries that place a bit of load on them to get a much more realistic reading than open-circuit voltage. I don't discuss the presence or characteristics of any of these or most other features not aimed at the van and particularly at the AFM and O2 sensor. I've barely glanced at AC ranges. Also keep in mind that I've never laid eyes on most of them.

One thing I neglected and am not going to go back and sort through now is whether there is a DC range on a given meter that is convenient for working with 12V systems. Your digital multimeter will no doubt take care of that splendidly, but it doesn't hurt to be aware of the question. And you should be aware that analog meters have much larger errors in general than digitals; and accuracy is specified in such a way that they can be effectively huge at low readings. I hope and believe that although it could be better organized, almost everything I've written here is useful and the rest is at least interesting. But if you feel like skipping all the background at least be sure you understand the accuracy part.

I put a lot of work into this, but it's basically a draft that outgrew its beginnings and I know it could be much better organized and lose some fat. But I'm done with it for the moment. If any decent writer cares to have a whack at editing it I solicit your suggestions. Likewise of course anyone please point out any errors or gaps or bad explanations so I can fix them. I'm past seeing any myself at this point, other than mentioning sensitivity a lot but not thoroughly explaining it. I hope what I say in context works well enough without the full explanation, but I'll fix it if I have to.

Mister Squirrel started this but all the explanations aren't for him because I'm sure he could have written them himself.

Yours, David

At 07:56 PM 7/3/2014, Rocket J Squirrel wrote: >The question is -- who has a good value in an analog multimeter these >days? Simpsons were my go-tos in Ye Days of Olde when Knights Were Bold >and I had a budget.

SMALL METERS and a touch of explanation:

The little pocket multimeters for $20 or so from anybody would be ideal for this use except that their lowest DC range is probably six volts or so which is a bit high for this. Also they'll have sensitivity (internal resistance) of 2,000 ohms per full-scale volt (e.g. 6V range would have resistance of 12,000 ohms) so they'd be useless for checking oxygen sensor activity in closed-loop mode, which is the other place on the van where you want an analog meter or scope.** However here's one from Extech with a 2.5VDC scale for $20. No good for O2 sensor but great for AFM. Extech has been around for a long time now, with various kinds meters that are comparatively low priced, at least if you compare them with Fluke. I've had at least one or two of their digital meters over the years with no complaints. The one I'm sure of was stolen, which I can't blame on the meter. Markings are clear, meter face crowded and wastes space on an anti-parallax mirror which is just silly for a meter like this, some ranges are not factors of ten of the 10/50/250 meter scales, which will drive you nuts. Test leads appear to be built in. http://www.transcat.com/catalog/productdetail.aspx?itemnum=38073 http://www.transcat.com/PDF/38073.pdf

Here's a throwaway one with a 2.5 VDC range, $9 at Amazon: http://www.amazon.com/Mastech-YG188-Pocket-size-analog-multimeter/dp/B00064CH6A/ref=pd_sim_hi_4?ie=UTF8&refRID=13CK3Z0VH2A2X08XNYT7 . Marking readability is mixed and I can't find a good enough image to distinguish the scale numbering, however ranges are some variation of 2.5/5/10 so the scales are likely easy to understand. Test leads are built in.

Elenco M105, $17 at Amazon. This is an exception to the others in this section, as it is 10,000 ohms per volt sensitivity instead of 2,000. On the ten volt range it should be usable to get an idea what the oxygen sensor is doing. 2.5V DC lowest range. The ohmmeter is very limited, 100 ohms midscale on the low range and 10 kohms on the high range. But it's compact and well built and can serve both our prime Vanagon applications. The reviewers love everything about it. Markings are clear, meter is easily readable and all ranges are a factor of ten of the 10/50/250 main scales. Well worth considering if you want a quite compact meter that's still quite sensitive, and can live with the +/- 5% FS DC accuracy and the crappy ohmmeter. There's no range that's good for 12 VDC, but that's what your little digital is for. 4.5x2.7x1.2 inches. http://www.amazon.com/Elenco-M105-Range-Compact-VOM/dp/B0002HQVFO/ref=cm_cr_pr_sims_t

**Or an indicator based on the National Semiconductor LED bar graph driver chip (LM-can'trememberthenumber), like the little indicator board Ken Lewis sells. It's truly ideal for the purpose. It won't load the sensor, it responds in milliseconds, gives you a ten-segment bar or dot graph, draws practically no current if it's not lighting up a segment; and if you want more resolution you can cascade as many chips as you want. You might even be able to cascade Ken's boards with a little creative wiring and a few resistors.

======================================================================== ========================================================================

LARGER METERS and a lot more explanation:

All the meters below are at least 20 kohms per volt on DC ranges, so on a 5V scale they'd have 100K resistance which shouldn't drag down the oxygen sensor *too* badly but still lets you see the meter pointer move. Most do not have a high-amps range and max out at a quarter or half amp.

People without analog meter experience should at least see the note on quoted accuracies under the Radio Shack entry and the note on ohms scales under the UEI meter.

All or almost all of these meters use the same jacks for the current ranges as for voltage. If you hook them to twelve volts or really any voltage source when they're switched to a current range either a fuse will blow or the meter will try to bend itself around the pin (and might succeed) and the current shunt for that range will smoke. Try to do better than my record of about five minutes when I was given my first meter at age six or seven. Smoke *and* bent pointer , whee. And of course I didn't dare admit it.

Digital meters these days use very low voltages and maximum currents on the ohms ranges. Very hard to hurt things with them, and the voltage isn't high enough to turn on semiconductors, which can be a convenience (that's why they have to have a separate diode check range). These analogs are different, they apply anywhere from 1.5 volts up through 9 or even 22 on the various ranges, and many can probably put 150 mA or more through zero ohms on the x1 scale. They *will* turn on semiconductors, and can potentially overheat or smoke small components on the low ranges. The ones using more than three volts on the high ranges can potentially ruin sensitive semiconductor components by overvoltage. And they have to be re-zeroed on each use and each time you switch ranges, especially when switching to a range that uses a different battery. You can still check diodes but have to choose the range thoughtfully, and the reading won't tell you what the forward voltage is without either measuring it with a second meter or working up a table of resistance readings vs voltage on the ranges of interest for your particular meter.

Any meter you look at, check the correspondence between the ranges and the scales printed on the meter. Any range that isn't a factor of ten of one of the printed scales will drive you mad. I have a little Radio Shack clamp meter around somewhere that's a clever little thing, but hardly any of the ranges are direct reading and it's impossible to use. Multiply this scale by two, divide that one by three...wrong. You'd have to tattoo the manual on your wrist.

Radio Shack's $30 22-037 meter (20 kohms/volt DC, 2.5 VDC range, +/-3% of full-scale DC accuracy,*** no high-amps range, 50 ohm continuity buzzer and uses 3 volts for ohmmeter so it should light up red/green/orange/yellow LEDs) sounds good but the recent reviews on both RS analog meters point to dreadful build quality/QA or accuracy troubles. Markings seem readable but the AC ranges are marked in red and to me have lower contrast against the case. All voltage/current ranges are powers of ten of the 10/50/125/250 scale markings. But given the reviews I'd stay clear, especially since reviewers compared unfavorably against previous RS meters.

==================================================

***(Long note, got even longer.) Analog meter accuracy is specified much differently from digital meters and it can bite you if you're not used to it. Digital meters on a given range are specified as +/- m% of the reading, +/- n counts on the display. That implies that a low reading on a given scale will have an error of similar proportion to the actual value as a high reading will (the lower the reading, though, the more the +/- so many counts part will influence it, so high-count readings are still the most accurate).

But on an analog meter, the voltage and current scales will be specified as +/- m% *of full scale* so that the allowed percentage error gets completely ridiculous as you get near the bottom of the scale. A ten volt scale with +/- 3% would allow an error of +/- three tenths of a volt anywhere on the scale, which would be a 30% error at one volt. For this reason people are advised to distrust the actual values of readings below about one-third or even one-half scale (however a higher input will always produce a higher reading than a lower input, which is not always strictly the case with a digital meter). Also, +/- 1.25 of FS on DC is the best spec I'm aware of for an analog meter (Simpson 270-5/5RT), and only high-class meters meet 2% (3% is more common); whereas even a very cheap digital meter may well have a basic DC accuracy of 1% of the reading +/- one or two counts and fancy ones run a quarter per cent or better. Harbor Freight's $6 meter is specified +/- 0.5% +/- one count on the 200 mV DC range and +/- 1% +/- two counts on its other DC voltage scales. Both analog and digital meters are generally most accurate on their DC voltage scales.

The analog ohms scale is odd because it runs backwards and has much better resolution on the low-resistance end of the scale which is also the high end of the meter scale where the movement is most accurate. Accuracy is specified in +/- m% of the total length of the scale rather than in terms of a specific value, so you have to measure the scale to know the accuracy in ohms for a given resistance being tested.

Typically analog meters will do noticeably better than the spec on low readings, but there's no promise. And errors on the high end won't necessarily be linear. Inaccuracy in the network of resistors used to change ranges will cause errors that may be different on different ranges, but on a given range will be the same percentage of different readings, high or low. But non-linearities in the meter movement will cause varying errors that affect every range, causing the pointer at a given point on the scale to deviate *by an angle* that may be different from that at every other point but doesn't change when you switch ranges. My hand-held analog meter which as I recall cost about eighty bucks ten years ago has a very irritating bump right in the vicinity of 12v (on a 15 or 20 volt scale, I forget) and of course a similar location on other scales. Ten volts reads as ten, fourteen volts reads as fourteen; but twelve reads as 12.3. It's in spec, but it bugs me.

Errors in zeroing the pointer (the real mechanical zero, not the backwards ohmmeter zero) will cause an angular error that diminishes proportionately at higher readings. All problems are exaggerated around the zero point because in theory everything is perfectly balanced and perfectly frictionless, and an infinitely small input will cause an infinitely small force that will move the pointer an infinitely small amount. In practice of course it doesn't work that way.

Most analog meters are specified to be read in horizontal position, whether or not they have a tilt stand or feet on the short side of the case. The zero may shift if you set them at other angles depending on how and how closely the pointers are balanced, and the accuracy will probably decline.

The next bit is about how meter movements are built,, how to recognize a certain sign that you just paid too much for a fancyish shock-resistant meter that turns out to be counterfeit, and the compromises and interactions among price, sensitivity, physical size, and how the pointer responds (the last is called ballistics). I think it's both interesting and useful, but if you want to skip to specific meters head down to the next double line of ==============.

The movement in most meters is of the type called D'arsonval, because he invented it. The pointer is typically made of extremely thin aluminum tubing, sometimes with the far end squeezed shut to make it yet thinner. It's mounted on a shaft which seats in jeweled bearings at either end, and there are two hairsprings that make electrical connection to a coil of wire wrapped around a rectangular form that's mounted surrounding the shaft. The pointer has a short stub extending to the rear, and two more stubs extend to the sides. These three stubs carry small coils of wire wrapped around them to balance the pointer in all directions so that ideally it does not change reading when the meter is held at a different angle. Adjusting these weights is very delicate work, so most pointers probably aren't quite balanced and will shift zero a bit when you hold them at different angles. The bigger and more sensitive the meter, the more critical the balance.

The entire assembly is placed inside the gap in a circular magnet so that it tends to rotate when current passes through the coil, and the hairsprings also provide the restoring force that the magnetic field works against. The upper hairspring has an adjuster attached so you can adjust the pointer zero point. This movement serves very well, but the jewels are easily cracked which makes the pointer stick or otherwise misbehave, or they can get sticky for other reasons. And at very low readings the tiny friction may be enough to disturb the reading slightly. Here's a photo of what's visible from the front of a D'arsonval meter with its cover/window removed. There's a pin mounted eccentrically on a screw-head in the meter cover, and this engages the fork you can see marked as Zero Adjust. As you rotate the screw-head the fork moves first one way, then the other, so there are two points in its rotation that will leave the adjuster in the same place. https://www.google.com/search?q=d%27arsonval+movement&rlz=1C1KMZB_enUS518US518&es_sm=93&tbm=isch&tbo=u&source=univ&sa=X&ei=hta2U_PEJ8-NyATAq4HICg&sqi=2&ved=0CCoQsAQ&biw=1746&bih=905#facrc=_&imgdii=_&imgrc=povq0GZ2s8KY7M%253A%3BUtR2wXBFMGNlMM%3Bhttp%253A%252F%252Fwww.repairfaq.org%252Fsam%252Fmmanat1.jpg%3Bhttp%253A%252F%252Fwww.repairfaq.org%252Fsam%252Ffaqfil.htm%3B603%3B390

There's another movement that's used in meters needing shock resistance, excellent linearity, better accuracy at low readings and suchlike. It's called a taut band movement, and it's just like the D'Arsonval except there are no bearings and no hairsprings and no shaft as such. Instead a thin springy ribbon is mounted to the top and bottom of the coil form, and the two ends stretched across a stiff C-shaped spring. The force of the spring keeps everything stretched out and the ribbons lie straight and flat with the needle at the zero point. The ribbons also supply the connections to the coil. When current passes through the coil it rotates, swinging the pointer and twisting the ribbons, which makes them a tiny bit shorter. This shortening pulls the ends of the spring together very slightly but proportionately to the pointer rotation, and this supplies the restoring force to bring the pointer back to zero. This is an expensive way to do it, but the resulting assembly is very resistant to shock and has no friction whatsoever. Here's a photo of one of the first taut band movements, built by HP in 1964. It's rotated 180 degrees from how you'd usually see it -- the short white thing pointing down and to the right is the stub of the pointer, and the adjuster yoke is sticking straight up. But you can clearly see the lack of jewels and hairsprings. I'm making a point of this because if you buy a meter that uses a taut band movement and it turns out to be a counterfeit, almost surely the actual movement will be of the D'Arsonval type which you can instantly recognize.

Every meter movement is a compromise. It's easy to make a slow-responding pointer that doesn't tend to overshoot its reading, not so easy with a fast one. It's easy to make a fast-responding movement that uses a lot of power and drives a short pointer, but the longer the pointer and the more sensitive the movement must be, the harder it gets to make it respond quickly. So big sensitive 50 microamp meters tend to be slow, and big *cheap* sensitive meters downright leisurely; but small 500 microamp ones can easily be reasonably brisk. But a meter (even a fairly small one) that meets the technical specs for a VU meter (90% of full reading in 300 milliseconds, and no overshoot at all) is going to be quite expensive and not very sensitive.++

And if you for some bizarre reason want a big meter that is quick but overshoots like mad, can hardly stand still -- you go to an outfit in say New Hampshire that makes big quick expensive meters and special order some with most of the damping taken out, and you take these now pretty useless but impressive movements and mount them in something called an E-meter that you sell for big money to every other Scientologist in the land who wants to amount to anything. The underdamped pointers make them considerably more impressive to watch. The first few seconds of this may give you an idea of it: http://youtu.be/_YCoqYyQXqc?t=2m42s

++There are (or were) meters labeled VU on every tape recorder and all sorts of other audio gear, but very few of them met both specs to legitimately wear the label. If they weren't slow they overshot. And if they did either they weren't very useful for their main purpose, which was to enable the operator to use a meter that responded only to average levels *and his own knowledge of the characteristics of what was being recorded* to set the levels properly so that the peaks didn't distort but the average level was as high as possible to minimize tape hiss. If you were recording piano music for example, you had to set the levels to read very low because each piano note starts with an extremely loud peak that quickly decays; but for a group of singers you could run the needle right up. If the meter didn't behave the way it was supposed to you didn't know what the readings really meant even though the steady reading would be the same as a "real" VU meter. All this quickly went away as soon as we learned to make various kinds of electronic peak-reading meters which could be both extremely fast and quite cheap since they didn't have to swing a big heavy pointer all about, though there was a stage when you still had an analog meter but also one or two LEDs that would flicker if the peaks got too high.

End of part 1, completed in part 2.


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