Analogue or Digital?

[Red]

Greetings to all

Having indulged to excess, as one should over Christmas, I decided to catch up on some overdue reading, now when I say overdue I am talking about the November issue of RCM&E. I came across an article entitled "Loose Connections" penned by the entity that is Alex Whittaker. I need to say at this juncture that I am, and will continue to be, an avid reader of this contributors musings. The article is misleading in some places and inaccurate in others. Part of the article tries to persuade the reader that the analogue meter is better for quick checks than a digital meter. How so? The range on most DIY analogue meters is too wide and therefore not accurate enough. By range I mean the scale used is too wide, it will not measure any electrical property that is not a whole number.

Now we all know on modern on-board power systems that measuring electrical properties needs to be extremely accurate, even when making quick checks, and this will almost always include the need to measure values less that a whole number, an analogue meter of the type in the article will not do this. The article implies the use of an analogue meter to measure a batteries voltage under load and then presents some voltage readings that include values of less that a whole volt. How were these readings obtained? They were not taken with the analogue meter featured in the article, they would have most likely been taken with a digital meter. This area of the article is hugely misleading in my opinion.

The next area of contention concerns checking continuity. As the previous sentence states, you "check" for continuity, you do NOT "measure" continuity. The article suggests using ohms for checking continuity, this is plain wrong, wrong, wrong. Ohms is a measure of resistance, which can, by default, indicate continuity or an open circuit condition. I will concede that in a closed circuit there will be a measurable resistance, however, the resistance is likely to be small and therefore beyond the scope of the analogue meter. I also agree that in an open circuit there will be an infinite resistance. The analogue meter in the article has a separate function specifically designed to check for continuity. If you are checking a closed circuit which is good, a buzzer will sound, if the circuit should be closed but is not (open circuit) then the buzzer will not sound, simple. This is one function of the analogue meter that is equal to a digital meter since you are not trying to measure a value.

The article also suggests that you can measure battery charge voltage against time or a capacitor discharging against time. What? You would need an oscilloscope to do that, or a separate piece of equipment to measure the time component.

I would like to reiterate the point in the article that states NEVER to test for resistance in a "LIVE" circuit, this applies to continuity testing as well.

Alex, you have let me down on this occasion, it is however, my new years resolution to forgive you.

RED

[Simon Chaddock]

Red

I have not read the article but I thought an analogue meter has a pointer scale driven by a moving coil arrangement. Even a simple one has a 50 division scale with 5 ranges covering 0.25V to 1000V full scale deflection.

Or am I missing something?

[avtur]

I have to say that I agree with 'Red' in my disappointment on reading the article.

Given the price of digital meters I am surprised to see the case being made for an analogue device.

I accept that less expensive digital meters may not offer the 'absolute' accuracy of more expensive devices but I would expect consistency and a level of accuracy which is acceptable for modelling purposes.

Most of us seem happy to accept the accuracy of measuring devices contained in modestly priced battery charging equipment which I'm sure compares well with the accuracy of modestly priced digital meters.

Maybe I'm missing something but I am at a loss to think of any use where I would prefer an analogue meter.

 

Edited By avtur on 26/12/2013 00:59:16

[Allan Bennett]

Like the others, I didn't really agree with what the article seemed to be saying, that you're better off with an analogue meter than a digital one. I got the impression the writer had found an analogue meter with a host of features that, admittedly, aren't usually found on budget digital meters (transistor testing, for instance), and was plugging it.

But I've got a small, very cheap, analogue one that I sometimes use in preference to my digital one and, as has been mentioned, even it has switched scales that allow very low voltage readings. No guarantee of its accuracy, but at least a small movement in the needle indicates there's something there!

[John Privett]

I've just read the article, and apart from the bit about "measuring" continuity in ohms found it to be reasonably correct. I'm puzzled though by a couple of Red's statements;

The range on most DIY analogue meters is too wide and therefore not accurate enough. By range I mean the scale used is too wide, it will not measure any electrical property that is not a whole number.

and

The article implies the use of an analogue meter to measure a batteries voltage under load and then presents some voltage readings that include values of less that a whole volt. How were these readings obtained? They were not taken with the analogue meter featured in the article, they would have most likely been taken with a digital meter. This area of the article is hugely misleading in my opinion.

Maybe, like Simon, I'm missing something, but why on earth would I not be able to measure, for instance 0.4v or 2.7v on an analogue meter? Perhaps Red is, like many other, confusing accuracy with precision?

[Bob Cotsford]

I'm with John (and Alex), you really don't need the precision of a digital meter for models, especially at the field. One decimal precision is fine to see if your receiver battery is good or not, and if you can't read an analogue meter to that accuracy then it's on the wrong scale or maybe a trip to Specsavers would be in order?

I thought most cheap meters had transistor/diode testers and the like these days?

[Wingman]

Analogue? You mean one of those cute little artefacts from last century with a little needle mounted on teeny weeny bearings located by a teeny tiny little spring thingy which is wrecked the first time it receives a significant knock? Don't be daft, spend your money on the best digital meter you can afford (and it doesn't have to be very much) which can be knocked about on your workbench and dropped into your field kit and still be absolutely accurate each time you switch it on

[Biggles' Elder Brother - Moderator]

I simply don't get this comment of Red's that analogue meters can only measure whole numbers? If I put an analogue meter on a 500mV range - then full deflection is nonimally 500mV, one tenth full deflection is 50mV - 0.05V - what's the problem?

Now if the comment had been made in the context of a digital meter - although not strictly correct - I could at least have understood what was being said. If we assume a 3 digit display then such a metter can only offer a precission of approximately 1 part in 1000 - it can never exceed that because of the nature of the device. The actual accuracy this equates to would depend on what scale the meter is set to. The equivant accuracy determination for an analogue device would be +- half the smallest division. The value of one division - and hence the determining factor on accuracy - would again depending on the scale that is selected.

But the point is that both devices can measure perfectly well - and either is in any way limited to "whole numbers" - where that idea came from I don't know!

But, does it matter? In my view, no. I would disagree with Red's comment that "modern on board power systems require extremely accurate precision" - I don't believe they require any greater level of precession than is easily attainable with any reasonable quality meter - analogue or digital.

What do I use? Digital. Why - because when I was buying a new meter (having dropped my old analogue one on the floor and broken it) the digital one was a good price. It is possible to make a good digital meter for less than the cost of a equivalently good analogue one. That's the reason I would recommend digital - but its got nothing at all to do with accuracy or being able to read whole numbers!

BEB

[Plummet]

To be a bit supportive of Alex...

There are times where analogue is better than digital. I f you are measuring DC, but the current is switched, as is the case with our ESCs, or if it is not smooth, for example the output from a transformer and rectifier, a digital multimeter can get confused. The meter samples its input at some frequency. If that frequency matches or is a multiple of the switching frequency then it can be difficult to get a steady reading.

Also it can be easier to get a quick idea of a measurement with an analogue meter - when accuracy is not important. A digital meter can be difficult to read in sunlight, and if its reading is changing too quickly it can again be hard to read.

Digital meters can have a very high input impedance. This means that taking the measurement does not significantly effect the thing being measured. I remember having to do some tricky calculations (with my slide rule) in order to get the true readings when using an old AVOmeter because the meter itself was putting a significant load on the circuit..

BUT...

Do I use an analogue meter? No. I have three assorted digital ones (somewhere.)

Plummet

[fly boy3]

An Avo meter,now that brings back some memories. Cheers

[GONZO]

Point 1/ If there is any possibility of measuring voltages where there may be RF present then an analogue meter is required to help avoid false readings.

Point 2/ Digital meters are +/- 1 on the count and +/- 1 on the display giving the possibility that the last digit could be out by +/- 2

Point 3/ For analogue meters the terms interpolation and extrapolation plus parrallax (assuming you've bought one with a mirror on the scale) are key.

HTH

Edited By GONZO on 26/12/2013 14:03:38

[Peter Beeney]

I made a New Year’s Resolution to myself years ago never to read electrical articles in aeromodelling magazines, much better to consult electronics magazines; or perhaps even better, a text book. I’ve not read that particular missive, and indeed I’m not likely so to do; but it certainly sounds about par for the course.

Aeromodellers in general have long seemed to me to want to bend the occasional electrical rule, I’ve noticed this for some time. Sometimes when I read about things like instructions on say aerodynamics I might think that it’s questionable but I don’t really know enough about it; however, instructions on many engineering practises that I have experienced I would definitely consider are at best misleading, if not completely wrong. At least in my opinion, anyway. This, for me, is really the all important fact. Years ago there was a fair degree of electrical modelling DIY, you could even make your own radio, with the likes of Pecon and Micron, perhaps even today this does still exist in a limited fashion, but I’m really wondering to what extent multimeters are now used? I suspect that very few of the reprobates at our strip own one, but there are a fair amount of electric models. I’m always being asked to check something or other.

With regard to analogue verses digital, very accurate voltage measurement is very easy, and cheap, these days. So I’m sure a digital would be fine. The only trouble might be with a real cheapo is the reliability of the mechanical bits, one segment of the display disappearing from time to time, say, this can be confusing to say the least; and this definitely can happen! Does the article mention and explain the ‘ohms per volt’ input impedance of the analogue meter? This can be significant, as mentioned by Plummet. Or the fact that when checking a diode say, the positive (red) lead is negative, thus making the negative return positive? This might lead the uninitiated on a merry dance for a while; fortunately this is not so on a digital. A quality analogue meter will have a fast dropout mechanism, so that in the event of the wrong connection it will quickly disconnect the meter. A cheaper variety doesn’t have this, so the indicating pointer will indeed bend itself around the stop pin at the end of the scale at a high rate of knots; again, it definitely can happen! But, nevertheless, an operator familiar with his high class AVO or equivalent, with it’s mirror-backed scale etc. will be able to take very accurate readings indeed.
But also by the same token, the very high input impedance of a digital can have a slight downside, when working in the close proximity of high voltages and currents this can results in some extraneous readings appearing on the display, plus RF, this can radiate slightly from other equipment rather than transmitters, you have to be careful of what you are looking at. I’m sure that won’t apply to modelling, though. Perhaps not very significant here, but back around 1980, when some super-duper new electrical power supply equipment came on the scene the only meter to fall within the fairly stringent test equipment specification was a very expensive (back then) digital meter.

Incidentally, a high class digital meter will measure the AC on the motor leads perfectly, the problem I always have is finding a suitable point to connect to…

I, too, won’t be changing my digital for an analogue any time soon, if ever even, now…….

PB

[Masher]

"I made a New Year’s Resolution to myself years ago never to read electrical articles in aeromodelling magazines, much better to consult electronics magazines; or perhaps even better, a text book. I’ve not read that particular missive, and indeed I’m not likely so to do; but it certainly sounds about par for the course."

Seems a good idea!

[John Privett]

Posted by Peter Beeney on 26/12/2013 14:34:58:

Does the article mention and explain the ‘ohms per volt’ input impedance of the analogue meter? This can be significant, as mentioned by Plummet.

Yep - that gets a mention, though the explanation is brief and perhaps a little "dumbed-down" - though I think that is deliberate, to suit the audience, rather than through any lack of knowledge on the part of the author.

[PatMc]

Haven't seen the article & TBH I wouldn't have read it if I had.
About the only practical advantage of an analogue over a digital that I can think of is that you can measure voltage & current without a battery in an analogue meter. There are digital devices for measuring the voltage of lipos at the balance leads (& probably other similar devices) without their own battery but they are of limited use.

[Peter Beeney]

‘Yep - that gets a mention, though the explanation is brief and perhaps a little "dumbed-down" - though I think that is deliberate, to suit the audience, ’

Yes indeed John, I have to say, and with the greatest respect, of course, I think these might have been the very words I was looking for! It would appear to very conveniently sum the whole thing up and then pop it neatly in a nutshell!

Talking of digital meters, years ago our Boss was in the habit of handing out Crippy prezzes to our little team, one year it was a Beckman DM78 digital multimeter. This was ultra thin, a touch bigger than a standard playing card in a plastic wallet; and it was designed to fit in a shirt pocket. Driven by a couple of coin cells, which only want changing every other year or so, it has self-ranging DC and AC scales up to 250 volts, a resistance measurement and a continuity/diode check with a beep. The wallet is now showing signs of wear, but the permanently attached leads are still like new; it even chirps at you if left unattended for a while, a switch off reminder. About the only thing that’s missing is a backlight…

The little Beckman now lives in the flight box, and is now getting on for 40 years old. It’s an RS stock number, 621-994. Still keeps perfect time and I would not be surprised if there were not a modern version. There is quality about; like all my old kit, it still keeps going strong. Even the Nitech torch, another Santa Supplement, with it’s original nicad battery that as yet shows no sign of flagging what so ever! I’m considering the situation is now becoming ever so slightly serious, I have this feeling in me water that there is now every chance it will collectively see me off the premises, eventually….

PB

[michael cawood]

this is what I use. well I am quite old!

[buster prop]

I think that meter is the Avo 'Minor', lovely! The one I used (professionally) years ago was the Avo 8, a big square job. That was 20,000 Ohms/Volt and had a 1.5 Volt D battery and a 15 volt one for the high resistance range. I don't know if you can get those 15V batteries any more. The 8 had a mirror scale to eliminate parallax errors. Like everyone else I now use digital meters, a Fluke that I inherited from a radio ham and a cheapie Chinese one. Both excellent and reliable. To get back to the OP, an advantage of analogue meters is that it's easy to detect a peak or dip in current or voltage, as you would need to do when tuning radio circuits. Its easier to see a peak meter needle movement than reading changing numbers. No need to do that any more so digital meters are better for general measurements. One of my meters has a 'beep' continuity function.

[Phil Green]

Haven't read the article, but... "Digital" doesn't mean "accurate", the dual-slope integration method used by almost all "digital" meters is an analogue process using constant current generators and op-amp comparators. Neither does "Resolution" have anything to do with accuracy! When Maplins had a BOGOFF offer on their little yellow DMM, I bought 4, and with all 4 leads paralleled up, there was a mismatch of anything up to 200mV when measuring a common 9v PP3 battery!

I would suggest that when measuring a low impedance source an AVO 8 is equally as accurate as some cheap DMMs, possibly more so (not HP, Wayne Kerr, etc obviously). On the other hand, digitals are quicker and easier and have a high input impedance with no ohms/volt compensation needed.

But at the end of the day, its horses for courses. Try peaking a transmitter strip or a receiver IF with a digital

Cheers

Phil

 

Edited By Phil Green on 28/12/2013 16:29:41