Help on buying a multimeter

[mhennessy]

The best voltage resolution is 1 microvolt. The best current resolution is 10nA. Hence the shunt is 100 ohms.

Inside the meter, R23 and R24 form the shunt - they are 99 ohms and 1 ohms respectively. The reason for the split is to provide an option to do milliamps on the main input, but they have chosen to use the 0.01R shunt on the 10A socket to do millamps as well. They could have replaced the 1+99 with a 100R, but they made a decision not to.

As a result of this, there are gaps in the current ranges which have upset some folk. Basically, it goes something like this:

Range      Resolution   Socket
-----      ----------   ------
99.99µA    10nA         Shared +ve input
999.9µA    0.1µA        Shared +ve input
999.9mA    0.1mA        Dedicated 10A input
9.999A     1mA          Dedicated 10A input

So 2 shunts (100R and 0.01R) and range switching is effected by switching in x10 gain in the front end of the IC.

Back to the point, 100 ohms isn't a short circuit (hence why I said "near short-circuit" earlier), but it could seriously upset the operation of a DUT - especially a valve circuit with high voltages and high impedances. The shunt resistors (MELF types) are in series with a 200mA fuse which will hopefully protect the meter and circuit in low-energy situations, but note that these AN8002/8/9 meters use miniature fuses - the larger ZT301 uses conventional 20mm types (yet another reason to pick that over the AN8002/8/9). These miniature fuses are why the manual states a voltage limit of 25V AC or 36V DC when measuring current - they're not warranting that the fuses will safely interrupt the current at higher voltages.

None of the above is saying the AN8008 is a bad meter; it isn't - I have one and I use it often. But for someone who is new to electronics, I would always recommend a meter that does not put current measurements onto the same socket as all the other measurements like volts and ohms. It's perhaps easy for some of us to forget a time when all this stuff wasn't second-nature to us - but we all start at the beginning.

[philthespark]

I had an interesting argument regarding multimeters with a so called fellow professional, he argued that a fluke for around £200 would survive being connected to a live circuit while being on the ohms range, whereas a cheap meter wouldn't, in my eyes a pointless argument.When I was taught about meters as an apprentice it was drummed into me to ALWAYS make sure it was on the correct range for what you are testing. As a professional I have a few meters, some AVO's, a couple of Flukes, and a couple of cheap ones, a lot of problems people have with meters is not knowing how to "drive" them.
I'm used to using an expensive meter, but if I was in a situation without my Fluke, I could on over 90% of situations locate the problem with a meter costing a fiver. Sometimes depending on where I'm working I may actually chose to use a cheaper meter, working on a bench, or in a nice clean control panel then out comes the Fluke, put me in a dirty or muddy environment, or somewhere with a chance of stuff getting nicked, and out comes the cheap one.
It's horses for courses, I use mine on 415V quite a bit and due to H&S I have to use a meter that meets certain standards, if you are just playing at home then a cheaper meter will do, however I would be wanting fused test leads and these can often be dearer than the meter itself if you are buying a cheap one. Normally they are rated in milliamps so it's important not to use them when measuring currents, however when measuring anything over about 50V then, in my opinion, they are a must, even more so if working on the mains.

[paulsherwin]

Professional electricians have different needs to hobbyists working on old radios though. I would never use a very cheap meter to work on mains wiring, other than to make a basic check of the voltage coming out of a domestic socket. There's a good reason why these guys charge so much

[Mistermal]

Thank you guys for your input. A bit overwhelming for a beginner but I’ve read, read and read and I’m doing my best to take it all in to make an informed decision. Thanks again guys

[ColinTheAmpMan1]

I think most of us here did the same thing with regard to buying a first multimeter.

I bought the best I could afford when I was still at school, dreaming of being wealthy enough to buy a really good one. In my case, that would have been an AVO, but some of our younger members might have yearned for a Fluke of some type. Well, all good things come to those that wait and I eventually got my AVO 8 Mk. V and also a Fluke 175. Along the way I also got a couple more multimeters from Maplin or elsewhere.

I tend now to consider the Fluke 175 as the standard by which the others are compared, even though it isn't calibrated.

I think electronics hobbyists have much more choice than I had all those years ago, what with sales over the internet and suchlike. Not quite as simple as looking in the shop-windows on Dale Street.

I don't think you can go wrong with any of the suggestions that have been made, but don't forget to dream of sometime owning a top-of-the-range Fluke, or whatever, one day......

Colin.

[Mistermal]

Thank you Colin. I’m going to get what I can afford one day and then one day the fluke and a vintage avo just because I can. Thanks again.

[Radio Wrangler]

You can sometimes get lucky. I bought an older model Fluke at an amateur radio rally. four and a half digits (so 10 times better resolution than usual) True RMS etc etc, but no autoranging on this one. Twenty quid!

I checked it over rather carefully before it even went near any high voltages.

Maybe a second hand meter and its attendant uncertainties is not the best choice for a beginner, but once you get some experience, you'll find more opportunities.

David

[Mistermal]

Thank you David, I’m sure I will one day. Hopefully find a good fluke or a vintage Avo just for the sake of having one.

[music-centre]

I would check out the display on a second hand Fluke if you intend to buy one - I saved up some years ago and bought a new Fluke meter after using cheap ones for years and its been getting the faded digit/display fault for ages - wish I had stuck with the cheap ones
Steve.

[The Philpott]

Yes, an AVO Model 8 Mk II or Mk III (or Model 9 Mk II) does tricks that a digital multimeter won't (and of course the reverse applies) Purchased online from an auction/buying/selling website will invariably get you one which requires a little attention- but more often than not if the movement is satisfactory they can be fixed given knowledge/advice and a steady hand. Your chances of damage in transit are reduced if the vendor is asked to set the meter to DC 50uA and short between the + and - terminals. Ask to see the battery compartment before buying- some of them are green by now.

I didn't want to recommend a 'sixties Avo until someone else did, as i am prejudiced in their favour....and similarly prejudiced AGAINST the later models of Avo.

There's something a bit screwy about having to mend the equipment that you purchased in order to mend something else - but it is rewarding.

Dave

[Mistermal]

Thanks Steve and yes Dave I know what you mean. I’ll probably just keep an eye out for one as and when but I won’t go out to buy one specifically. I go to some vintage and antique shops aswell as Car boots so who knows what I might find. Thanks again.

[David G4EBT]

Originally Posted by music-centre

I would check out the display on a second hand Fluke if you intend to buy one - I saved up some years ago and bought a new Fluke meter after using cheap ones for years and its been getting the faded digit/display fault for ages - wish I had stuck with the cheap ones
Steve.

Flukes are legendary for fading displays. A search on 'fading fluke displays' threw this up - 4.9 million hits:

https://www.bing.com/search?q=fluke+...&setlang=en-US

Repair kits are available for a range of Flukes and there are lots of youtube videos on how to effect repairs for those so inclined. Owning and repairing aging Flukes and Avos, which can be rather 'needy' is something of a hobby in its own right, enjoyed by many, but is something of a distraction for hobbyists with other interests, such as restoring vintage radios. I find the striving for accuracy to several decimal places really quite surreal. I'm not in a lab helping to put rockets into space - I'm in a shed, restoring radios as a hobby.

I've got quite a mixture of digital and analogue meters some were literally 'buy one for a fiver, get one free' from Maplin, just for in the car boot etc. On the voltage ranges, using a homebrew EPE ('Silicon Chip') precision 10V reference they're all very close to one another. A pic of my Toolzone digital multi-meter is attached, which as I said earlier, for a sub £15.00 meter, ticks all the boxes for hobbyist use. I could easily afford a new Fluke, but can't think why I'd want one, less still need one.

Most diagnostic work on old radios involves continuity and resistance checks, often on components which - when new had a tolerance of 10 0r 20% +/-. Likewise, stated voltages on service data vary according to mains voltages and mains transformer/dropper settings, so 10% either way is generally neither here nor there, and will vary according to whether a digital or analogue meter is in use.

Just musing really.

[Mistermal]

Thank you David, it’s good to hear all angles of choice. Thanks a lot. Makes my choice much easier.

[AC/HL]

I read on here once, when you have one meter you know the voltage. When you have two you can't be quite sure!

[60 oldjohn]

If it has not been mentioned already, I would put "Auto Off" high on the list of features. So easy to miss switching off when leaving the shack.


John.

[Mistermal]

Excellent, thanks John.

[Craig Sawyers]

Originally Posted by AC/HL

I read on here once, when you have one meter you know the voltage. When you have two you can't be quite sure!

There is a lot of debate about the maths of this, which is a re-statement of Segal's law (which was for watches and time).

The best way of thinking about this is in terms of random walks. First you have a meter, which itself may well have an error in terms of standards for that measurement. So your single measurement rather than giving you a known voltage, might be quite wrong. Having a second meter that reads differently gives a probable voltage equal to the root sum of squares of the two readings halved. So suppose one meter reads 1.5V and the second one reads 1.75V, the most probable voltage is root((1.5^2 +1.75^2)/2)) = 1.63V

Providing the meter readings are normally distributed.

So rather than being uncertain, you are actually more certain!

The more meters you include, the certainty of measurement improves as root ((sum of squares)/(number of meters))

Craig

[philthespark]

Originally Posted by paulsherwin

Professional electricians have different needs to hobbyists working on old radios though. I would never use a very cheap meter to work on mains wiring, other than to make a basic check of the voltage coming out of a domestic socket. There's a good reason why these guys charge so much

you think we as professionals are expensive, wait until you hire an amateur.lol

[mhennessy]

Originally Posted by Craig Sawyers

The more meters you include, the certainty of measurement improves as root ((sum of squares)/(number of meters))

Absolutely - it's the same basic principle that is used when putting multiple resistors (for example) together to result in a better overall tolerance.

I'd say you need a minimum of 3 meters, on the assumption that it's unlikely that 2 meters would develop exactly the same problem at the same time.

And quite apart from all that, you often need to measure more than one thing at once.

Just about all multimeters have auto power-down. The cheapest DT830-style meters tend not to, but then there's already enough reasons to avoid those

To the OP, there's a danger of "paralysis by analysis" with these sorts of things. You're going to end up with more than one meter anyway if you stick at the hobby, so don't sweat too much about buying the first. When you come to buy another, you'll have a much better idea about what to buy. My suggestion is this:

1. Decided whether you want manual or auto-ranging. I recommend the latter - almost no-one winds their own windows down any more! But make sure it comes with a manual over-ride of those odd occasions when you need to fix range.

2. Just buy one!

Personally, at the bargain end of the market, for auto-ranging, I recommend the BSIDE ZT301 (around £16). It's true-RMS, does everything you could need and more, has a nice, large display, 8000-count, reasonable leads, good battery life. If you really want manual ranging (why?), then the BSIDE ADM08A is worth a look. There's a full review on my website. It's a much better meter than the Toolzone, having true-RMS, 6000-count, capacitance, frequency, and even one of those dreaded transistor hFE testers

Also on my website is an article about multimeter safety, with links to lots more reading. Personally, I don't care if you buy a Chinese special or an expensive Fluke - provided you've made an informed decision. Despite what some repeatedly say, there are real benefits to an expensive meter from Fluke or Keysight, but everyone is different, and everyone should be free to make their own mind up in full possession of the facts. This is engineering, so there should be no place for tribalism or inverse-snobbery here.

As I say, Just Do It

[paulsherwin]

Actually, the last couple of DT830s I bought seem to have auto power off, so maybe the chip inside has been updated. I've certainly left one on for a few days and got away with it, whereas the earlier ones would just flatten the battery.