Buying a Multimeter

[khaffra]

Fluke 70 series are classed as 3&3/4 digit as they measure to 3200 digits before ranging up, as far as I am aware and I have calibrated thousands of them they are all this, Earlier versions 8020's etc are 3&1/2 which has a full scale of 1999. Newest versions 175 etc have a full scale of 6000.

 

Flukes never go out of spec and are always miles better than the spec qouted they will always be only a maximum of a couple of digits out and almost never go wrong.

 

Cheap meters sometimes struggle to be within spec from new and if used often the switches tend to wear causing intermittent readings especially on ohms, a lot of them go out of spec quickly so the specs given are meaningless.

 

For Boat use I would have a fluke everytime, but a cheap meter is fine for 95% of everything you are ever going to measure and much better than nothing.

[Gibbo]

Fluke 70 series are classed as 3&3/4 digit as they measure to 3200 digits before ranging up, as far as I am aware and I have calibrated thousands of them they are all this, Earlier versions 8020's etc are 3&1/2 which has a full scale of 1999. Newest versions 175 etc have a full scale of 6000.

 

I'll take your word for it. There's just summat in my head telling me a very early one was actually only 3 1/2 digit and when it got to say 19.99 volts it swapped over to 20.0 volts. It maybe my memory failing me. I think it it was a model 73 but could be mistaken.

 

Flukes never go out of spec and are always miles better than the spec qouted they will always be only a maximum of a couple of digits out and almost never go wrong.

I think you're being pessimistic there! If I put *all* of mine on a calibrated 12.60V supply they *all* read *exactly* 12.60 volts. Not even a single digit out! Everyone of them.

 

Cheap meters sometimes struggle to be within spec from new and if used often the switches tend to wear causing intermittent readings especially on ohms, a lot of them go out of spec quickly so the specs given are meaningless.

 

For Boat use I would have a fluke everytime, but a cheap meter is fine for 95% of everything you are ever going to measure and much better than nothing.

 

You reckon he's convinced he needs a Fluke yet?

 

Gibbo

[blackrose]

What about a Fluke 15B like this one. Any good?

 

http://cgi.ebay.co.uk/PROFFESSIONAL-FLUKE-...1QQcmdZViewItem

Edited February 3, 2007 by blackrose

[anhar]

...I keep a cheap meter on the boat (two for £5 at Maplins last year, including the 9v battery) and I check it against the Fluke about twice a year. I've just thrown one away because it failed that test, and replaced it with another. I looked at the spec for it, on the box it says "basic accuracy 0.5%" which sounded pretty good. Then I read the leaflet that came with it, where it quotes accuracy as plus or minus "0.5% of range maximum, plus 1.25% of displayed reading, plus 10 displayed digits". So if you're on the 20v scale and it's reading 12.00, the accuracy is within +/- 100mV plus 150mV plus 100mV, which gives +/-350mV, a total spread (ie the possible difference in readings between two identical meters) of 700mV in 12V, nearly 6 percent. On the same battery at the same time, one would say it was 90% charged and one would say it was 20% charged.

 

Allan

Hello Allan

 

After reading your above message I checked again the spec. of the cheap meter from Machine Mart I have which I mentioned above. For the 20V range, accuracy is quoted as ±0.5% ±2 digits with a resolution of 10mV.

 

Taking your example then, a reading of 12.00V would be within ±60mV and ±20mV, a total of ±80mV in the worst case where the two accuracies were adding to each other, equivalent to a maximum spread of 160mV or 1.3% of the 12.00V reading. This is substantially more accurate than the 5.8% of your cheap meter although at £7.04 it is a lot dearer

 

I can't know whether my meter actually sticks to the claimed accuracy ranges or whether they are being somewhat economical with the facts, but for a complete electrical airhead like me and most other people apart from the pros and dedicated amateur types around here, it's probably good enough I suspect, as Gibbo suggests.

 

Steve

Edited February 3, 2007 by anhar

[blackrose]

This is *the* workhorse of multimeters.....

 

http://cgi.ebay.co.uk/FLUKE-75-MULTIMETER-...1QQcmdZViewItem

 

I'm bidding on this, so if anyone else is please PM me & I'll drop out so we're not bidding against each other.

 

Thanks

[dor]

Just a comment: make sure the meter you buy has auto-power-off. Otherwise if you are like me it will cost you a fortune in batteries!

[chris w]

Hello Allan

 

After reading your above message I checked again the spec. of the cheap meter from Machine Mart I have which I mentioned above. For the 20V range, accuracy is quoted as ±0.5% ±2 digits with a resolution of 10mV.

 

Taking your example then, a reading of 12.00V would be within ±60mV and ±20mV, a total of ±80mV in the worst case where the two accuracies were adding to each other, equivalent to a maximum spread of 160mV or 1.3% of the 12.00V reading........................ it's probably good enough I suspect

 

Steve

 

1.3% on an indicated measurement of say 12.4v +/-2 digits means the actual reading is somewhere between 12.3v and 12.5v which equates approximately to a charge state spread of 60%-80%. This will give you a reasonable guide as to charge state.

 

Chris

[Theo]

John, you are totally incorrect in that accuracy is not an issue. It's very important on boats with regard to the battery state.

 

Yesterday, for example, I met up with Mike on "Blackrose" and he asked me that very question in regard to his digital multimeter which looked the business. However, on checking the manual, the accuracy on the DC ranges was quoted as +/-2%. This is OK for a general measurement but is totally useless for checking battery terminal voltage in order to ascertain the state of charge. One needs a meter for this purpose with an accuracy of at least +/-0.5% because a 0.1v difference in terminal voltage equates to approximately a 10% charge difference.

 

So, if you measured for example a battery voltage of 12.4v with a +/-2% meter, the actual reading could be anywhere from 12.65v (100% charge) to 12.15v (50% charge). ie: useless information.

 

You don't have to pay the earth though for a good enough meter. Maplin sells a range (the Uni T range) which have +/-0.5% accuracy and even includes things like temperature measurement, frequency measurement and capacitor measurement plus all the usual stuff for around £25.

 

If you just want to know whether 12v is getting to a particular point, a 12v "screwdriver" tester for £5 or less is much more convenient to use.

 

Chris

 

One very useful and very cheap bit of test equipment:

 

40' length of black multistrand test lead fitted with a 4mm plug at each end and a croc clip for each end too.

Ditto Red

1 12v bulb with test leads attached in some way. (I soldered mine on) also 4mm plugs and croc clips. The plugs plug into the croc clips that I have. They are just little ones.

 

I have but this lot together because there are going to be times when I am going to have to test the continuity of a wire that starts at the fuse board and ends in a light fitting way along the boat.

 

Nick

 

Edit for typo

Edited February 4, 2007 by Theo

[andywatson]

If you missed it, please do read my post a few back regarding the importance of meter accuracy with regard to boats. Basically without +/-0.5v accuracy or better on the 20v DC range it's pretty useless other than for a go/no-go test on voltage.

 

Chris

 

Thanks. I've just had the instruction manual out that came with it and checked. The claimed accuracy at 20V is +/- 0.5%.

[Theo]

Thanks. I've just had the instruction manual out that came with it and checked. The claimed accuracy at 20V is +/- 0.5%.

 

 

Hm...

 

It looks as if I need a Fluke and a cheapo!

 

Nick

 

Thanks everyone.

 

Edit to add thanks

Edited February 4, 2007 by Theo

[Blue_skies]

Mine is a modern digital AVO (Multimeter M2006). I don't know if they still make them but this one is very well built and reliable. I've had it about 15 years. It was probably about £100/£200 from RS, but I can't remember. I think it's worth paying for a quality one that will last and be accurate. I'm currently getting rid of a lot of equipment, downsizing ready to make the move to a boat in the Summer. However, I think I will invest in a digital oscilloscope for the boat. I used to use a hand-held one at work and found them very good for fault finding. I've a couple of CRT oscilloscopes but will probably sell them as they're quite bulky.

[khaffra]

Mine is a modern digital AVO (Multimeter M2006). I don't know if they still make them but this one is very well built and reliable. I've had it about 15 years. It was probably about £100/£200 from RS, but I can't remember. I think it's worth paying for a quality one that will last and be accurate. I'm currently getting rid of a lot of equipment, downsizing ready to make the move to a boat in the Summer. However, I think I will invest in a digital oscilloscope for the boat. I used to use a hand-held one at work and found them very good for fault finding. I've a couple of CRT oscilloscopes but will probably sell them as they're quite bulky.

 

That is a pretty good multimeter not quite as foolproof as most flukes but you sound like you know how to use it so it should be fine for many more years.

Digital scopemeter for fault finding on a boat sounds a bit over the top to me, but if you have one then that is brill, excellent bits of kit.

 

I can't say to much really as I did use a very expensive datalogger on my fridge to test its internal temps while doing some ventilation experiments over a couple of days.

[Gibbo]

The other day I just happened to be doing something at work that required a few multimeters. Whilst they were on the bench I stuck them all on a single PSU. They are all Fluke.

 

 

Try that with other meters and you will get anything between 12.50 and 12.70 on the display. Try it with cheapo meters and you will get between 12.35 and 12.85.

 

The ones showing 12.59 and 12.61 (less than 0.1% error) have been sent off for calibration because that's just not good enough for a Fluke.

 

Gibbo

Edited February 11, 2007 by Gibbo

[Ernie]

The other day I just happened to be doing something at work that required a few multimeters. Whilst they were on the bench I stuck them all on a single PSU. They are all Fluke.

 

 

Try that with other meters and you will get anything between 12.50 and 12.70 on the display. Try it with cheapo meters and you will get between 12.35 and 12.85.

 

The ones showing 12.59 and 12.61 (less than 0.1% error) have been sent off for calibration because that's just not good enough for a Fluke.

 

Gibbo

What makes it even more impressive is the age of some of those meters !!

 

I remember using the ones on the right some 20 years ago........Makes me feel old !!

[khaffra]

What makes it even more impressive is the age of some of those meters !!

 

I remember using the ones on the right some 20 years ago........Makes me feel old !!

 

yep, all those meters are at least 15-20yrs old I would estimate.

[Keeping Up]

The ones showing 12.59 and 12.61 (less than 0.1% error) have been sent off for calibration because that's just not good enough for a Fluke.

How do you know that one of them wasn't the only one showing the right voltage?

 

Allan

[Gibbo]

yep, all those meters are at least 15-20yrs old I would estimate.

 

Yep, and they just keep on working. Hence my preference for Fluke. Absolutely nothing else lasts like they do.

 

Gibbo

 

How do you know that one of them wasn't the only one showing the right voltage?

 

Allan

 

Well I don't. Apart from 11 meters and 1 PSU all showed exactly the same voltage and two showed different. Just playing the numbers game really. The chances of all the others having drifted by exactly the same amount in the same direction must be pretty close to zero I would have thought.

 

Gibbo

 

What makes it even more impressive is the age of some of those meters !!

 

Exactly. I have some other Flukes that make those in the photo look modern. They still read accurately.

 

Gibbo

[Keeping Up]

The chances of all the others having drifted by exactly the same amount in the same direction must be pretty close to zero I would have thought.

I agree with you 100% (but that isn't a high enough percentage to prevent me adding the odd comment like that)

 

Allan

[khaffra]

I agree with you 100% (but that isn't a high enough percentage to prevent me adding the odd comment like that)

 

Allan

 

TBH if you sent them for calibration then they would come back exactly the same, A calibration company would only adjust them if they were out of spec or very close to the spec, Not wishing to get to technical but your uncertainty of reading is +/- 1digit anyway as they could be about to flash across one way or the other. Therefore they are all as good as you can expect.

 

If you want to have a play there are normally only 2 main pots inside them anyway, one will adjust AC and one DC so if you have a fiddle you can get them all spot on.

[Theo]

TBH if you sent them for calibration then they would come back exactly the same, A calibration company would only adjust them if they were out of spec or very close to the spec, Not wishing to get to technical but your uncertainty of reading is +/- 1digit anyway as they could be about to flash across one way or the other. Therefore they are all as good as you can expect.

 

If you want to have a play there are normally only 2 main pots inside them anyway, one will adjust AC and one DC so if you have a fiddle you can get them all spot on.

 

 

But you do need to have a reliable standard to check them against!

 

N

[Gibbo]

TBH if you sent them for calibration then they would come back exactly the same, A calibration company would only adjust them if they were out of spec or very close to the spec, Not wishing to get to technical but your uncertainty of reading is +/- 1digit anyway as they could be about to flash across one way or the other. Therefore they are all as good as you can expect.

 

Actually last time I sent some for calibration because they were one digit out they came back spot on.

 

If you want to have a play there are normally only 2 main pots inside them anyway, one will adjust AC and one DC so if you have a fiddle you can get them all spot on.

Indeed. But because of some of the markets we sell into we get the odd customer who wants to see calibration certs.

 

Gibbo

 

I agree with you 100% (but that isn't a high enough percentage to prevent me adding the odd comment like that)

 

Allan

 

It's exactly the sort of comment I would have made

 

Gibbo

[khaffra]

Actually last time I sent some for calibration because they were one digit out they came back spot on.

 

Indeed. But because of some of the markets we sell into we get the odd customer who wants to see calibration certs.

 

Gibbo

It's exactly the sort of comment I would have made

 

Gibbo

 

Which cal lab do you use?

[Gibbo]

Which cal lab do you use?

 

The current ones have gone through Farnells. I'd thought I'd give them a try as I use them for all sorts of other supplies. I *think* the previous lot went to Haven but I'm not 100% sure. I'd have to check at the office.

 

There also used to be a small (3 blokes) calibration lab local to me who did a brilliant job but they aren't UKAS accredited even though they do a better job than some that are.

 

Gibbo

[scribe]

I am picking up on needing an accurate meter to measure battery capacity. OK I need to catch up fast.

I am just about to set off on a pre-loved boat with 3 meters for power management. 2 Ammeters, analogue. 0-10amp for wind vane generator and solar cell input. Then one similar meter, analogue, 0-15volt which is telling me battery condition. This boat is a "budget" price aquisition, and quite a number of things are scheduled to be "fixed" on my voyage!

I have just the one 245 Ah gel leisure battery, and I`m very keen to keep an eye on its charged state.

The volt meter I just mentioned may be a bit of a noddy device because from what I have just read I will need to have a good pair of glasses to tell if I have 10 or 90% charge available.

Can anyone lead me to a table reading off voltage against %age capacity available, please? I have learned that +/- 0.1 volt tells me a lot, and for the time being its a magnifying glass task.

Down the track a bit, is it a smart idea to solder a resistance across the meter terminals to modify the range of this analogue instrument so that its full scale deflection is useful-not just the top end?

Many thanks

[chris w]

I am picking up on needing an accurate meter to measure battery capacity. OK I need to catch up fast.

Can anyone lead me to a table reading off voltage against %age capacity available, please? I have learned that +/- 0.1 volt tells me a lot, and for the time being its a magnifying glass task.

Down the track a bit, is it a smart idea to solder a resistance across the meter terminals to modify the range of this analogue instrument so that its full scale deflection is useful-not just the top end?

Many thanks

Re changing the range: A resistor is not what you want. A resistor will change the range BUT the meter will still read from zero up to some value. You want it to read a restricted range of say 10v- 15v.

 

You need to solder a zener diode in series with the meter. So if, for example, the meter currently reads 0-15v and you want 10 - 15v then solder a 10v zener in series with the meter (the cathode of the zener towards the plus of the meter). You will probably have to use two 5.1v zeners in series to get 10v.

 

However, if the meter is less than +/-0.5% accuracy, it is useless for measuring battery state of charge. If it's +/-2% for example, a reading of 12.4v (nominally 70% charge) could be anywhere between 12.65v (fully charged) and 12.15 (50% charge), viz: useless.

 

The table of battery charge vs voltage is as follows:

 

12.7v = 100% charge

12.6v = 90% charge

12.5v = 80% charge

12.4v = 70% charge

12.3v = 60% charge

12.2v = 50% charge

 

Do not let the batteries fall below 50% charge or you will compromise their life. Also, the above readings MUST be carried out with NO LOAD on the battery or the result is meaningless. Do not take a reading immediately after charging because it will be artificially high - switch on your tunnel headlight for 10-15 minutes first, switch the light off again and then take the reading.

 

Chris