ebay panel voltmeter accuracy and a question..

[mjcoon]

And if you connect them [high and low sensitivity voltmeters] at the same time?

As I suspect you already knew, you get the same reading on each, slightly lower than the lower of the two separate ones.

Mike.

 

[GHA]

If you don't understand the difference between precision and accuracy, it's probably best you leave the measuring to someone who does.

I do understand the difference between precision and accuracy.

Perhaps you could explain then, with reference to measuring boat battery voltage, how a error of 0.02v is inaccurate.

And that's assuming the Fluke 110 is the more accurate.


And you failed to answer the question, where will I find a voltage source that has a low (internal) resistance onboard, it would be nice to see the difference in readings,.

 

[GHA]

As I suspect you already knew, you get the same reading on each, slightly lower than the lower of the two separate ones.

Mike.

With the eBay meter I suspect the backlight would have a greater effect in pulling voltage down a little than any current draw from input impedance.

 

[JumbleDuck]

this has sparked (ha!) a memory of Pico oscilloscopes.


A piece of hardware with various probes which you attach to a laptop.

I have a Bitscope 10 (http://www.bitscope.com/product/BS10/), a staggeringly tiny device which does everything. For simpler stuff I use xoscope (Linux) which works very nicely.

 

[elton]

I do understand the difference between precision and accuracy.

Perhaps you could explain then, with reference to measuring boat battery voltage, how a error of 0.02v is inaccurate.

And that's assuming the Fluke 110 is the more accurate.

You wrote

How can 2 meters read the same and one be inaccurate ?

- which indicates that you don't understand the difference between accuracy and precision, so I tried to explain. An error of 0.02v gives the impression that the meter is accurate, but it depends upon what is being measured. It won't necessarily be accurate under all conditions, dependent upon its internal resistance. Please don't touch my boat's electrics.

 

[JumbleDuck]

- which indicates that you don't understand the difference between accuracy and precision, so I tried to explain. An error of 0.02v gives the impression that the meter is accurate, but it depends upon what is being measured. It won't necessarily be accurate under all conditions, dependent upon its internal resistance. Please don't touch my boat's electrics.

You seem to be using a rather odd definition of "accurate" here. Precision: number of decimal places. Accuracy: correctness of these decimal places. Whether the act of measurement has affected the quantity being measured: something else entirely.

Even if the meter draws 10A and drags down the voltage it's supposed to measure, it's still accurate as long as it displays that dragged down voltage correctly. Whether it was in that case the right tool for the job is another matter.

All that aside, have you had many problems on your boat with varying voltmeter internal resistances?

 

[elton]

I disagree. It will accurately (and precisely) measures something which may, of course, be affected by the act of measurement.

Well of course the measurement may be accurate, but the purpose of measurement is defeated by the meter lowering the measured voltage.

Have you found varying internal resistances of voltmeters to give you many problems on your boat?

No, in fact most measurements are of a go/no go nature, and a 12v bulb with flying leads would suffice.

 

[GHA]

You wrote



- which indicates that you don't understand the difference between accuracy and precision, so I tried to explain. An error of 0.02v gives the impression that the meter is accurate, but it depends upon what is being measured. It won't necessarily be accurate under all conditions, dependent upon its internal resistance. Please don't touch my boat's electrics.

You are resorting to misquoting now.
20mA difference on the boats main batteries. It's obvious accuracy is being discussed in terms of measuring a boats battery systems.

And where will I find a voltage source that has a low (internal) resistance onboard.

Your boat is safe, close to zero chance of me setting foot onboard with a condescending grumpy git already there.

 

[JumbleDuck]

Well of course the measurement may be accurate, but the purpose of measurement is defeated by the meter lowering the measured voltage.

Almost all meters do that. It's a matter of knowing how much, and whether it matters.

 

[elton]

You are resorting to misquoting now.
20mA difference on the boats main batteries. It's obvious accuracy is being discussed in terms of measuring a boats battery systems.

And where will I find a voltage source that has a low (internal) resistance onboard.

Your boat is safe, close to zero chance of me setting foot onboard with a condescending grumpy git already there.

The boat's lead/acid battery is a voltage source that has a low internal resistance.

I don't know what you're referring to with "20mA difference on the boats main batteries".

 

[GHA]

The boat's lead/acid battery is a voltage source that has a low internal resistance.

I don't know what you're referring to with "20mA difference on the boats main batteries".

Are you suggesting the eBay voltmeter could pull down the voltage on the boats batteries? Because it doesn't.

20mA is the difference between the reading on the Fluke meter and the eBay meter, the Fluke meter reads the same if the eBay meter is connected or not.


And anyway you completely missed the backlight, likely to pull much more current the a lowish impedance.

Where is the inaccuracy?
We're measuring on a boat not a pcb.

 

[vyv_cox]

I fitted one of those £8 jobs from EBay and it seems to work OK. Access to the batteries with the multi meter was/is poor so I fitted one instead. I fitted a switch so I could test the engine battery or the domestic one.

I was told that I really needed a £100+ battery monitor but I ignored that advice and bought a beer, and quite a lot of other stuff too.

Me too. I bought a combined volts/amps meter from a UK supplier, unfortunately not around any more. It has been excellent. I also have a combined V/A meter from China, (on another boat) about £8, that also seems to work well and gives me the info I want. I'm not too bothered about the difference between 12.6 and 12.62 volts, just between 12.6 and 12.0.

 

[GHA]

A few more measurements...
The Fluke draws about 1.5microA at 14.5v which I make to be 9.8Mohm, close to the spec sheet 10Mohm.
14.5/(1.5 * 10^-6)

The eBay meter pulls 9.8mA with the backlight on or 5.3mA without. But as the power for the meter comes from the circuit being measured I can't see how to calc the input impedance from that.
A meter across the leads shows 2.5Mohm.
But whatever it is it's going to do nothing to the voltages being measured from a boat battery.

Feel free to check the calculation and highlight any errors, as elton so selflessly points out, I'm a Billy no nothing who shouldn't be allowed to turn on a nav light.

 

[elton]

Are you suggesting the eBay voltmeter could pull down the voltage on the boats batteries? Because it doesn't.

No, I'm not suggesting that. I mean it will, but not to any measurable degree.

20mA is the difference between the reading on the Fluke meter and the eBay meter, the Fluke meter reads the same if the eBay meter is connected or not.

You're not measuring current though, are you? So a mA reading is meaningless.

And anyway you completely missed the backlight, likely to pull much more current the a lowish impedance.

Where is the inaccuracy?
We're measuring on a boat not a pcb.

Both the boats I've owned, and many I've looked at, had electrical faults that demonstrated an amateur had carried out some really shoddy work. A little bit of knowledge seems to instil a lot of misplaced self confidence. Electrical theory isn't that difficult to grasp; it just requires a serious intent to understand, and a bit of basic maths.

 

[GHA]

You're not measuring current though, are you? So a mA reading is meaningless.

typo, meant mV.

Both the boats I've owned, and many I've looked at, had electrical faults that demonstrated an amateur had carried out some really shoddy work. A little bit of knowledge seems to instil a lot of misplaced self confidence. Electrical theory isn't that difficult to grasp; it just requires a serious intent to understand, and a bit of basic maths.

So what you're saying is i'm have no maths or electronics skills and should give up before someone gets hurt.

All because of an accurate ebay meter.

There's nothing to learn from your posts, another one for the ignore.

 

[mjcoon]

20mA is the difference between the reading on the Fluke meter and the eBay meter, the Fluke meter reads the same if the eBay meter is connected or not.

Since we thought the discussion was about voltmeters it is confusing to report current...

(Millivolts "mV" isn't much better; that's a jolly flat battery!)

Mike.

 

[GHA]

Since we thought the discussion was about voltmeters it is confusing to report current...

(Millivolts "mV" isn't much better; that's a jolly flat battery!)

Mike.

Yep, typo, meant mV . 20mV is the difference between the reading of the eBay volt meter and a Fluke. So plenty accurate enough for a boat.

 

[aquaplane]

Me too. I'm not too bothered about the difference between 12.6 and 12.62 volts, just between 12.6 and 12.0.

Fit for purpose then, what more could a man ask for?

 

[chrisbitz]

Feel free to check the calculation and highlight any errors, as elton so selflessly points out, I'm a Billy no nothing who shouldn't be allowed to turn on a nav light.

This conversation is hillarious!

You could supply all the proof and common sense in the world, and the naysayer(s) will STILL try and argue that your 12v meter for measuring whether a light is on or off is *WOEFULLY* inaccurate and a waste of £8! And if you don't understand where you're completely wrong, I'm not going to tell you!

 

[elton]

This conversation is hillarious!

You could supply all the proof and common sense in the world, and the naysayer(s) will STILL try and argue that your 12v meter for measuring whether a light is on or off is *WOEFULLY* inaccurate and a waste of £8! And if you don't understand where you're completely wrong, I'm not going to tell you!

You can't read.