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I find myself in need of some way of calibrating a couple of ammeters so they read the same. I will need a load of about 25amps or more (they are 300amp meters).

Back from the days when I used to build 12v PSU I seem to remember load testing using 0.5ohm 100w tubular wire wound resistors in single or parallel to give 25 or 50amps load, when in use these were left in a bucket of water which could boil after a while. I don't think the modern encapsulated ones will do as there wont be enough thermal transfer and they will self destruct.

Does anyone have a cheap source of these resistors or another way of loading 25/50 amps. I have found some chinese ones but on long delivery times :(

 

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3 minutes ago, Jess-- said:

light bulbs are a common choice and are usually close enough to their rating to be usable for calibration

Would have to be headlight bulbs to get the current required, or 15/30  21w indicator bulbs. ;)

 

6 minutes ago, Onewheeler said:

A few m of 22 swg wire in a bucket of water? (Other gauges are available).

Much the same as the resistors but can my brain calculate the correct length. 

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1 minute ago, WotEver said:

300W 0.5R available ex-stock from Farnell:

https://uk.farnell.com/multicomp/mc14654/wirewound-resistor-0-5-ohm-300w/dp/9456805
 

That’d give you about 25A. 

Glad I was sitting down when I saw the price but that is exactly what is needed 

Just now have to justify the cost.

Its only happened because my Handheld meter and the two meters all have different readings for the same load :(

 

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1 hour ago, TheBiscuits said:

Immersion heater ...

We can work this out, that Ohm bloke told us how

 

Maybe 2kw at 240 volt so about 8.3 amp

so resistance is 240/8.3 = 28 ohm   so just half an amp at 12 volts

 

If the boat is available then the charge current from the alternator would be about right. Doe the op need a calibrated current, or just an arbitrary current to compare the two meters???

 

.................Dave

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33 minutes ago, dmr said:

If the boat is available then the charge current from the alternator would be about right. Doe the op need a calibrated current, or just an arbitrary current to compare the two meters???

 

Boat is 250 miles away it would be easy to do it if I was there, so many choices, charging current, immersion on inverter, the list is almost endless.

Its just me being picky really at 60amps the two meters and my UT203 are out by 5 amps from each other, this will annoy my OCD to a point where it will drive me mad. I will be calibrating making the two meters  read the same as my UT203. This may or may not be totally accurate but I trust the UT203 more than a couple of cheap Chinese meters. I did the voltage against my fluke before I got them up to the boat and that is pretty much the same as the UT203 reads.

I sometimes wish that I was normal when it comes to numbers and such, but that would take a lot out of my life ;)

 

ETA

In the glovebox of one of our cars there are two good headlamp bulbs left over from when I swapped to Osram megalight, this will give me about 18.5amps at 12.5v which is a good enough place to start.

 

Edited by Loddon
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?

14 minutes ago, Loddon said:

Boat is 250 miles away it would be easy to do it if I was there, so many choices, charging current, immersion on inverter, the list is almost endless.

Its just me being picky really at 60amps the two meters and my UT203 are out by 5 amps from each other, this will annoy my OCD to a point where it will drive me mad. I will be calibrating making the two meters  read the same as my UT203. This may or may not be totally accurate but I trust the UT203 more than a couple of cheap Chinese meters. I did the voltage against my fluke before I got them up to the boat and that is pretty much the same as the UT203 reads.

I sometimes wish that I was normal when it comes to numbers and such, but that would take a lot out of my life ;)

 

ETA

In the glovebox of one of our cars there are two good headlamp bulbs left over from when I swapped to Osram megalight, this will give me about 18.5amps at 12.5v which is a good enough place to start. I

 

Are the inaccurate meters digital or proper moving needle jobbies?  If they are proper analog then offset (incorrect zero) and even hysteresis (sticky needles) can be an issue as well as "calibration". If they are cheap digital meters then there still might be an offset error. Ideally you should check the calibration at two different currents.

Are these meters Hall effect devices?

 

The spec on some of the cheapo ammeters is quite poor, the voltage is usually half ok but the current can be something like +/- 3%, so if its a 300amp range then thats an error of 9 amps.

 

A little bit of OCD is not a bad thing, maybe it should be called ATD (attention to detail) or CAW (careful and accurate workmanship) "

 

................Dave

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Both meters are cheap chinese digital shunt jobbies accuracy is quoted as 1% +/- 2 word

One thing I have just thought of is that I only have one shunt here as the other is in circuit on the boat  so both will be calibrated to the one I have here

Knowing my luck the shunts will be slightly out but at least I will know and so long as I use the shunt that was used for calibration on the domestics I will never see a difference as two meters are on the domestic shunt with one switchable between domestic and start.

I am ignoring the fact that one meter will have 8m of cable on it and the other 1m hoping that their impedance is high enough to negate any voltage drop ;)

 

 

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32 minutes ago, Loddon said:

I am ignoring the fact that one meter will have 8m of cable on it and the other 1m hoping that their impedance is high enough to negate any voltage drop ;)

Yes, that at least should be a reasonably safe bet.
 

A few years ago I adjusted everything to match what I believed to be a calibrated Avo. It was a few months later that I compared my Avo to my Fluke which had just been calibrated and found it was a fair bit out. Like you though, I was happy that all my meters agreed (except for the Fluke) even if they were a few percent out.  I never did get around to tweaking them all again. 

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1 hour ago, WotEver said:

Yes, that at least should be a reasonably safe bet.
 

A few years ago I adjusted everything to match what I believed to be a calibrated Avo. It was a few months later that I compared my Avo to my Fluke which had just been calibrated and found it was a fair bit out. Like you though, I was happy that all my meters agreed (except for the Fluke) even if they were a few percent out.  I never did get around to tweaking them all again. 

Well at least the 3 meters on the boat will now all agree. 

I seem to have lost my two Flukes one in the move last year and one I mislaid somewhere, both were 35 years old so have served me well, I found the battery cover for one :(

The general house meter reads 0.06v at 12v higher than the one on the boat, don't know which one is correct but as they wont meet again its not a problem to my brain.

Now the question is would the output of a 7812 regulator be 11.96v or 12.02v?

To much time on  my hands ;)

 

Must go searching for my Flukes.......

 

Edited by Loddon
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28 minutes ago, Loddon said:

 

Must go searching for my Flukes.......

Found the true RMS Fluke it says 11.985v so  in the middle of the other two

That's 3 meters 3 readings all of the same source.

Can I justify the cost of getting the Fluke calibrated now I am no longer working.

Or shall I just keep the meters apart.

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19 minutes ago, Loddon said:

Found the true RMS Fluke it says 11.985v so  in the middle of the other two

That's 3 meters 3 readings all of the same source.

Can I justify the cost of getting the Fluke calibrated now I am no longer working.

Or shall I just keep the meters apart.

Get it calibrated. You know it makes sense :)

 

Betcha it comes back from calibration reading exactly the same...

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12 minutes ago, WotEver said:

Get it calibrated. You know it makes sense :)

 

Betcha it comes back from calibration reading exactly the same...

Every one I have had done over the years has not been changed all they do is send you a cert saying whether its within spec,  unless its 30% out when they will change it.

What I need is a known voltage source then I will know how far out it is or not.

This

https://www.avdweb.nl/tech-tips/multimeter-calibration-diy#h1-why-calibrate-a-multimeter

Looks interesting, I may investigate it, then I can DIY ;)

 

Edited by Loddon
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20 minutes ago, dmr said:

A voltage reference is better than a voltage regulator:.

I got myself a cheapo chinese voltage reference from eBay, a bit like this:

That's a cool idea I think I will go down a similar route.

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1 hour ago, Loddon said:

Found the true RMS Fluke it says 11.985v so  in the middle of the other two

That's 3 meters 3 readings all of the same source.

Can I justify the cost of getting the Fluke calibrated now I am no longer working.

Or shall I just keep the meters apart.

Old Chinese proverb, Man with working watch know the time, Man with two working watches never knows the time.

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