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Amperage conversions when measuring on 20 vs 200

Postby NotScienceOriented13 » Thu Jan 14, 2016 6:57 am

My daughter compared penny batteries in different liquids to measure the voltage and microamps. I am not a parent that knew anything about a voltmeter or is good in math so I need a little help to help her finish this. She just saw a video about a remote run on penny batteries and kind of merged some of these ideas.

In doing this if she measured some on the 20ma setting that would give her a reading of .53 but if at 200ma 1.02 and 1.17. My question is how should the decimal point be moved and label correctly to average and make the chart and average.

Im sure we did a lot wrong on this part but it was a nice learning experience. She took this very seriously and had so much fun taking things apart to light things up - Id just like some help from someone that could explain everything a little better than what we can read and learn from google.

Also curious during our first trial we had a cheaper Amazon model that was easy to use but she picked one of the leads and the wire basically feel out of the holder. What caused this? An overload or just cheapness?

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Re: Amperage conversions when measuring on 20 vs 200

Postby HowardE » Thu Jan 14, 2016 7:56 am

The only difference between the 20mA and the 200mA setting on a good meter is accuracy. The 20mA setting will report from 0 to 19.99 mA. The 200mA setting goes from 0 to 199.9mA. The idea is that if you set the meter for 200 and it reads less than 20, you can switch to the 20 setting for more accuracy.

In your case you were getting readings near 1mA and that meter probably can't measure that low with any accuracy. So the data doesn't really mean anything except that it was a low amount of current. If you want to convert milliamps to microamps, multiply by 1000. So 0.53 mA = is 530 microamps. 1.02mA = 1002uA.

Having the test leads come apart is a sign of cheapness to be sure. You can just reattach it (soldering it on - which if you don't know how to do that, a computer or electronics shop will probably do that for free).


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Re: Amperage conversions when measuring on 20 vs 200

Postby rmarz » Fri Jan 15, 2016 12:32 am

NotScienceOriented13 - Adding to Howard's comments, an inexpensive multimeter may have a few drawbacks. In general, experiments with coin batteries involve very low voltages and very low currents. It is always best to stay with one range setting of the meter, so that your readings are relative to one setup of the meter. If you were to get a 10 mA reading on the 20 mA scale, you should logically expect to see a 10 mA reading on the 200 mA scale. But, at very low voltage and current levels, as you change meter settings, the changing internal shunt resistances of the instrument become a source of error, similar to what you perceived. Stay with one setting and your readings will be consistent.

Rick Marz

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