multimeter measuring a bit weird

Discussion in 'General Electronics Chat' started by bleu, Jan 3, 2011.

  1. bleu

    Thread Starter New Member

    Jan 3, 2011
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    I am using an analog multimeter with copper and zinc electrodes to find the voltage of liquids with different pH levels. (for my Science Project)

    Nothing seemed wrong when I got the voltage from lime juice, around .95V

    But I get about .75V when I put the electrodes in WATER. I know the water is not perfectly neutral, it is a little acidic because it's not totally pure... but I don't expect it to even give off that much voltage. Is this normal?

    I did test a couple of batteries using the probes, and I got 10V for the 9V one and 1.75V for the 1.5V one. So, I know it is slightly off...

    Help would be greatly appreciated... thanks! :)
     
  2. Markd77

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    Sep 7, 2009
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    It's more to do with the metals than the liquid (electrolyte). Anything even slightly conductive like tap water or a potato will start to approach the electrode potential.
    The less conductive the electrolyte, the lower the current can be. With a digital multimeter (with a higher input resistance than the analog one) you will probably see a higher voltage for water.
    Batteries are commonly different voltages from what is written on them so your multimeter may be more accurate than you think.
    http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/electrode.html
     
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  3. beenthere

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    One suspects that if your water were freshly distilled, the voltage would have been very close to 0. But you have electrolytes dissolved in the water, so it has some conduction.

    As a check, what does the meter indicate for voltage when the leads are shorted together?

    Try adding a bit of vinegar and see what it does.
     
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  4. bleu

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    Jan 3, 2011
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    I'm going to dinner right now so I'll try to reply when I get back... thanks for the help!
     
  5. #12

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    My definition of a battery is: any 2 dissimilar metals in an ionic solution with a Ph that is not 7. (This definition helps me when I'm thinking about corrosion in a water system.)

    As has been said, the most important component is, which two metals? There's some room for exploration!

    You can try vinegar as your electrolyte, then dilute it with 50% water, then 90% water. You will find out how much the acidity affects the voltage by knowing that the Ph of the liquid gets closer to 7 as water is added. Then you can think about the alleged 100% water that you measured.
     
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  6. jpanhalt

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    I don't understand why you say pH not equal to 7. Please explain?

    John
     
  7. #12

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    Probably just a false belief based on the idea that I've never heard of a battery with water as the electrolyte.
     
  8. jpanhalt

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    You need to read a bit more.

    John
     
  9. Markd77

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    Good point. Sodium Chloride in water is pH 7 but works as an electrolyte.
     
  10. #12

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    What are these neutral Ph batteries called, and are they of any practical use?
     
  11. jpanhalt

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    You missed my point. It's like a football game. Who wants to watch a game without the cheerleaders? But, they don't affect the score. pH has nothing to do with the definition of a battery. John
     
  12. #12

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    Excellent. A battery can be made with a neutral Ph.

    I'd still like to know what Ph neutral batteries are called and if they have any practical use.
     
  13. #12

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    Nostalgia...I worked in a factory that made Ph meters. In order to input a calibration signal we used a battery powered box that provided 0.0 volts to cause the meter to show Ph=7 This is where I deduced (wrongly) that a Ph of 7 produced no voltage.

    The standard sensors consisted of a salt in a porous container and a glass Ph electrode, in case that explains anything.
     
  14. thatoneguy

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    What is the input resistance of your meter?

    If input impedance is extremely high, the meter draws virtually zero current. The meter may show a voltage, but there would be nearly zero "useful" current available.

    When testing cells, you should always apply a load. For what you are measuring, something like 1/2-1mA (1k-2k Ohms/volt) between the two electrodes.

    If you don't put a load on standard batteries, such as AA, they will almost always read as around 1.5V good or bad, even though bad cells drop to 0.9V when given a 10mA load.
     
  15. beenthere

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    At the U we made a box (with mercury cells) that let us set all the pH voltages, 0 - 14.
     
  16. #12

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    The box I used applied 357 mv/Ph if I remember correctly after 38 years. Positive for one direction and negative for the other direction with 0.0 volts registering as 7Ph.
     
  17. beenthere

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  18. #12

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    Looking at my notes, I seriously doubt my memory. I'm picking up notes like, +- 294 mv for a span of 2 to 12 Ph.
     
  19. #12

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    So...according to the Wiki article, Confirmed: the Ph probe puts out 0 volts at 7Ph. I would come to the conclusion that Ph7 does not produce voltage [in a Ph probe!]. What voltage does Ph 7 produce in a battery? Depends on the electrodes. Right? Are there metalic electrodes that produce voltage in a Ph7 solution?
     
  20. bleu

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    Jan 3, 2011
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    This is some interesting stuff… I was thinking that anything with a pH of 7 would not be able to produce voltage.

    @Markd, you said the digital meter (with higher resistance) will probably show a higher voltage of water… I’m guessing this is because of Ohm’s law? I’m pretty sure I set my meter’s resistance to 0, though hopefully I did it right… no, wait a second… if my resistance was 0, if you put that in V=IR you would get V=0! Now I’m confused. Although, there could be an internal resistance in the electrolyte… :eek: There is a setting on my meter to measure DCmA… could I use that to get the current and calculate resistance?
    I sort of see what you’re saying about the metals… like, that the difference in electrons wouldn’t even be there if the metals weren’t different... and the only reason the voltage is lower in weaker electrolytes is because the current is lower? Sorry if I'm not making much sense!

    @beenthere, I did notice that the needle immediately went to 0 when the metals touched each other. I guess this is because when they are touching, there are no two points where a difference can occur…

    @#12: Ok, I just tried starting with some water, and I put the electrodes in. As I slowly added vinegar the needle on the meter did move up quite a bit…
    It’s odd because earlier I tried adding water to the lime juice… at some point, it was probably at least 80% water, and there didn’t seem to be a significant change in voltage. Though, I was doing it a different way (not just watching the needle move as I poured more water in… each time, I took the electrodes out, added more water (and poured some juice out) and put the electrodes back in to measure).

    Well then… this just gets more complicated as I find out more! But I think I understand better now :)
     
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