Topic | Negative resistance

Home Forums Mooshimeter Support Negative resistance

  • This topic has 0 replies, 2 voices, and was last updated 6 years ago by Anonymous.
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    • #19840 Reply

      Dear James
      Congratulations for your fantastic idea.
      I bought several Mooshimeters with Robotshop and Elektor for a research project.
      But I met several issues :
      -They show sometimes”negative resistances” where standard multimeters work without problems.
      -When you insert a microSD card, closing the cover of the mooshimeter push the micro SD so it is ejected when you open the cover again… so I had a problem of microSD bad contact on one mooshimeter I have been obliged to create a hole to be able to insert the sd directly wiihout opening the cover.
      -The reset button was partially missing on another one
      – Short circuit with battery contacts

      As the most important problem now is the negative resistance do you have any solution?
      thanks for your help

    • #19841 Reply

      Better than 1% accuracy over 20 Ohms – 20 Megaohms

      So, there are no promises of resistance measurements under 20 ohms. You can still measure those, but need to at least zero your leads (and meter itself) to get about realistic values.

      There’s (a little bit) more about it:

    • #19842 Reply

      I had to test that memory card ejecting by itself situation and had hard time to reproduce it (pcb revH). Then took my second mooshimeter (revJ) and card popped out when i just opened the thing.

      Only difference i could find was maybe just under half a millimeter difference in card holder placement. I have to wiggle back cover a little to get it past memory cards edge. Strange. Maybe i have not had to open my newer meter after i put that memory card in to it and just got lucky when closing the whole thing first time.

      Battery contacts had a problem in early mooshimeters, when there was sharp burrs that could break the battery wrap and positive battery shell would get into contact with negative battery holder. If this is the case, you could (and should) dull those with just about any tool that would feel right in your hand. I got even more paranoid and wrapped sides of those battery holders with kapton tape.

      More about battery contacts can be found here:

      Short Circuiting Battery Problem

    • #19844 Reply

      Thanks for your reply.
      I was aware of the resistance measurement range but my problem occurs with
      kOhms measurements!
      So it does not appear to be a problem of range at all, and I met these negative resistance measurements with different Mooshimeters (upgraded with latest firmware).
      Any idea would be welcome as this problem is a major failure for my use.
      I have sent an email to the support 15 days ago but no reply.
      best regards

    • #19845 Reply

      Could you tell more about your measurement setup? If it’s something that others could reproduce, we might be able to narrow possible reasons for this..

    • #19846 Reply

      It is self potential measurements (a very old school geophysical method):
      2 electrodes in the soil with a certain spacing can mesure a potential in millivolts created by the soil itself.
      It is often a very small voltage from tenth of mV to hundreds of mV rarely reaching 1 volt.
      Measuring the resistance between these 2 electrodes gives also at least an acurate information about your contact between soil/electrode.
      On a volcanic medium it can be kOhms to hundreds of kOhms.
      This processing is often done with a standard multimeter with no problem (even the ones very cheap).
      I tested 2 configurations :
      – DC measurement with the main V channel, and then jumped to the Ohm channel with a switch
      – DC measurement with the Ohm channel on auxiliary mode, then resistance measurement with the same channel.
      You have to notice that once we have choosen the place of the negative electrode, the measured values in mV can be positive or negative according to ions migrations in the soil, this is normal and in fact this is one of the targets to get these kind of variations.

      But why does the Ohmmeter give negative values, that is the problem…
      and these negative resistance values always occur when we have some negative mV readings, but they are not all the time negative.
      With negative mV readings we can have positive resistance readings but they are non sense according to comparative measurements with standard voltmeter.
      So to summarize :
      – negative millivolts can give negative ohms, but not at 100%
      – in some cases of negatives mV, some resistance values look bullshit even when these resistances are positive
      – in some other cases the “absolute value” of negative resistance look good!

      thanks for helping.

    • #19847 Reply

      Hmm.. I measured what mooshimeter outputs when it is on 10 kohm range.

      1,79 volts when open, 99 microamps at “short circuit” (100 ohm shunt in another multimeter).

      With 4k7 resistor and 100 ohms extra from a cheap multimeter at microamp range, mooshimeter was showing 4796 ohms, current was 99,56 micro amps and voltage at mooshimeter connectors 478 mV. Mooshimeter seems to use constant current and then check how much voltage drops with measured resistor.

      Same setup with (yet another) cheap multimeter, it seems that there is nothing constant in resistance measurement. Voltage and currents are all over the place when trying different resistors.

      So, what have we learnt about this? Probably nothing. I should try with my fluke 87v, but sleeping sounds much more fun right now, as i need to be an functional electrician in the morning.

      For a conclusion, voltages and currents for resistance measurements are quite small. Voltages or currents in soil might be higher than the ones multimeters are using. That has to produce errors. Devices might handle those situations differently. I think that multimeter resistance measurements in soil cant produce any useful data. Maybe you could get better results with much bigger currents, like 200 or 250 mA, which are what most installation testers use when measuring continuity of protective earth conductors around a house (in Finland, how should i know what others do).

    • #19849 Reply

      If the soil is generating a voltage, it will also be generating a current. If this current is comparable to the meter’s probe current in resistance mode, you’ll get significantly wrong resistance readings. Essentially it’s like trying to measure a resistor whilst it’s in a circuit and with the circuit switched on.

      The solution is to measure the dynamic resistance: apply two different test currents and measure the voltage drop for each. The resistance is the difference between the two voltages divided by the difference in the two currents. The Mooshimeter would be fine for measuring the voltage drop, but you’ll need an external current source.

    • #19850 Reply

      Hi Tim
      are you a member of the support Team?
      The fact is I sent an email to the support at least 15 days ago but they never reply…
      I had an issue with the SD card which is ejected systematically when I open the case (on all my Mooshimeters) and actually nobody gave me explanations about this. Another one has a missing reset button…

      About my main concern : I am not at all convinced by your reply.
      Any basic multimeter with a correct input impedance (10 MΩ) even the cheapest ones that cost a few tenth of bucks are working for my measurements (I tested plenty of them), just because they have an input impedance at least 10 times higher than the ones measured and so the internal circuit of the voltmeter does not interfer with the measurement.
      The higher the input impedance of the volmeter is, the better it is for these measurements.
      The manual gives >100MΩ for the Mooshimeter on the Ohm terminal so that was meaning some good results because in my cases the measured values are in the kΩ range.
      So my question remains : why it is not working compared to a basic multimeter?
      thank you for your reply

    • #19851 Reply

      Even if basic multimeter gives you value in ohms, it does not mean that the value has anything to do with the soil you are measuring. I would say that negative values are even better, so its clear that you are doing something wrong.

      Multimeter, in resistance mode, always generates current through the resistor you are measuring and does maths for the resulted current and voltage. If and when your measured thing, in this case soil, has any own electrical action going on, it will mess the result. With basically all multimeters, those currents both on the meter itself and soil are really close together, so that you would get better scientific results by throwing a dice.

      Input impedance for voltage measurements has nothing to do with measuring resistances, two completely different measurements. High input impedance does “help” you to measure very low voltages, but as it does so, your measurements with different meters are not that directly comparable. Very small input impedance would short circuit your measured voltage and high one, like in most of the voltage measurement devices would let external errors in. As you have seen, just waving your test leads in air produces random voltages to your screen. And now, we are in situation that you might have 5 different voltage meters and all of them show different voltages, if you try them separately.

      You could probably see the difference, if you set your highest input impedance meter in your experiment showing voltage from soil and then add your lowest impedance one directly in parallel with the first one. Voltage on first meters display will also go down when you add that second meter. If you now remove the high impedance meter from the setup, voltage on the second meter does not move up as much (if at all).

      Sorry, this was me trying to be brief :D

    • #19854 Reply

      Geoeric, let me try to explain in simpler terms:

      There is a current flowing through the soil. You know this because there’s a voltage across the soil, and the soil has a finite resistance. Ohm’s Law tells you that current equals voltage divided by resistance.

      The Mooshimeter uses a very small test current in resistance mode. This is a sign of a *good* meter. Cheaper ones may use a larger current, and normally this is not a good thing.

      The relatively tiny test current of the Mooshimeter may be overwhelmed by the soil’s own current, and that disturbs the resistance reading. You can test the truth of this by switching the Moosimeter’s leads round and seeing if you get a different reading.

      Finally, just because a cheaper meter gives you a believable resistance value, that does not mean that you have made an accurate measurement.

      And no, I’m not associated with the creator of the Mooshimeter.

    • #19856 Reply

      I just got home from a gig that had a problem which did bring this thread to my mind. Partly broken cable under concrete floor, with some moisture. It somehow generated its own voltage, so that i got negative resistance in one way, positive in another. Fluke 87V handled it exactly the same. So both meters gave somehow reasonable value in one way, but when crossing the leads i just knew that both values are useless.

      Now that i got more involved in this, i wanted to try this in more controlled environment and realized that an empty AA-battery could do the trick. First one i grabbed had ~0,7 volts in it, nicely low and in range that i felt comfortable to poke with my meters at ohm-setting.

      As I said, mooshimeter and fluke “warn” about problems in resistance measurement, if i happen to measure in “wrong” polarity, or try both ways. So 50% change to find out of problems at first try.

      5 cheaper multimeters that i had at my hands reach all gave positive resistance readings, no matter what polarity i gave them. There were quite big differences in how they handled that situation though. Some went totally crazy and just flashed from range to range, some seemed quite smooth, but did not seem to stop wandering at all. Different polarity values were way off of each other, which is a good sign as its possible to realize screwed results with double checking. If measuring totally unknown things, you could believe your first result and just think that it needed some time to settle for a reading.

      So, mooshimeter works like the big boys and thats a good thing. I’m positively surprised also about those cheap meters. Now that i know how they behave, I’ll probably remember to check at different polarity, if something seems odd and i cant get stable reading. I have done that in the past, but never had interest to examine what is really happening.
      Now i know a little bit more how my tools behave.

      My final verdict is: Measuring resistance is like measuring speed for land speed records. It’s only official when done in both directions in quite short time window.

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