Science Buddies: "Ask an Expert"

My results for this experiment don't make sense. I measured the amps for distilled water, gatorade, powerade, and tap water with an Innova Equus 3300 Digital Multimeter. I set it up just like the diagram on the project page. The distilled water shouldn't have a current that similar to the sports drinks.

9.398 milliamps Distilled Water
9.447 milliamps Tap Water
9.458 milliamps Powerade
9.458 milliamps Gatorade

mb9595 - Clearly you should not be getting the readings you listed. With readings of 9.xxx it seems to me you are using the DC voltage range, not DC current range. If your setup is correct, make sure your meter is set up for a current range of 200 milliamps DC. This range is good for the more conductive drinks like Gatorade or orange juice. Is your sensor and battery connected to the negative (COM) and the positive VΩmA jacks? Are you sure there is no short circuit in your sensor assembly? Was the sensor wound on an insulating core? You can do a simple test on the sensor using the 20K Ω range on the resistance scale. There should be no reading when attaching the two multimeter leads of the sensor. When you read current when testing distilled water it should be barely readable, down in the microamps range. Check these items and you may find the bug in your setup.

Rick Marz

I tried the DCA settings for Gatorade and got O. The black area is labed DCV with 200m 2 20 200 500. The green area is labeled DCA with 200m 20m and 2000U.

mb9595 - I can't see everything of your setup in the photo, but what I see appears correct. The meter should be set to the green area at 200 milliamps for the highly conductive drinks. If this doesn't fix the problem you may have a problem with the meter. Check it in the 20 mA range with your 9 volt battery and a 1KΩ resistor in series with the battery (effectively in place of the sensor). You should read about 9 mA. Check our the DC voltage range with your 9 volt battery. If this doesn't work, take a picture of the whole setup and send it along.

Rick MArz

Thanks for your support so far. When I tried measuring the battery directly the amps is 0 and the volts are 9.54 volts. I'm not sure what you mean by the 1K resistor. Could my digital multimeter be broken?

mb9595 - I think your difficulty is in using the meter. Your comment of reading 9.4 milliamps when reading for voltage is a mixup of terms. Possibly, the meter is not working correctly. When I suggested testing the meter using the 9 volt battery, the meter should be set to the DCV setting at the 20 volt range, you should read something close to 9 volts depending on battery condition. In this situation you are reading voltage, in volts, not current in amperes. When the meter is set at DCA you are reading current in amperes (or milliamperes, depending on range selected). If you had access to a 1KΩ (1000 ohm) resistor, you could test the current range, using the DCA setting at the 20 mA range. Ohm's law states that a 9 volt potential applied to a 1000 Ω load would pass a current of approximately 9 mA. The formula is I=E/R, or Current=Voltage/Resistance. As a final test, you could use the same 1000 Ω resistor with the meter in the OHM setting using the 20K range. It should read a resistance of 1000 Ω. I think you are mixing the terms voltage and current, and that may be causing erroneous meter settings and/or readings. It's possible that a fuse may have blown as a result of selecting an improper range and subjecting it to the 9 volt battery.

Rick Marz

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