Science Buddies: "Ask an Expert"

What are some reasonable readings on the multimeter for the electrolyte challenge? We have tried everything and cannot get a reading passed 0.07, and that is only when the multimeter is set up to read 10A and not in the V0mA socket. When we set up the multimeter to the correct settings, as explained in the procedures, we get 0 for readings. Here are all of the things we have done to troubleshoot:
-tried several new batteries
-tested the batteries with the multimeter
-tested the fuse on the multimeter by turning it to ohms and touching the leads together
-tested the copper wires with a battery and light bulb
-measured the current on a series circuit with a light bulb
-measured resistance with a 1000 ohm resister with the multimeter
-changed gauge of copper wire
-tested the alligator clips
-tested several different beverages- we still only get 0.07 and less, and only if plugged into the wrong sockets.

We still get zero when we have the multimeter set to 200mA and the red lead in the V0mA socket. Suggestions would be greatly welcomed!!! Thank you!

lundeenj - It would appear that you have virtually tested everything that could be tested. It really sounds like your meter is not functioning in the lower current ranges, particularly the 20 and 200mADC settings. A reading of 0.07 Amps (70 mA) read on the 10 ADC scale is in the expected range for an ionized sports drink. In order to use the 10 Amp range, the probe must be inserted in the special 10 Amp socket, and the meter set to the 10 ADC position. Here are my comments on your debugging efforts to date:

-tried several new batteries - Good, batteries are not a problem
-tested the batteries with the multimeter - Good, batteries still not the problem
-tested the fuse on the multimeter by turning it to ohms and touching the leads together - This will not test anything except the continuity of the test leads and the ohmmeter function
-tested the copper wires with a battery and light bulb - Good, sounds like a continuity test of all your wiring
-measured the current on a series circuit with a light bulb - Good, but was this done with the meter in the 10 ADC setting? I'm pretty sure that range works OK, it's the 20 and 200 mADC ranges that are questionable
-measured resistance with a 1000 ohm resister with the multimeter - Good, but just a test of the Ohmmeter function of the meter
-changed gauge of copper wire - Good, but probably not a significant factor in your result
-tested the alligator clips - Good, one more variable eliminated

As you apparently have a 1,000Ω resistor at hand, the best test of the 20 and 200 mADC current ranges of the meter is to place the resistor in series with the 9 V battery in series with the multimeter leads with the meter set to 20, then 200 mADC. You should read about 9 mADC in either range. If no reading is obtained it is almost certain that the meter is not functioning in these ranges. See diagram. Even though you are possibly getting current measurement in the 10 A range, you need to be able to measure much lower currents, even in the microampere ranges (measuring distilled water for example). That's why functioning in the 20 and 200 mADC ranges is important.

Rick Marz

Rick gave you an excellent answer; if you don't see about 9 mA in the test setup he described, your multimeter is broken. But before replacing your multimeter it might be a good idea to replace the fuse in the volt-ohm-mA path. It's not totally clear to me that your previous measurements entirely rule out a blown fuse. At the very least examine the fuse carefully; there should be a fine wire stretching from one metal cap to the other -- it's visible through the glass. If there is no wire, the fuse has blown. You can buy fuses at Radio Shack or other 'bricks-and-mortar' stores for a couple of bucks-- at Amazon or eBay the the shipping will make the fuses more expensive.