Science Buddies: "Ask an Expert"

I am doing the electrolyte challenge project and I am not sure what setting I set the digital multimeter to. There is a knob in the middle and I don't know what number to turn it to.

cmiln1 - On most multimeters you will find four types of measurements. DCV or DC Voltage, ACV or AC Voltage, DCA or DC Amperes and Resistance. You will need to select a value in the DCA group. Most current measurements will be under 200 milliamperes for very ionized electrolytes, so the best meter setting to start with is 200 mADC (or 200 m). For distilled water or other slightly ionized solutions, the readings will be in microamperes, so the lowest setting is appropriate, probably 200u.

Rick Marz

Thank you for helping me with this. Now I can start my project!

Hello my name is Olivia and I am a student currently in 7th grade. I started this project off easy, but as soon as I got to the part after where you make a conductance sensor with the straw and copper wire, things started to fall apart because I wasn't sure what any of the things were. The instruction manual was no help at all. My project is due next week and I seriously need help. I started going off of the real life image in figure number three, but then I got lost and thought I had gone too far ahead of the directions. So when I thought I had scrolled through all of what I already did, it seemed like it was time to use my drinks. First drink I used, my multimeter started acting up and was very indecisive about what my measurement was going to be. I need to fix this, fast!

Hi Olivia-

The procedure is quite straightforward. The meter, battery and sensor are connected in a big loop, just as shown in the picture. You want to make sure that the black lead is plugged into the COMMON connection on the meter, and the red one is plugged in to the jack indicated for uA or mA. Once you have that, you simply place the sensor in the dish and add your drink. Any electrolytes present in the drink will cause current to flow through the circuit - the meter will indicate how much. Distilled water is a terrible conductor and the current will be very low. The other, sugary, drinks will conduct a lot better and the meter will measure higher current.

The meter readings should NOT bounce around. If you pour in the drink and take your hands off the sensor, the meter will simply display a number. When you say "started acting up and was very indecisive", can you be more specific about what the meter was doing? How did you have knob set and where were the leads plugged in? Your meter may have one red connection for low current like you're measuring and a second one for higher current. The Velleman DVM810 pictured in the photo for example has one jack with just an 'A'. That one is for measuring up to 10 amps and is NOT the jack you want to use.

Howard