Q: How many alligator clips do I need? Do I need insulated clips (i.e., clips with a rubber coating)?
A: If your digital multimeter does not have alligator clips on it, you will probably need at least two alligator clips, as shown in Figure 3 in the Experimental Procedure. If your multimeter already has alligator clips, then you should only need one alligator clip (to connect the conductance sensor to the positive terminal on the 9 V battery). Because this experiment deals with very low voltage and current levels, insulated clips are not necessary, but be sure not to let the connections touch!
Q: Why is it important to keep the wires on the conductance sensor from moving?
A: If the wires on the conductance sensor move while you are taking measurements, your measurements may be inaccurate. Make sure the wires are tightly secured on the ends of the conductance sensor by attaching the short end of the wire to the longer end by twisting the two together or by using a very small drop of super glue to hold the wires in place.
Q: What is the purpose of dipping the sensor in distilled water? Should I replace the distilled water between tests?
A: Dipping the sensor in distilled water removes all of the ions and other liquids from the sensor. Not rinsing the sensor will cause the sensor to become contaminated with different liquids between the different tests, which could make your results have higher or lower conductance values than they actually do. Although it is not necessary, changing the distilled water in the rinsing bowls between tests may improve accuracy.
Q: What does it mean if I am getting a negative current reading on my multimeter?
A: The wires in the circuit may be connected incorrectly. Make sure that the positive node of the battery is attached to the red wire (+) from the multimeter. If you have a negative connected to a positive (for example, a black wire [-] connected to the positive node on the battery), it will give you a negative current reading.
Q: I'm not sure if my multimeter is set up correctly. How should it be configured?
A: For this project, the black multimeter probe should be inserted into the hole on the bottom of the multimeter labeled "COM" or "-" and the red multimeter probe should be inserted into the hole directly above it labeled "'V Ω mA" or "'V Ω LOGIC" or something similar. To measure the current of your samples, make sure the multimeter dial is set to the DCA area (which stands for DC Amps), not DCV (which stands for DC Volts). Set it to the 200 milliamps setting to measure your samples (except for measuring the distilled water, when it should be set to the 200 microamps setting). You may also want to refer to the multimeter's instructions for more information on how to take readings. If you purchased a Science Buddies kit, then you can look at Figure 1 below to see how to set up your multimeter.
Figure 1. Proper setup for the multimeter included in the Science Buddies kit. Other multimeters may be similar. The black multimeter probe goes in the lowest of the three holes (the hole labeled "COM") on the bottom right of the multimeter. The red multimeter probe goes in the middle of the three holes (the hole labeled VΩmA). For measuring your tap water, orange juice, and sports drink samples the meter should be set to DCA 200m, which means the meter is measuring current in the 200 milliamp range. For measuring distilled water, the meter should be set to DCA 200µ, which means the meter is measuring current the 200 microamp range.
Q: I'm not sure if the values I am getting are correct. How should I be making my calculations and what is the range that my results will probably fall in?
A: If you take your measurements using the 200 milliamps setting, your current readings will be in milliamps. (If you used the 200 microamps setting with the distilled water, your current readings will be in microamps.) For this experiment, current readings in the range of 0 (for distilled water) to 100 milliamps are expected.
To calculate the conductance of your different samples, use Equation 1 from the Introduction: Convert your current readings (in milliamps or microamps) to amps and divide this by the voltage of your battery (which should be about 9 V, but you can measure this with your multimeter to be sure). This will give you conductance in siemens (which you can convert to millisiemens by multiplying by 1,000). For this experiment, conductance results up to around 2.0 millisiemens are expected.
Q: The current readings on my multimeter seem very low for all of my samples and there is not much variation between them. What should I do?
A: Your multimeter may not be configured correctly. To check this, see the answer for the question above on "I'm not sure if my multimeter is set up correctly. How should it be configured?" Alternatively, the 9 V battery you are using may be dead. To test the battery, see the third answer for the question below on "Why am I getting a reading of 0 from the multimeter?"
Q: Why am I getting a reading of 0 from the multimeter?
A: A number of different issues may result in a reading of 0 from the multimeter:
- One or more of your connections may not be attached securely. Double-check all the connections.
- Your multimeter may not be set to a sensitive enough setting. The currents flowing through the liquids in this experiment are very small, so your multimeter must be set at a high sensitivity, such as 200 milliamps (mA) (or 200 microamps [uA] for distilled water).
- Your 9 V battery might be dead. You can check whether your battery still works by setting your multimeter to a scale that can read 10 volts (possibly a 20 V scale) and placing the positive (red) multimeter terminal on the positive battery node, and the negative (black) multimeter terminal on the negative battery node. If the reading is below 6, your battery may not have enough power for this project and you should use a fresh battery.
- Your multimeter may have blown a fuse. Check the fuse inside the meter. Check your instruction manual for details on replacing the fuse. When the experiment is set up as described but the two sensor wires touch (the ones in the liquid), it will blow the fuse, so be careful that they do not touch. If your multimeter was working well and then suddenly starting reading 0 all the time, then you probably blew the fuse in your multimeter.
- The wires on your conductance sensor may have become compromised in some way. There should be no material collected on them; if there is anything collected on them, clean and rinse them well and try again.
- Make sure that the wire you have wrapped around the ends of the conductance sensor is "bare" and has no insulation on it.
Q: Why are my readings going up and down?
A: A few possibilities could explain why your readings are fluctuating; you can determine what is happening in your experiment by how much the readings are changing.
- It is normal to have very small fluctuations (i.e., the reading stays around the same number but increases or decreases slightly). In these types of experiments with multimeters, it can be very difficult to get an entirely stable current.
- If your measurements decreased quickly, you may have encountered a problem with electrolysis. Electrolysis is when water is broken up into hydrogen and oxygen gas by an electrical current. If electrolysis is occurring, there will be little bubbles collecting on the wires on the ends of the conductance sensor. Electrolysis will result in a smaller surface area on the wires on the conductance sensor, and your readings will decrease.
- If the wires on the conductance sensor move while you are taking measurements, this can make your measurements randomly vary from sample to sample. To fix this, see the answer for the question above on "Why is it important to keep the wires on the conductance sensor from moving?"