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Abstract Did you know that firefighters never spray water on an electrical fire? That's because ordinary tap water conducts electricity. This project will show you how to measure the conductance of water, and how to relate your measurements to water quality.Objective The goal of this experiment is to assess the purity of water samples by measuring their conductance (the inverse of electrical resistance) using a low-cost data acquisition device that you control using your personal computer. Introduction Electrical conductance, which is the inverse of electrical resistance, is a measure of how easily current can flow. The higher the conductance, the more easily current can flow. Conductance is very useful when testing water purity. You can use conductance to estimate the amount of total dissolved solids (salts) or ions in water. The more dissolved solids that are present in the water, the higher the conductance of the water. This is because the solids dissolve into positively and negatively charged ions that can conduct an electrical current proportional to their concentration. Water that has no dissolved solids or ions (i.e., "pure" water), conducts electricity several orders of magnitude less readily than regular tap water. Measuring the conductance of water is only one of many tests that exist to determine water purity. Performing this test would tell you that there are solids present, but not what the solids are. With a home water test kit (about $15) you can also measure the levels of bacteria, lead, pesticides, nitrates, nitrites, chlorine, and pH. As mentioned above, conductance, G, is the inverse of resistance, R. So G = 1/R. You can measure resistance using a data acquisition device. Connecting the leads from CH1 and Gnd (see the diagram below) will give you a reading of 0 volts, V, which is equal to no resistance. Placing material between the leads will lead to a voltage reading, which will determine resistance (and thus, conductance). Measuring the voltage of a battery is an example of a simple reading to get because it doesn't require any instruments. In this project, you will be measuring conductance using a data acquisition device that you connect to your computer. The data acquisition device (see the Materials and Equipment section below) measures analog signals, which are continuously variable, and converts them into digital signals, which are the sequences of 0's and 1's used in computer circuits. Conductance is just one example of an analog signal. Analog signals may come from instruments, sensors, or transducers that convert a huge variety of measurements into a change in voltage. Examples include: load, pressure, torque, frequency, strain, temperature, linear speed, rotational speed, flow, relative humidity, resistance, and current. Terms, Concepts, and Questions to Start Background Research To do this project, you should do research that enables you to understand the following terms and concepts:
Questions
Bibliography
Materials and Equipment To do this project, you will need the following materials and equipment:
Experimental Procedure
Variations This base experiment can lead to many variations and more-advanced experiments.
Credits Data Acquisition by DATAQ Instruments. StyrofoamTM is a registered trademark of the Dow Chemical Company.
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