Conductance as a Water Quality Measurement

Abstract

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.

Credits

Data Acquisition by DATAQ Instruments.

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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 and Concepts

To do this project, you should do research that enables you to understand the following terms and concepts:

• Electrical conductance
• Water purity
• Total dissolved solid (TDS)
• Ion
• pH
• MicroSiemens (μS)
• MicroSiemens/cm

Questions

• What materials will raise TDS?
• Can we reasonably assume what solids are in the water we are testing?

Bibliography

• This website is an excellent source of information regarding water purity:
  Lake Access. (n.d.). Electrical Conductivity: Measuring Salts in Water. Retrieved August 16, 2005 from http://lakeaccess.org/russ/conductivity.htm.
• This site explains the pH scale:
  • Decelles, P. (2002). The pH Scale. Virtually Biology Course, Basic Chemistry Concepts, Johnson County Community College Retrieved March 14, 2006 from http://staff.jccc.net/pdecell/chemistry/phscale.html.
• TransEra Corp. (2007). Data Acquisition Tutorial. Retrieved March 27, 2008 from http://www.htbasic.com/support/tutorials/hardware/data-aquisition.html.
• DATAQ Instruments. (2005). DATAQ Instruments Application Notes and Article Reprints. DATAQ Instruments Retrieved August 16, 2005 from http://www.dataq.com/applicat/index.htm.

Materials and Equipment

To do this project, you will need the following materials and equipment:

• A Windows-compatible personal computer and one of the following:
  • If your computer has a serial port (RS-232), you will need the Data Acquisition Starter Kit (DI-194RS)
  • If you have a computer with a USB port, you will need the Data Acquisition Starter Kit (DI-148U)
• WinDaq Acquisition and Playback Software (free) must be installed on the computer (administrator privileges are required)
• Two insulated 16-gauge solid copper wires of equal length and type (at least 18 inches each)
• A straw
• Electrical tape
• Styrofoam^TM cups (12; 3 cups for each liquid sample)
• Pen or marker
• Four different types of water or liquids

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Experimental Procedure

1. Cut two pieces of insulated 16-gauge copper wire that are each at least 18 inches long. Strip 1 inch off an end of both wires; do not strip the ends that will go on the probe (there should be no bare wire in the liquid, other than the very ends).
2. Now you will build your probe. Cut the straw so that it fits into the cups, as shown in the diagram below.
   1. Position each wire on the straw, so the ends are facing each other and so the ends are 1 inch apart.
   2. Then securely tape the two unstripped signal leads (wires) to the straw. The tape must securely fasten the wires so that they do not move around when you handle the probe. If the wires move, it could change the distance between the two ends, which could unexpectedly alter your measurements in the middle of your experiment and lead to bad results.
   3. Bend the wires 90°, as shown in the diagram.
   4. Now connect the other, stripped end of the signal leads to the data acquisition device. Connect one wire to Gnd and the other to CH1.
   5. Use the following diagram for connections:
      
3. Label all of your cups with the name of the liquid and a letter, such as Tap Water A, Tap Water B, etc. Take your first liquid and pour an equal amount into three Styrofoam cups.
4. Set up parameters (sample rate, number of channels, etc.) in the WinDaq Acquisition Software. Then place the probe in the liquid, and record the data in a data table. Use the User Annotation function in WinDaq to label the data (i.e., "Sample 1 - Tap Water_A", "Sample 1 - Tap Water_B") to easily identify the samples. Voltage readings should be between about 0.125 and 2.5 V (in our tests, tap water readings were around 0.6 to 0.7 V). Record the data repeatedly until you get consistent results to ensure accuracy. WinDaq Data Acquisition Software Manual and Multimedia demonstrations are available with the included CD or via the Internet at http://www.dataq.com/products/software/acquisition.htm.

5. Wipe the signal leads dry and repeat step 4 for each of the two remaining cups of the first liquid sample.

6. Disconnect your probe, rinse out the straw, and dry the leads. This is an important step because it ensures there is no residue left from the previous liquid, which could alter the results. Repeat steps 3–5 for each different liquid.

7. Analyze the recorded data using WinDaq Playback Software and/or Excel. Average the values for the three replicates for each liquid to get a single data point for each type of liquid. A special formula is required to convert voltage readings to microSiemens (μS) with the data acquisition device. Use the calculator at http://www.dataq.com/science-fair/calc-siemens.php to convert volts to microSiemens.
   • If you are using the DI-194RS, the formulas are as follows. Keep in mind that V = voltage in volts, Ω = ohms, and ʊ = mhos.
     Resistance, R, (Ω) = 137,000 x V / (2.5 – V)
     Conductance, G, (ʊ) = 7.3 x (2.5 – V) / V

     Special notes for advanced students:

     • The voltage output will read zero for the DI-194RS, with a short or 0 Ω applied to the inputs.
     • 2.5 is the voltage output reading for the DI-194RS in free air or infinite resistance.
     • 137,000 is a constant for the DI-194RS, used to convert the formula's output to resistance in ohms. This number is the effective input impedance of the units.
     • The conductance formula is the reciprocal of the resistance formula, with an output in Siemens—or mhos—(ʊ), and is multiplied by 1,000,000 to give an output in microSiemens (μS).

     If you are using the DI-148U, the formulas are as follows. Keep in mind that V = voltage in volts, Ω = ohms, and ʊ = mhos.

     Resistance, R, (Ω) = 225,000 x V / (1.39 – V)

     Conductance, G, (ʊ) = 4.44 x (1.39 – V) / V

     Special notes for advanced students:

     • The voltage output will read zero for the DI-148U, with a short or 0 Ω applied to the inputs.
     • 1.39 is the voltage output reading for the DI-148U in free air or infinite resistance
     • 225,000 is a constant for the DI-148U, used to convert the formula's output to resistance in ohms. This number is the effective input impedance of the units.
     • The conductance formula is the reciprocal of the resistance formula, with an output in Siemens—or mhos—(ʊ), and is multiplied by 1,000,000 to give an output in microSiemens (μS).

8. Use the following chart to view the relationship between voltage and microSiemens:
   
9. Note that for voltages less than about 0.125 V, the relationship between conductance and voltage changes very rapidly. Conductance values in this range are likely to be unreliable.
10. WinDaq Playback Software Manual and Multimedia demonstrations are available with included CD or via the Internet at http://www.dataq.com/products/software/playback.htm.

11. Note: Measurements should be taken at the same temperature.

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Variations

This base experiment can lead to many variations and more-advanced experiments.

• Use pH and other water-quality measurements to determine total pollution in rainwater or lake water.
• Electrical conductivity as a general water test is just for taste and is not necessarily a health hazard. Measure the total dissolved salts in multiple liquids (e.g., coffee, tea, cola, etc.).
• How does temperature affect the conductivity of water?
• Since "lite" soy sauce contains less salt than ordinary soy sauce, if you add equal amounts of lite and regular soy sauce to equal amounts of distilled water, will you get different conductance values for the two samples?
• Add known amounts of different materials (e.g., table salt (NaCl), Epsom salts, potassium chloride) to dissolve them in equal amounts of distilled or deionized water. Is the measured conductance the same for each material? Does the molecular weight of the added material matter? (Do background research on molarity, which is how chemical concentrations are measured in the lab.)
• Measure the same samples over a period of time (days or weeks)—how is TDS affected and what could be the possible reasons for this?
• Does "pure" water exist? Try to create water with no solids or ions.

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