(Note: Using gas sampling tubes is expensive. If you can get a donation, or arrange teams to plan their sample taking carefully, it may be possible. It certainly can be rewarding (See A moment of Glory in San Antonio.) In 2008 one could obtain 10 tubes for $59 from QA supplies.)

Measuring CO2 with Gas Sampling Tubes

Gas sampling tubes such as those made by Drager or QA can be used to measure CO₂ accurately. The operation of the tubes and air pump are simple. The pump draws a measured amount of air through the tube. The CO₂ in the air reacts to a chemical lining inside the tube, causing the chemical to change color. The degree of color change corresponds to the level of CO₂.

Materials

safety glasses

glass gas sampling tube

pliers

TERC air pump

GL Journals

Measuring CO2 with Gas Sampling Tubes

1 Work safely

Put on safety glasses. Carefully break both ends of the

glass gas sampling tubes. Use a pair of pliers to carefully snip only the tips of each end.

2 Set up the equipment.

Insert one end of the glass sampling tube into the flared open end of the air pump. Make sure the direction of the arrow on the gas sampling tube points toward the air pump. Remember, air is pulled through the gas sampling tube by the air pump and exhausted into the atmosphere through the rubber one-way valve on the air pump. The arrow points in the direction of this airflow.

3 Sample the air.

Pull 1 liter (1,000 ml) of air through the CO₂ tube with the plastic air pump. Since the pump can draw a maximum volume of 140 milliliters, you must use 10 strokes of 100 milliliters each to draw a total of 1 liter of air. The scale of many CO₂ gas-sampling tubes is calibrated for 1 liter of air. Pull each stroke with a slow, deliberate, even pressure on the plunger of the pump so that it reaches the 100- ml mark in 10 seconds. Hold the plunger there for approximately 10 more seconds, at which point the plunger should no longer be pulling back against you. At this point, all the air for that stroke has been pulled through, and the plunger is ready for the next stroke.

4 Read the findings.

After pumping 1 liter (1,000 ml) of air through the gas sampling tube, disconnect the tube from the air pump and determine how far up the tube the color change extends. Read the value of CO2 on the scale at the color transition. This is a qualitative measurement. You will need to use your judgment.

Different people will see the color change in slightly different places. To get a more reproducible result, have three students independently look at the tube and determine the CO₂ reading. Each student should make his or her determination without telling the other two students. After each student has made a best estimate, share the results and calculate the average. This number takes into account the random differences between students in their reading of the color change in the gas sampling tube.