Submitted to: Agronomy Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/7/1999
Publication Date: N/A
Citation: Interpretive Summary: Carbon dioxide is a critical variable in many fields of research, partly because it is the fundamental constituent of all plant growth, and partly because its atmospheric concentration is increasing at a rapid rate, leading to concerns about potential global warming. Frequently researchers need to measure differences in CO2 concentration between two air streams in order to compute CO2 production or consumption. Unfortunately, the instruments that are typically used for this (infrared gas analyzers) are very expensive. In this paper, I have explored an inexpensive alternative, in which two air streams are bubbled through two separate cells of deionized water. Some of the CO2 dissolves and ionizes, creating a measurable conductivity in the water that is proportional to the CO2 concentration in the air. It is shown the difference in CO2 concentration between the two cells can be determined form the ratio of the econductivities of the two cells, without the need for empirical calibration. Further, the conductivity ratio can be easily measured with a standard datalogger. The method appears to offer a promising alternative that is much less expensive than current instruments for measuring differential CO2 concentration, and thus may be useful in a broad range of research areas.
Technical Abstract: There are many applications in the fields of agronomy, plant physiology, ecology, meteorology, and others where it is necessary to measure the difference in atmospheric CO2 concentration [CO2] between two points. This is commonly done with an infrared gas analyzer, but such instruments are expensive, representing a substantial, and sometimes prohibitive share of the cost of CO2-related research. In this project, a simple inexpensive alternative was explored, in which the difference in [CO2] between two air streams is determined by bubbling the air through cells containing deionized water and measuring the ratio of the resistances of electrode pairs suspended in each cell. The underlying principles are presented, and it is shown that: a) differential [CO2] and the resistance ratio are directly proportional to one another; b) the coefficient of proportionality can be closely approximated as twice the mean [CO2] of the two air streams; ;and c) that the coefficient of proportionality is temperature-independent, provided both cells are at the same temperature. A system was designed and constructed to test these principles, and the results confirmed them. Dynamic response was characterized, and shown to be proportional to Q/V, where Q is the air flow rate and V is the water-filled volume of the cell. Differential resolution was found to be in the range of 0.4 -0.8 ppm, but it may be possible to improve this with design changes. Differential measurement of [CO2] by conductimetry shows considerable promise, partuclarly considering the straightforward nature of the relationship and the relatively low cost of the required components.