Understanding the dynamics of respiratory gasses gives critical insight into the metabolic processes of a biological system. An efficient and effective means of measuring respiration is essential to understanding aspects of the biosphere. Many methods have been developed for measuring changes in CO2 and O2, both as integrated systems and as individual components. Many experiments use an alkali trap with subsequent titration as an inexpensive method for CO2 measurement. Haney et al. (2008) compared the titration method against infrared gas analysis (IRGA) and found them to be highly correlated, with r2=0.95. The use of IRGA provides the potential for automation in a system due to the electronic output. Bowling et al. (2001) described a system using a pneumatically driven piston to inject sample air, which gave consistency and high accuracy (a coefficient of variance of 0.05%). This and similar systems afford accurate measurements by incorporating complex mechanics, but can be expensive and time intensive to maintain. An emphasis was placed on simplicity and cost effectiveness, while still allowing for some degree of automation and instantaneous measurement results.
When measuring respiration, the majority of systems used focus on CO2 as the measured gas, generally due to the differences in proportional gas changes relative to background concentrations. Blonquist et al. tested the validity of using an oxygen sensor to measure the respiration of soil and found it to be possible given corrections for temperature, pressure and humidity. The study also found oxygen measurements to be less affected by solubility in water and therefore advantageous in aquatic or semiaquatic environments. While assessing the biodegradability of hydrocarbons in soil, Miles and Doucette (2001) found that a measurement of respiration by oxygen depletion was comparable to measurements by known chemical depletion and collection of marked 14CO2.
While the respiration rate can be measured by either CO2 or O2, the measurement of both gives further understanding to the underlying biological processes. Respiratory quotient (RQ), the ratio of CO2 produced to O2 consumed, is often used to determine the type of substrate being consumed and presence of anaerobic conditions. While some make the assumption that the RQ value is approximately 1 under aerobic conditions and therefore interchange respiration rates as determined by CO2 evolution or O2 depletion, Dilly (2003) found that RQ is rarely steady and is often well above or below 1 for microbial populations. Therefore respiration rates cannot be assumed comparable when measured by different respiratory gasses and an incorporation of both may be necessary to allow for proper understanding and comparison of metabolic processes.
Four types of sensors were tested: syringe IRGA injection, and oxygen probe, and two types of CO2 probes. Each was combined with a data acquisition system, to created a measurement system capable of recording the gas concentration. To allow for better assessment of respiratory processes, each system was tested to compare its strengths and limitations in a closed system application.
Nelson, Jake, "A Comparison of Methods for Measuring CO2 and O2" (2012). Techniques and Instruments. Paper 4.