| Gas exchange: compensating for CO2
Compensating for RQ: One of the many troublesome complications in respirometry is that organisms seldom exchange just one gas at a time: instead, they are simultaneously consuming oxygen and producing CO2 (we'll leave water out of this discussion because -- usually -- it can easily be scrubbed from air streams prior to gas analysis). Since the ratio of CO2 produced / oxygen consumed (the Respiratory Quotient, or RQ) is seldom exactly equal to 1.0, respiration causes a net change in the total gas volume. As a result, gas concentrations are affected. For example, consider a calculation of oxygen consumption: If more oxygen is consumed than CO2 is produced, the total gas volume is slightly reduced, which results in an increase in the concentration of O2. This seemingly trivial imbalance will influence exchange calculations and needs to be accounted for (startlingly large errors can result if you don't do this correctly). The necessary equations can be derived from simple algebra, but are tedious in practice. Fortunately, equations for most respirometry systems are built into LabAnalyst (see below for details).
If you are calculating oxygen consumption and you use a flow configuration that requires compensation for CO2 production, you are asked if you want to use a constant RQ value, or a previously computed channel containing VCO2 (note that VCO2 must be in units of ml/min, or you may get extremely inaccurate results), or CO2 concentration data (in % deflection from ambient CO2 concentration). If you elect to use VCO2 or %CO2 , you will be asked to select the channel containing CO2 data. For maximal precision, be sure to use the lag correction function to synchronize the timing of oxygen and CO2 data (especially important if there are large short-term fluctuations in gas concentrations).
Similarly, if you are calculating CO2 production, you will always need to compensate for oxygen consumption (since animals rarely produce CO2 unless they are also consuming O2) and you are asked if you want to use a constant RQ value, or oxygen concentration data (in % deflection from ambient O2 concentration), or a previously computed channel containing VO2 in units of ml/min.
In most cases, use of a constant RQ will produce quite good accuracy.