Respirometry fundamentals
Respirometry (calculation of metabolic rate from changes in concentration of the gases -- oxygen and/or carbon dioxide -- involved in cellular respiration) is simple in principle, but can be a real headache to do correctly.  The major pitfalls stem from the following issues:

These issues -- and how LabAnalyst deals with them -- are addressed in more detail below.

 Note: if you are a respirometry geek -- and who else would waste time reading this? -- you may have noticed that I'm not using the correct abbreviations for rates of oxygen consumption and carbon dioxide production.   Proper usage includes a dot over the V in VO2 and VCO2 to indicate rates instead of volumes.

     Alas, I haven't found a convenient way to make the 'Vee-dot' symbol in html.

  •    Open versus constant volume systems:   There are two methods in routine use for measuring rates of gas exchange: open system respirometry and constant volume ('closed system') respirometry.  In the former, a continuous flow of air (or other fluid) moves past the animal, and you measure the incurrent vs. excurrent difference in gas concentrations.  Most of the calculations and discussion on this page concern open systems.

    In constant volume respirometry, the organism is placed in a sealed chamber, and over time its respiration changes the gas concentrations in the chamber.  You measure rates of gas exchange by determining gas concentrations (O2 and/or CO2) at the start and end of a period of measurement, and then using the cumulative difference in concentrations and the elapsed time to compute the average rate of change.  The most straightforward way to handle constant volume calculations with LabHelper and LabAnalyst is as follows:

    A more versatile method for computing C.V.R. is in the SPECIAL menu. This offers more options than the C.V.R. window in the maximum value calculator

    If you have an oxygen or CO2 sensor within the sealed chamber (or if the respiratory fluid is circulated past a sensor), you can measure the decline in pO2 or increase in pCO2 continuously, and then take the derivative of the change to get time-specific rates.

  •    With either open system or constant volume respirometry, if your animal is an air-breather you need to think about the possibility that its metabolism involves gases other than water, O2, and CO2.   For example, ruminants and other herbivores may emit large quantities of methane and other organic gases.  These result largely from the microbial fermentation in specialized gut regions.   However, the effect is not limited to plant-eating species with specialized fermentation chambers.  Any animal with bacterial flora in the gut may produce organic gases (humans are well known for this...).   Predators such as snakes can emit substantial amounts of various decomposition gases (H2, etc.) during the digestive breakdown of prey.
                These additional gases have two possible effects, both of which can compromise measurement accuracy:

    • With any gas analyzer, 'extra' gases dilute the concentrations of O2 and CO2. Unless you quantify this, you might not calculate VO2 or CO2 accurately.  Most animals don't emit enough of these gases to cause much of a dilution problem, but you need to be aware of the potential.
    • With some oxygen analyzers that have high-temperature measurement cells (like the zirconia cells used by Applied Electrochemistry/Ametek S-3As), another error can result when organic gases oxidize -- combust -- in the cell.  The S-3A's cells operate at more than 500 °C, so there is plenty of potential for this to occur.  Oxidization removes oxygen from the gas stream.  It also produces CO2, water, and other reaction products that lower the concentration of the remaining O2 and further increase the error (generating an artifactually high VO2). 

    If your animal does produce substantial quantities of non-standard respiratory gases, the solution is to remove the problematic gas species from analysis air by using an appropriate scrubbing filter upstream of the gas analyzer(s).   This, too, incurs operational penalties, such as reduced response time and more scrubbing tubes to keep fresh.

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