Respirometry without scrubbing CO2 and water vapor

If you are unfamiliar with the fundamentals of respirometry, you might want to read this page before diving into the rest of this section..   Also, John Lighton's book, Measuring Metabolic Rates - A Manual for Scientists is an excellent reference.

This page covers a special case of 'constant flow' (or 'open sytem') respirometry, where a respiratory gas -- typically air -- is flowed past the animal while excurrent gas concentrations are continuously monitored.
      In 'classical' constant flow systems, water vapor and/or carbon dioxide are 'scrubbed' out of the analyzed gas stream prior to measurement, making for relatively simple calculations and avoiding the difficulty of an unknown dilution of O2 and CO2 concentrations by water vapor.   But as discussed here, if the water vapor content of analyzed gas is measured, it is possible to accurately determine O2 consumption and CO2 production on a 'wet' gas, as well as the rate of water loss.   Note that:

  • Excurrent oxygen concentration must be in % and stored as the deflection from baseline (i.e., the difference between incurrent and excurrent O2)
  • O2 deflections from baseline must be positive-going (i.e., higher [O2] indicate more deflection and hence increased O2 consumption)
  • Excurrent CO2 concentration must be in %
  • Water vapor data must be in units of kilopascals of water vapor pressure
The equations used to compute VO2, VCO2, and mH2O are on this page.

After selecting the No-Scrub option from the Toolbar or the Respirometry menu, this window appears:

     

You need to select the channels for O2 deflection (∆ O2), CO2, and water vapor pressure.
          It's best to measure all of these parameters, but if you want to calculate VO2 but do not have a channel for CO2, select the 'Estimate CO2 from RQ' option.   Conversely, if you want to calculate VCO2 but do not have a channel for O2, select the 'Estimate O2 from RQ' option.   However, you MUST have EITHER a channel with O2 data, OR a channel with CO2 data, or both.   You ALWAYS need to have a channel containing water vapor pressure measurements, in kilopascals.
          Before proceeding, make sure the values for FiO2 (incurrent O2 concentration), FICO2, and incurrent water vapor pressure are correct.


The next window is for selecting the channel with flow rate data (or use of a constant flow rate), along with STP data:

     

Important:   The program expects flowrates to be in units of ml/min, unless set to liters/min in this window.


Next, you need to indicate the arrangement of the respirometry system 'plumbing':

     

Note that since CO2 and water vapor are not scrubbed from the analyzed gas, the arrangement of the sensors for O2, CO2, and water vapor is not important (and either the O2 or CO2 sensors are not necessary if those data are estimated from RQ).   Similarly, it is irrelevant if the flow through the gas sensors is subsampled.   What IS important is the position of the flowmeter with respect to the animal chamber (or mask):   upstream versus downstream.
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For orientation, the overall direction of gas flow in this diagram is from left to right.


Finally, you need to select which of the three computed gas exchange rates are to be saved.   Note that these variables are always stored in NEW channels, so you must have less than 38 existing channels if you wish to save all of them.   You must select at least one channel to save.

     

The default output units for these channels are ml/min for VO2 and VCO2, and mg/min for mH2O.   You can use the pop-up buttons to output results in a variety of other units.   If you select heat-related units (e.g., watts, kilojoules/day, etc.), you will need to specify either the heat equivalent of VO2 and VCO2 or (for evaporative water loss), the temperature of the evaporating surface.   The latter has a small effect on the heat of evaporation.

       

                  

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