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• Respirometry
  • Respirometry basics
  • Common functions
  • Compute VO2
  • Compute VCO2
  • sensor humidity compensation
  • Compute EWL
  • VO2 in water
  • 'Instantaneous' correction
  • O2 heat equivalence
  • CO2 heat equivalence
  • Effective volume
  • Flow measurement accuracy
  •           Open systems design
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  •           Closed systems design
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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 'constant flow' (or 'open sytem') respirometry, where a respiratory fluid -- typically air or water -- is flowed past the animal while excurrent gas concentrations are continuously monitored.   For the alternate method of 'constant volume' (or 'closed system') respirometry, see this page.   Different means of calculating gas exchange with constant volume respirometry are shown here.

These functions convert gas concentrations -- O₂, CO₂, H₂O -- into rates of gas exchange:   VO₂, VCO₂, evaporative water loss (EWL).   Gas concentrations should be in units of % or for EWL, % RH, vapor pressure, or dew point temperature.   The program expects gas concentration to be expressed as difference from reference levels, with reference (baseline) set to zero (in the PREFERENCES option in the LabHelper menu, you can select automatic lag and baseline correction prior to gas exchange calculations).

Choose one of two computation modes:

• Positive-going deflection:   Changes in gas concentration are positive with respect to baseline (the default).

• Negative-going deflection:  Changes in gas concentration are negative with respect to baseline. This can be used if -- as in oxygen consumption -- gas exchange is measured as a depletion of gas concentration.   Many users find positive-going deflections more intuitive, but the negative-going option is available if desired.

Results are stored either in the source channel or optionally (if there are less than 40 channels) in a new channel.

Gas calculations begin with flow rate source selection.  Flowrate is entered from the file or by the user, or is obtained from a channel (i.e., recorded from a flow meter).

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

For VO₂ and VCO₂ , you need to specify:

• whether incurrent CO₂ is absorbed – note that this refers to gas entering the metabolism chamber, NOT to gas entering the analyzers.   If you are using ‘standard’ air (where the CO₂ concentration is about .04%), this option has little effect.

• concentrations of O₂ and CO₂ in ambient gas (FiO₂ and FiCO₂; default 20.95% and 0.04%, respectively – note that the program expects these to be entered as percentages, not fractions).

For EWL calculations (VH₂O), and for VO₂ and VCO₂ if you did not use dry incurrent gas, you will also need to indicate the type of sensor used to measure the water vapor content.   Most lab-grade humidity sensors output relative humidity (the current humidity relative to saturation water content), dew point (the temperature to which air must be cooled to become saturated with water vapor), or water vapor pressure in units of pressure (usually pascals or kilopascal) or mass/volume (usually micrograms/mL, mg/L or grams/m³; all of these are equivalent).

For relative humidity sensors, you will also need to indicate operating temperature of the sensor (this is done later; see below).

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Channel selection:  In the next window (below,left) you select the channel containing gas concentration data, and if you are obtaining flow from a channel, the flow rate channel:

Flow configurations:  Next you select a flow configuration ('Mode' 1, 2, etc.).  This specifies the conversion equation to be used, based on where flow rate is measured (upstream or downstream of the animal chamber and gas sensor) and how (and if) the the gas stream is dehumidified and scrubbed of CO₂.

The 'masks' button (VO₂ and VCO₂ only) opens a similar window with a selection of mask configurations (example for VO₂ shown at right).  A mask system is defined as one in which unscrubbed ambient gas - usually air - is pulled past the animal to capture exhaled gas (i.e., gas is sucked past the animal under negative pressure).  In mask systems all pumps, flow meters, gas analyzers, etc.  are downstream.
       Some mask systems are functionally identical to some 'regular' flow configurations and use the same conversion equations. The '?' button describes these overlaps, and also certain configurations to avoid if possible.

Pick the configuration and mode closest to your own respirometry system.  It's quite important to give LabAnalyst the right information about your flow arrangement.  In some -- but not all -- cases, with 'normal' incurrent concentrations of CO₂ and O₂ (about 0.04% and 20.9%), different modes yield fairly similar results.  However, in some cases, serious errors (20% or more) can result if the wrong equations are used (this is most problematic for VO₂).  Selecting the appropriate configuration is most critical if the sample gas from the animal chamber has large deflections from ambient concentrations of CO₂ and O₂.  An important and easily avoidable error will result if the gas stream at the flow meter contains a significant (but unknown) fraction of water vapor.  Therefore it's good practice to use DRY gas whenever possible -- and if you can't, be sure to compensate as described in the next section.