| Gas exchange: Sensor humidity compensation
Note: this is related to, but not the same as, the effect of water vapor on measurements of flow rate, as discussed on this page.
In order to accurately compute gas exchange rates from undried gas, it is necessary to remove the dilution effect of water vapor in the gas stream (which slightly reduces the concentration of O2 or CO2 in the gas analyzer). This causes an underestimate (VCO2) or overestimate (VO2) of 'true' gas exchange rates. The resulting error is small if the temperature and relative humidity of the measured gas are low, but increase rapidly as temperature and humidity climb (see the figure in the section on water vapor correction showing the volumetric contribution of water vapor at different temperatures and humidities). Low barometric pressure increases the error still further.
You use several methods for compensating for measurements on wet gas, but for all of them you must provide information of the temperature and water content of the measured gas stream. You can use recorded channels of relative humidity or dew point, or provide a constant value. Similarly, you can use recorded or constant values for temperature. These are selected through a series of windows. Note that some combinations require extensive vapor pressure calculations for each data point, which slows the computation speed; in these cases a progress bar is shown.
Note that these routines compensate only for the passive dilution effect of water vapor. They cannot rectify any active interference with the sensor caused by water. In other words, if a sensor detects water vapor and 'confuses' it for the gas of interest (CO2 or O2), there is no alternative to completely drying the gas stream prior to measurement. Fortunately, most O2 and CO2 analyzers are relatively immune to this problem.
Entering the required data involves several successive windows, the first of which is shown at right. You need to choose between VO2 or %O2 deflection (the difference between breathed and inbreathed air) versus VCO2, %CO2, or absolute %O2 content (i.e., the 'raw' concentration of O2 prior to zeroing to baseline). This selection is necessary because water vapor in the sensor dilutes the concentrations of other gases. If you calculated VO2, or zeroed O2 content relative to reference, that dilution will artifactual increase the calculated VO2 (because it increases the difference between the O2 content of dry, unbreathed air and humid breathed air). However, for VCO2, or %CO2, or unadjusted O2 content, water vapor tends to decrease the calculated VCO2 (or VO2) because it reduces concentrations. This seems confusing, but remember than animals consume oxygen (i.e., lowering O2 content of air going to the sensor) but produce CO2 (i.e., raising CO2 content of air going to the sensor). Finally, you can select if you want to store the result in a new channel (if the number of channels is less than the maximum channel count).
The next step is to specify whether sensor gas water content is measured as vapor pressure (kiloPascals or micrograms/mL) or as relative humidity or dew point temperature:
Note that it's usually necessary to adjust to ambient barometric pressure and temperature (even if flow rate has already been set to STP). This is because the partial pressure of water vapor is constant at a given combination of relative humidity and temperature, regardless of the partial pressure of the rest of the gas mixture. You also have the option of not compensating, if you wish.
You are next asked for a channel containing humidity (or water vapor) data; alternately you can use a constant RH (the default is 20%). This is shown below, at left. Next you are next asked whether the humidity data are in units of relative humidity or dew point temperature (below, at right):
If the data are in units of relative humidity, you are asked to select a channel containing humidity temperature data (as shown at right), or a constant temperature for the RH sensor:
Once all these data are entered, the software will compensate for temperature differences between the humidity sensor and the flowmeter.