| Respirometry: VO2 in water
- For closed-system measurements, LabAnalyst estimates the amount of dissolved oxygen in the container at the start of measurements from container volume, temperature, solute concentration, initial oxygen saturation, and the partial pressure of O2 in the gas phase (calculated from barometric pressure and the percentage of O2 in the gas mixture, and accounting for the water vapor content of air at 100% RH).
- For open-system measurements, LabAnalyst estimates the amount of dissolved oxygen in the water flux through the system from temperature, solute concentration, initial oxygen saturation, and the partial pressure of O2 in the gas phase (calculated from barometric pressure and the percentage of O2 in the gas mixture, and accounting for the water vapor content of air at 100% RH). Flow rates are obtained either from a channel or from keyboard entry (you can also specify whether deflections in oxygen content are positive or negative).
Most of these parameters can be edited with the EDIT FILE DATA option, including flow rate, barometric pressure, temperature, and container volume (= effective volume). Alternately, you can enter your own O2 content data (dissolved oxygen per unit volume) and click the 'net O2' button, or you can directly enter net O2 content (total dissolved oxygen).
If there are less than 40 channels in the file, you can store results in a new channel.
For closed-system measurements, there are three conversion options. For any of these, you can generate absolute units or mass-specific units (per mg, per g, or per kg). Note that all conversions are based on the value shown in the 'net O2 content' edit field. If you know that oxygen content differs from the computed value in this edit field, you can enter your own value prior to doing the conversion.
- cumulative oxygen content. This simply converts the input data (% saturation, pO2, ppm, mg/L) into the residual oxygen content of the container. You can select several different output units, such as mL, microliters, mmoles, or micromoles. The slope of this line over time (multiplied by -1) is the average rate of oxygen consumption (VO2).
- total oxygen used. This option computes the cumulative amount of oxygen used during the measurement, again with a choice of output units. The slope of this line over time is the average rate of oxygen consumption. As for the previous option, several choices of units are available.
- oxygen consumption rate. This computes the rate of oxygen consumption (VO2; in a choice of units) by taking the point-to-point derivative of the relationship between oxygen use and time. Note: unless the input data have been highly smoothed, results may be noisy when averaged over short time intervals. However, long-term VO2 averages should be quite accurate, provided the O2 consumption of the organism was stable over time.
Here is an example showing conversion into consumption rate (ml O2 / [g . min]):
The Open-system (flow-through) routine will only compute oxygen consumption rate, in your choice of units. You can use either of two algorithms, selected with buttons in the initial window:
- In absolute value mode, input data are assumed to be absolute values, and VO2 is calculated by subtracting them from baseline O2 concentration (i.e., the amount of O2 in the incurrent water stream). Baseline O2 concentration is computed from pressure, temperature, etc. as described above. For example, if baseline O2 concentration is 100% saturated, an oxygen saturation datum of 1% is assumed to mean that 99% of the oxygen in the water stream has been consumed, and VO2 is computed accordingly.
- In change from ambient mode, baseline O2 concentration must be offset to zero (this should be done with the BASELINE option prior to oxygen calculations). In other words, an oxygen saturation datum of 1% is assumed to be a 1% change from baseline percent, not an absolute value of 1%.
Deflections in O2 concentration in the excurrent stream may be either positive or negative (if the latter, click the 'negative deflection' button in the initial window).