6.1.5. Impact of flow control and measurement chamber size on sensitivity and temporal resolution

Gas flow has to be accurately regulated by mass flow controllers (compare section The respiratory output signal, however, depends on several factors (Gray and Bradley, 2006; Lighton, 2008; Terblanche et al., 2010). Always consider that washout phenomena and signal delays influence measurements, depending on the volumes of the measurement chamber and the tubing. If you see respiratory peaks like in Figure 10, for example, keep in mind that signal rise and signal decay characteristics are always distorted. With proper calibrations and mathematical treatment (Z transformation; Lighton, 2008), the original signal characteristics may be restored to some extent for individual respiratory peaks or events.

Signal height is also influenced by several factors. In the first place it is influenced by the measurement chamber volume. Smaller volumes lead to a higher CO2 accumulation and O2 depletion. A low flow rate (recommended for small individuals) accumulates CO2 (decreases O2; Fig. 11) and therefore increases signal height. On the other hand, this reduces temporal resolution. A high flow rate decreases CO2 concentration (increases O2) and this way decreases signal height, but improves temporal resolution. A high flow rate may help to avoid signal overflow, for example if larger bee groups or whole colonies are to be measured. One has always to find a compromise between sensitivity and temporal resolution. Taking a measurement chamber as small as possible and increasing flow speed to a not too low value will increase sensitivity, and this way improve detection of small changes in the respiratory signal. A too high flow rate in a small chamber, however, will increase convective heat loss, especially in endothermic or flying bees. As thermoregulating bees probably counteract such additional losses, the measured metabolic rates may be overestimated as compared to natural situations.

Fig. 11. Effect of flow rate on the respiratory measurement signal. Graph by Anton Stabentheiner.

Figure 11