Measurement range selection

DIRGA measurement range and sensitivity is determined by the tube (or better: infrared beam) length. A frequently offered measurement range is 0–3000 ppm, well suited for the measurement of endothermic or flying bees.

0–250 ppm tubes in the DIRGA provide a high resolution, well suited for resting metabolism measurements of bees and for tiny honey bee larvae. With this measurement range, however, endothermy of adult bees will soon produce a range overflow. With a 0–10,000 or 0–20,000 ppm tube, the respiration of even whole colonies can be measured. A high resolution (small range) and a large range (lower resolution) system in series always provides a calibrated output (ABB, Inc.; Stabentheiner et al., 2012).

It has to be considered, however, that there exists an interrelation between the gas flow and the height of the measurement signal (Gray and Bradley, 2006; see section 6.1.5.). In endothermic foragers (in an 8-18 ml measurement chamber) for example a flow rate of 250 ml min-1 provides a sufficient signal height with a 0-3000 ppm DIRGA. During investigations of honey bee resting metabolism with a 0–250 ppm tube, a flow of 150 ml min-1 turned out to provide a good compromise between sensitivity and temporal resolution of CO2 production measurements in a wide range of ambient temperatures (2.5–45 °C; Kovac et al., 2007). With small honey bee larvae, the flow rate has to be reduced, for example to 20 ml min-1 (Petz et al., 2004).