188.8.131.52. Air drying, CO2 scrubbing and tubing
1. Chemical scrubbers: Drying of the air by some means is a prerequisite to obtain accurate results in insect respiratory measurements. Fuel-cell O2-measurement devices require completely dry air. Chemical scrubbers (desiccants) are inserted in columns, usually before the mass flow controllers. A commonly used desiccant is Drierite (contents: >98 % CaSO4 (gypsum), <2 % CoCl2). For special purposes, e.g. if CO2 has also to be absorbed, granular magnesium perchlorate, Mg(ClO4)2, Ascarite is an excellent CO2 scrubber.
2. Cool traps: For CO2 measurement with a differential infrared gas analyser (DIRGA) complete air drying is not necessary in all cases. Cool traps (regulated at temperatures of ca. 2-10°C, for example) can bring relative humidity to a low and constant level. This works well because DIRGAs are typically operated at internal temperatures of >50°C. Relative humidity during measurement is therefore low. Since the water content reaching the DIRGA is the same during calibration and measurement, and in the measurement and reference gas stream, differential measurements are accurate. However, independent cool traps are needed for the measurement and the reference gas streams (Lighton, 2008; Stabentheiner et al., 2010), which makes the equipment more expensive.
3. Tubing: Tubing materials are diverse. A flexible and very durable material well suited for respiratory measurements is Viton®. Specialists of insect respiration also use metal tubes (which of course are less flexible). Inner diameter of tubes should in any case be small, e.g. 2-4 mm, to achieve optimal results with honey bees.
User safety advice: Take care not to inhale Drierite. While fine gypsum particles may already impair respiratory functions, CoCl2, which is used as a humidity indicator, is extremely hazardous. Ascarite is also extremely hazardous. To quote from the Oxzilla 2 (Sable Systems International) manual: "Try not to breath too much of it if you plan to see your grandchildren."