18.104.22.168. Parallel measurement arrangement
In parallel measurement mode, two sets of pumps and mass flow controllers provide the reference and measurement sensors of the measurement device simultaneously with independent gas streams. This avoids delays between reference and measurement gas stream if the tubing of both sides has the same length. A well-tried sequence of devices and parts for the measurement stream is:
measurement/(reference) chamber(s) – drying columns – pumps – mass flow controllers – measurement device.
Inserting the columns for air drying before the pumps helps in this arrangement to considerably attenuate coupling of pump noise to the measurement chamber at the tube inlet (Stabentheiner et al., 2012).
The parallel mode is preferable with fuel cell devices (for O2 consumption measurement) because they are very sensitive to pressure differences between the reference and measurement channels (see section 22.214.171.124.).
The parallel mode is also preferable if the respiratory quotient (RQ, quotient VCO2/VO2) is to be determined with open-flow respirometry, with a combination of an infrared gas analyser (VCO2; section 6.1.3.) and a fuel-cell device (VO2; section 6.1.2.) in series. The need for frequent recalibration of fuel-cell instruments necessitates a possibility to bypass the bee measurement chamber. This can be done with manual three-way valves which allow smooth switching. If automated switching is intended magnetic valves are not suited because their pressure impulses lead to large unwanted signal offsets (Stabentheiner et al., 2012). Motor stepping valves are be an alternative.