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Stability and accuracy of CIRAS-2 Water Vapor (reference humidity): control option 1


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Rapid large-scale fluctuations of carbon dioxide and water vapor, although unnatural and artificial in the plant world, are desirable attributes for gas exchange systems from a sampling efficiency standpoint. As PP Systems’ benchmark IRGA system for gas exchange and photosynthesis research, CIRAS-2 was designed for and expected to have exceptional response times to user-initiated changes in reference gas concentrations of CO2 and water vapor. This document provides information and data specific to the most commonly used water vapor control option available with CIRAS-2, H2O control option 1 (‘Supply (Ref) % of Ambient’).

Supporting evidence is mainly in the form of simple timed profiles of reference water vapor, from a stable origin condition through the delta phase, to the target steady-state condition. This information, analogous to operator uncertainty about (leaf) physiological stability, can help CIRAS-2 users determine when the system is likely to be stable, and the degree of stability following experimental changes. It also indicates the target accuracy of selected changes below ambient, both in terms of measured output (partial pressure, mb) and calculated output (%RH).

CIRAS-2 has five menu options for operator control/manipulation of leaf chamber water vapor. For CIRAS-2 users, it is at times a source of confusion as to which option is best – generally speaking, the first two options (% of Ambient and Set mb) provide the most stable reference water vapor. The control itself is based solely on diversion (valve duty-cycle) of the reference gas through the air supply dessicant columns (orange color-indicating Envirogel), and not subject to feedback from the leaf in the chamber (Analysis gas control options).

Test Design
CIRAS-2 was allowed to warm up for 30 min. in the standard open-system portable photosynthesis mode, with an attached PLC6 cuvette. Within 5 min. the system was operating normally in Measure mode, having attained its stable IRGA temperature of 55 ºC and performed Autozero (Z) and Differential Balance (DB) functions. With the leaf chamber empty and closed and the cuvette temperature stabilized at 24.7 ºC, the stable reference partial pressure of water vapor was determined to be 19 mb, with a firmware-calculated RH of 60%.

It is important to note that this humidity level, sometimes referred to in CIRAS-2 operational literature as “ambient”, is in fact artificially elevated above true background ambient. This is due to unintentional humidification of the reference gas stream passing through soda lime, between the air forwarding and main air supply pumps. The water vapor equilibrator, next in line along this path, removes a large portion of this water vapor, but fails to remove it entirely. Although this is documented in the CIRAS-2 Operator’s Manual ver. 2.04 (2010), it remains a point of confusion for some CIRAS-2 users. For instructions on determining external ambient RH, see CIRAS-2 Operator’s Manual (Determination of Ambient RH).

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