PP Systems CIRAS-2 - Appendix 3. Photosynthesis Equations Used in CIRAS-2; Calculations; Step 1.0; Step 2.0

Appendix 3. Photosynthesis Equations Used in CIRAS-2
CIRAS-2 Operator's Manual Version 2.04
- 155 -
Appendix 3. Photosynthesis Equations Used in CIRAS-2
Calculations
Step 1.0
Calculate the mass flow of air (W) per unit leaf area entering the cuvette
The CIRAS-2 mass flowmeter is calibrated to read the volume flow at 20 °C and 1013.25 mb (V
20
). Molar
volume is 22.414 at 0 °C and 1 standard atmosphere (STP). Therefore:
Where: a is projected leaf area (cm
2
). The V
20
term is calculated above using cm
3
sec
-1
, but displayed by
CIRAS-2 as ml min
-1
.
Step 2.0
Calculate transpiration rate (E) from the partial pressures of water vapor of the air entering (e
in
)
and leaving (e
out
) the cuvette
(2.1) The molar flow of water vapor (mol m
2
s
-1
) into the cuvette is:
(2.2) The molar flow of air out of the cuvette (due to the addition of transpired water) is (W+E). Therefore,
the molar flow of water vapor out of the cuvette is:
(2.3) However, the difference between the molar flows into and out of the cuvette must equal the
transpiration, so: