Making Measurements Leaks

Transcription

1 Check the seal around leaves and stems When the gaskets are compressed around thicker leaves and stems, small gaps will be created. At high flow rates, this may not be a problem, but at lower rates, be sure to seal them with putty or gum. f bulk flow leaks are there, they will be a bigger problem at low flow rates, than at high flow rates. Diffusion CO moves from high concentrations to low concentrations. t does this not only through air, but also through solids, including many plastics and synthetic rubbers. CO can pass through Bev-a-line tubing (polyethylene, lined with ethylene vinyl acetate), through gasket material (ethylene and neoprene), and through O-rings (butyl rubber). t goes through just about everything thatõs not glass or metal. There are some thin-film materials that have very low permeabilities to CO, however, such as Teflon, Saran (polyvinylidene chloride), Mylar, and Propafilm (polypropylene coated with Saran). Diffusion of CO into or out of the L-64 leaf chamber is proportional to the difference between the inside and outside CO concentrations. t is useful to think of this diffusion leak as a flux of CO. When the bulk flow rate through the chamber is high, this diffusion flux will affect the chamber concentration very little. When the bulk flow is low, that same diffusion flux will have a much larger effect. This model is presented in Figure 4-9, and predicts Using the L

2 4 Making Measurements a linear relation when normalized leak is plotted against 1/flow rate. (The diffusion leak is ( c o Ð c i ); the normalizer is the gradient ( c a Ð c o ).) u c i c a c o k u c o rv c o t uc i Ð uc o + kc a Ð kc o uc ( i Ð c o ) + kc ( a Ð c o ) uc ( o Ð c i ) kc ( a Ð c o ) c o Ð c i k c a Ð c o u Figure 4-9. A model for diffusion in an open photosynthesis system, based on conservation of mass. The walls of the system have CO leak rate k (mol s -1 ). u is bulk flow rate (mol s -1 ), and c i, c o, and c a are incoming, outgoing, and ambient CO concentrations (mol mol -1 ). n steady state, the change of CO with time in the system To verify this model, we performed a simple experiment 7. We used the CO mixer to generate a variety of reference cell CO concentrations, and recorded the difference between the sample and reference cell concentrations at various flow rates. (No leaf in the chamber.) We took the following steps to ensure the ambient CO concentration was as stable as possible: 1) data collection was done with an AutoProgram, so no one had to be standing there breathing; ) the instrument was set up in a vacant, well mixed greenhouse; 3) an external fan continually ventilated the chamber; 4) the ambient CO concentration was monitored with a second gas analyzer. Sample cell CO concentrations were measured by the reference RGA using the match valve. This eliminated any potential errors due to RGA drift, since the same RGA 7. Simple in hindsight - it took a couple of weeks to get it right Using the L-64

3 (reference) was used for both sample and reference measurements. The results are plotted in Figure CO Diffusion Effect C µmol mol -1 r C s - C r (µmol mol -1 ) -5-1 C 4 µmol mol -1 a Flow Rate (µmol s -1 ) Figure 4-1. CO diffusion into a closed, empty L-64 chamber with black neoprene gaskets as a function of flow rate, for various reference concentrations C r. C a (ambient CO ) was 4 µmol mol -1. Diffusion effects are highest at low flow rates and large gradients. The lines in the figure connect data of common C r. The diffusion effect( C s Ð C r ) plotted in Figure 4-1 is equivalent to the ( c o Ð c i ) term in the model in Figure 4-9. Normalizing by the gradient ( C a Ð C s ) should make this data fall on one curve if the model is correct, and it does to a reasonable degree (Figure 4-11). Curve fitting yields a diffusion coefficient k of.46. (The easiest curve fit is to plot (1/Flow) on the X axis, rather than Flow; k is then the slope of the line.) Note that the outliers tend to be the data collected with the smallest gradient, so uncertainties in what the true gradient was are largest. Using the L

4 4 Making Measurements (C s - C r ) / (C a - C s ).1 Normalized CO Diffusion f(x).46 x C r µmol mol Flow Rate (µmol s -1 ) Figure Data from Figure 4-1 normalized by the CO gradient, (C a - C r ). 5 n order to correct photosynthesis calculations for the effects of CO diffusion, we re-visit the derivation of the photosynthesis equation. The mass balance equation ((1-11) on page 1-9) becomes sa uc i Ð uc o + kc ( a Ð c o ) (4-) The kc ( a Ð c o ) term accounts for diffusion. The final equation ((1-15) on page 1-1) becomes FC ( r Ð C s ) k A Ð C 1S s E ( C 1S a Ð C s ) (4-3) Note that there are now two Òcorrection termsó: one for transpiration, and one for diffusion. The diffusion correction term is insignificant for measurements with near-ambient CO concentrations in the chamber (Table 4-). Near the CO compensation point itõs a different matter; diffusion becomes signifi Using the L-64

5 Operational Hints Operational Hints cant, and failure to account for it will lead to a large relative overestimation of assimilation rate. Table 4-. Typical values of the three terms of the net photosynthesis equation under two sets of conditions Term Equation Near Ambient CO Uptake FC ( r Ð C s ) 3( 38 Ð 34) S Transpiration Correction Diffusion Correction kc ( a Ð C s ) S Net Photosynthesis A protocol for measuring at concentrations well away from ambient is: Minimize the gradient Keep high CO (breath) away from the chamber. Keeping the chamber well ventilated will help do this. f possible, collect data with AutoPrograms, so an operator doesnõt need to be nearby. Use the diffusion corrected formula for photosynthesis mplement the correction in the Compute List. Near CO Compensation ( 5 Ð 48).67 C s E ( 37 Ð 34) ( 37 Ð 48).1 Ð Ð Air Supply Considerations An open system, such as the L-64, is only as good as the incoming air stream is stable, especially with respect to CO concentration. When the incoming air is fluctuating in CO concentration, there will be phase differences as those fluctuations pass through the reference RGA and the sample RGA, resulting in fluctuations in the CO differential - even with no leaf in the chamber. There are essentially three options for making the incoming air stable: Using the L