The Wheatstone Bridge Porometer
| Autor: | O. V. S. Heath, J. Russell |
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| Rok vydání: | 1951 |
| Předmět: | |
| Zdroj: | Journal of Experimental Botany. 2:111-116 |
| ISSN: | 1460-2431 0022-0957 |
| DOI: | 10.1093/jxb/2.1.111 |
| Popis: | The main trend of porometer design has been to improve ease and rapidity of reading: thus the original porometer of Darwin and Pertz (1911), the Knight porometer (1915), and the 'resistance porometer' of Gregory and Pearse (1934), if calibrated in absolute terms, are all capable of yielding the same information, viz. leaf conductance or resistance to viscous air flow, but they represent successive improvements in convenience. Sensitivity is so much a matter of dimensions that comparisons between the different designs are difficult to make, but it seems probable that the Knight porometer is the least sensitive. A notable exception quite outside this main trend is provided by the 'diffusion porometer' of Gregory and Armstrong (1936) which yields information of much more fundamental importance but is certainly less convenient. The slight modification made in the resistance porometer by Heath (1941) and the more extensive alterations made by Spanner and Heath (1951) had for their object the reduction of various errors, although in the latter case the use of a 'gasometer' type of aspirator and manometers filled with paraffin also resulted in greater ease and accuracy of reading. The new porometer described below represents, it is thought, a further advance in the same direction, having proved in use to be of greater convenience than any other ; it can also be made to give reasonably rapid and sensitive responses over an extremely wide range of leaf resistance. It has been called the 'Wheat stone bridge porometer' since it is in fact nothing but a conventional Wheat stone bridge, with pneumatic pressures in a system of capillaries substituted for electrical potentials in electrical conductors. The Wheatstone bridge prin ciple applied to air flow has been employed previously in this Institute (Porter, Pal, and Martin, 1950) for standardizing capillary resistances'. In the present apparatus a null method of reading is made possible by the use of a continuously variable resistance—in this case a fine control needle valve. The apparatus is shown diagrammatically in Fig. 1. The aspirator. A 'gasometer' type aspirator working in medicinal paraffin (Spanner and Heath, 1951 ) can be used to provide either a negative or positive pressure difference, but the latter method is preferable for reasons discussed in the paper cited. The aspirator may then be used to supply moist, but not saturated, air to the system, thus reducing errors due to augmentation of the mass flow with water vapour during passage through the leaf. The 'circuit' is completed by the atmosphere. Since air escapes through both pairs of arms of the bridge, at low resistances the aspirator requires rather frequent replen ishment. It is therefore convenient to effect this through the tube t, by means |
| Databáze: | OpenAIRE |
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