AGRICULTURE

date : 23/05/2015

PORTABLE PHOTOSYNTHESIS SYSTEMS 

MEASUREMENT THEORY

The PHOTOSYNTHESIS SYSTEM is a completely self-contained unit for measuring the CO2 assimilation (Photosynthesis and Respiration) and transpiration (water loss by evaporation of leaves of plants. These systems are designed specifically for use by students in Schools and Universities. It offers many of the facilities of instruments designed for research , but greatly simplifies the measurement procedure.
 
It operates on the Open System principle. The leaf is placed in a sealed enclosure with a window for illumination. This is referred to as the leaf cuvette. Through the cuvette is passed a measured flow of air. The CO2 / H2O concentrations of the air entering (reference air) and of the air leaving (analysis air) are measured. To measure the concentrations the PHOTOSYNTHESIS SYSTEM uses a single CO2 and H2O sensor and alternately switches the reference and analysis air. From the flow rate of air and the change in the concentration the assimilation and transpiration rates are calculated.

Though it is designed to supply ambient air to the cuvette, for the study of CO2 responses, it is possible to decrease the CO2 concentration in a series of steps. A similar provision is made for water vapour responses. It is supplied with a leaf cuvette which can be used for a wide variety of leaves. A light unit will be available shortly (LED) for use with the cuvette for manual control of cuvette light intensity.

CO2 /H2O ANALYSIS MEASUREMENT PRINCIPLE

Carbon Dioxide absorbs Infra-red radiation strongly at a wavelength of 4.26 microns. PHOTOSYNTHESIS SYSTEM uses this absorption to measure the CO2 concentration. The analyzer consists of a source of infra-red radiation (a small tungsten filament lamp) at one end of a highly polished, gold plated tube through which the air passes. At the other end of the tube is the infra-red detector which has a window through which only infra-red radiation at 4.26 microns can pass so that the responds only to the presence of CO2. The theoretical analysis range is from 0-100% CO2. However, because of the absorption characteristics of gases, the absorption path lengths, infrared source intensities, detector sensitivities and the S/N (Signal to Noise) ratio of the system define the effective range. The absorption path length of PHOTOSYNTHESIS SYSTEM is optimised for 2,000 volumes of CO2 per million volumes of air. This is correctly referred to as 2000 parts per million by volume or 2000 ppm. (Ambient air contains about 360 ppm.).
Temperature corrections are not required as the opto-electronics are thermostatted and the air is equilibrated to this temperature before entering the absorption cells. The built in transducer compensates for absolute pressure changes in the cell.
In part, the excellent stability of PHOTOSYNTHESIS SYSTEM is due to regular zeroing when CO2 free, air is passed through (referred to as ZERO). ZERO minimises the effects on span (gas sensitivity), of sample cell contamination, source ageing, and changes in detector sensitivity, amplifier gains, and reference voltages. It is done every approximately minute. The ZERO reading is used to compensate for changes in the signal level. From the relationship between absorptance and concentration, determined in the factory, and the current calibration factor, the sample concentration is determined.

Water vapor is measured using a high precision capacitive sensor. This consists of a small piece of glass coated first with a layer of metal, then with a polymer, followed by a second metal layer. Wires are soldered to the metal layers and the sensor is placed in a circuit that measures its electrical capacitance. The amount of water in the polymer depends on the water vapour content of the air and the electrical capacitance of the polymer depends on the water content. So with calibration, the water in the air can be measured. Water vapour concentration is again expressed as a volume/volume relationship but in parts per thousand, which is called millibars (mb)

Both CO2 and H2O measurements give the absolute concentrations for the reference air, and then the difference between the reference and the analysis concentration.
The complete PHOTOSYNTHESIS SYSTEM gas circuit with control valves is shown below.














Near-infrared reflectance (NIR) spectroscopy is widely used for the quantitative determination of quality attributes such as moisture, protein, fat, and kernel hardness in agriculture and food products . It is an approved method for quantitative measurement of wheat protein and moisture content. NIR instruments for grain and grain product measurement have predominantly used grating monochromators to obtain spectral information, which is measured with a single or diode array detector. NIR instruments use various hardware configurations to obtain spectral information, and Fourier-transform near-infrared reflectance (FT-NIR) instruments are only one method to do this. FT-NIR hardware is generally more complex but advances in electronics, methods, and manufacturing have significantly improved the detector sensitivities, resolution, and immunity from vibration effects .


FT-NIR records the intensity of the entire spectrum as a function of the optical path differences (OPD) between two NIR beams in an interferometer. The two beams are created by splitting the measurement beam, i.e., the beam that is transmitted through or reflected from the specimen. One split beam travels over a different optical path length, via a moving mirror, and is recombined with the second beam to create an interference signal. The total interference signal results from the mirror traveling through a range of wavelengths and is transformed to spectral components via a fast Fourier transform.  Gave a detailed
explanation of FT-NIR and cited its advantages over conventional grating NIR spectroscopy as 1) higher signalto-noise ratios, 2) extremely high resolutions, and 3) fast and accurate frequency determinations. The first advantage is realized because there are fewer optical components to attenuate radiation which leads to greater power reaching the detector and a better signal-to-noise ratio. The second advantage is that resolution at discrete wavelengths is much better, and thus elements which interact within very narrow
bands can be detected. The third advantage states that spectral collection is much faster than with a dispersive instrument, since all wavelengths are measured simultaneously. While this last advantage is somewhat true, it can also be misleading since simultaneous measurement occurs only at discrete mirror positions. The total interference signal must be measured at discrete points during the mirror’s travel, which requires a finite time period. After the entire time-domain interference signal is measured, it is then
transformed to the frequency (spectral) domain via a fast Fourier transform (FFT) to obtain discrete wavelength information. When using FT-NIR at the shorter NIR wavelengths, compared to mid-infrared, the
interference pattern is more influenced by misalignment and may cancel any added advantage FT-NIR may have over grating NIR instruments. Thus small perturbations in mirror positioning or misalignment are sources of error and as a result, lasers are commonly incorporated to provide a position reference. To achieve a position reference, monochromatic laser output is directed through the interferometer onto its own detector. The precision of this alignment is not generally specified by the manufacturer. Other literature indicated that the advantages of   FT-NIR/IR are in the mid-infrared region, not the NIR region, because of the higher signal/noise (S/N) ratio using detector noise- limited instrumentation. NIR instruments do not use
detector-noise limited instrumentation. Biological materials absorbs over broad regions in the NIR, not at discrete wavelengths. Thus, the advantage of FT-NIR being able to detect absorbance in very narrow bands may not be a significant benefit. 

FT-NIR has been applied to study various attributes such as fat, protein, cholesterol, lactose, and internal quality in products of agriculture and food industries.

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