Time of exposure

The required exposure times are difficult to estimate. They are best found by trial and error. Documentation of fluorescence quenching at A = 254 nm usually only requires one trial. The exposure time found to be adequate here is normally suitable for all following exposures of fluorescence quenching if the exposure conditions are maintained constant (camera type, film type, distance of objective and lamp, aperture etc.). The exposure time required for fluorescent chromatograms is primarily dependent on the intensity of the fluorescence and, therefore, has to be optimized for each chromatogram. It is best to operate with a range of exposure times, e.g. aperture 8 with exposures of 15,30,60,120 and 240 seconds. Experience has shown that one exposure is always optimal. 

Owing to the laborious task of obtaining corrosion rates from gravimetric measurements, data for the effect of exposure time on corrosion rates have been very limited. However, with the more recent use of polarisation resistance measurements it would appear that in the absence of macro-biofouling... 

The effect of exposure time on the corrosion of copper and aluminium is illustrated in Table 2.17. The results quoted by Southwell, Hummer and... 

This result gives the flux for a small portion of the surface that has been exposed for exactly t seconds. Other portions of the surface will have been exposed for different times and thus will have different instantaneous fluxes. To And the average flux, we need the differential distribution of exposure times,/(f). Danckwerts assumed an exponential distribution ... 

Fig. 19. Variation of exposure time associated with cell rupture and the threshold shear stress for various systems [112]...

After each series of experiments with beams of various intensity the section plate would be removed from the cell and disassembled, with radioactive silver washed out by nitric acid. Radioactivity of the solutions obtained was measured by a multichannel spectrometric scintillation y-counter with sensitivity of up to 10 G, i. e. around 10 of atoms which, according to calculations, is 10 times lower than sensitivity of ZnO sensor 10 G or 10 of Ag atoms respectively [28]. This difference in sensitivity lead to great inconveniences when exposing of targets was used in above methods. Only a few seconds were sufficient to expose the sensor compared to several hours of exposure of the scintillation counter in order to let it accumulate the overall radioactivity. It is quite evident that due to insufficient stability during a long period of exposure time an error piled up. 

Figure 1 is based upon experience gathered since the 1940s. Supporting data were obtained from a variety of commercial processes and laboratory experiments (see the references to Figure 1). While temperature and hydrogen partial pressure data were not always known precisely, the accuracy is sufficient for commercial use. Satisfactory performance has been plotted only for samples or equipment exposed for at least one year. Unsatisfactory performance ftom laboratory or plant data has been plotted regardless of the length of exposure time. The chemical compositions of the steels in Figure 1 should conform to the limits specified for the various grades by ASTM/ASME.

Figure 4. Effect of exposure time on CPE n-value and relative surface area.

When step (a) reaches equilibrium, both and 8 should have steady state values. To test this hypothesis, we determined the effect of exposure time on and 8 for N80 steel in 15% HC1 at 25 C and in 28% HC1 at 65 C, as shown in Figures 6 and 7, respectively. Here [octynol] = 3.5 x 10"% in 15% HC1 and 2.1 x lO"2 in 28% HC1. It is immediately apparent from these plots that neither nor 8 reaches a steady state value during the course of the experiments. Although appears to be relatively constant throughout the 15% HC1 test and during the first stage of the 28% HC1 test, it is continually increasing in both cases. 

A study of 137 lead-exposed workers found that of various indices of exposure, time integrated index of PbB was the best predictor of variation in serum P2 1 -globulin, serum -globulin, and urinary albumin... 

The application of a modified swelling test [3] allows us to estimate the network density as a function of exposure time and of the thiol content. Highest values are achieved in stoichiometric mixtures. 

Figure 5. UV absorption spectra of a thin film of an equimolar mixture of TTDBM/SEGDVE as a function of exposure time to a Pyrex-filtered medium pressure mercury lamp (I 19 mW cm"2).

Figure 19. Formation of major components as a function of exposure time for the reaction of 10% CO/H2 at 250°C over CCI fused iron catalyst (reduced at 450°C) (52).

The choice of exposure times is complicated when more than one reaction is taking place as the rates will be different. Generally, it is best if equilibrium absorption is reached relatively quickly in comparison to chemical changes and its effects treated separately from the subsequent chemical changes. This is not unlike the situation for physical and chemical stress relaxation. 

We also note data from atomic force microscopy (AFM) versus depth, carried out by using a diamond tip for scratching patterns into the surface [12], Because of the 2° microtoming method reported, these authors were able to examine the depth profile of brittle behavior in weathered samples with excellent resolution. The data showed a very rapid decrease in the brittleness with depth into the sample which, of course, was a strong function of exposure time. The brittleness was more in line with the IR data (see above) versus depth than the molecular weight data, hence suggesting that some chain scission and branching can be tolerated in the system before it manifests brittle behavior. 

Figure 18.14 Effect of exposure time on the color versus depth profile in Spectar copolymer sheeting (3 mm) exposed in New River, AZ [10]. From Photodegradation in a copoly(ethylene/1,4-cyclohexylenedimethylene tereph-thalate) with and without UV absorber, presentation given by D. R. Fagerburg at the 37th International Symposium on Macromolecules, IUPAC World Polymer Congress, Gold Coast, Australia, July 1998, and reproduced with permission of IUPAC...

Figure 3 Polymeriztion of 50/50 w/w methecrylic anhydride initiated with 0.5 wt% DMPA and 80 mW/em of ultraviolet light (a) Illustration of the temporal eontrol of the photopolymerization. Initiation is eeased at —30 s and reinitiated at —40 s (dashed line) eompared to eontinuous initiation (solid line), (b) Evaluation of the eompressive modulus as a funetion of exposure time.

LC50 (24, 38, 72, and 96-h) values, which were identical regardless of exposure time, were 0.36, 0.16, and 0.14 mg/L for Poecilia reticulata, Brachydanio rerio, and Paracheirodon axelrodi, respectively (Oliveira-Filho and Paumgartten, 1997). 

Figure 5. The relative change in the halogen concentration as a function of exposure time for two current densities differing by 2.5 orders of magnitude.

FIGURE 5.31 Change in surface concentrations of N(ls), OUs), and Cl(2p) as a function of exposure time of an NaCl(lOO) single crystal to gaseous HNO, (adapted from Laux et al., 1996). 

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