Noise control INDEX

The sound reduction, attenuation or insertion loss is defined as the difference in sound pressure level or sound power level before and after the enclosure (or any other form of noise control) is installed. The performance of the enclosure will be largely dependent on the sound reduction index (SRI) of the outer wall , assuming approximately 50% of the internal surface is covered with mineral wool or other absorption materials °. Typical values of sound reduction index for materials used for enclosures are shown in Table 20.3. Absorption coefficients are shown in Table 20.4. 

Refractive index detectors are not as sensitive as uv absorbance detectors. The best noise levels obtainable are about 1CT7 riu (refractive index units), which corresponds to a noise equivalent concentration of about 10-6 g cmT3 for most solutes. The linear range of most ri detectors is about 104. If you want to operate them at their highest sensitivity you have to have very good control of the temperature of the instrument and of the composition of the mobile phase. Because of their sensitivity to mobile phase composition it is very difficult to do gradient elution work, and they are generally held to be unsuitable for this purpose. 

Refractive Index Detectors These detectors respond to changes in refractive index (positive or negative) arising from the presence of a compound in the eluent. All the factors which can affect refractive index must be carefully controlled (e.g. temperature, eluent composition, pressure) otherwise noise and drift will limit the sensitivity. Thus the chromatograph is best placed in a thermostatically-controlled cabinet and good pumps are desirable to minimise pressure fluctuations. Changes in eluent composition will also cause spurious changes in refractive index. 

The noise level of detectors that are particularly susceptible to variations in column pressure or flow rate (e.g. the katherometer and the refractive index detector) are often measured under static conditions (i.e. no flow of mobile phase). Such specifications are not really useful, as the analyst can never use the detector without a column flow. It could be argued that the manufacturer of the detector should not be held responsible for the precise control of the mobile phase, beitmay a gas flow controller or a solvent pump. However, all mobile phase delivery systems show some variation in flow rates (and consequently pressure) and it is the responsibility of the detector manufacturer to design devices that are as insensitive to pressure and flow changes as possible. 

Comfort is influenced by temperature, humidity, air velocity, radiant heat, clothing, and work intensity. Psychological factors may mso influence comfort, but their discussion is beyond the scope of this handbook. The reader is referred to Chap. 42 of the HVAC Applications volume of the A.S.H.R.A.E. Handbooks for a full discussion of the control of noise, which must also be considered in air-conditioning design. Figure 5 in Chap. 8 of the HVAC Fundamentals volume of the A.S.H.R.A.E. Handbooks relates the variables of ambient temperature, dew point temperature (or humidity ratio) to comfort under clothing and activity conditions typical for office space occupancy. It also shows boundary values for ET, the effective temperature index. This index combines temperature and moisture conditions into a... 

A eonsequence of using a bulk property for detection is that this property of the solvent must be controlled very closely the refractive index of the eluant is sensitive to fluctuations in pressure, temperature and composition. Whilst the pressure and composition can be controlled using pulse dampners and reciprocating pumps, the limits of sensitivity and stability of the RI detector are determined by temperature. The temperature must be controlled to +0.0001 K for accepted noise levels. Fluctuations in the RI caused by temperature and noise changes are compensated for by use of a reference cell. 

DMC uses a direct, least-squares computational procedure to find the optimum values of future changes in the manipulated variables to match future output responses such that some performance index is minimized [8]. Panda applied DMC to control the fluid-bed dryer as mentioned earlier and proved that DMC gives better control in the presence of measurement noise [19]. The response in the exit-air temperature does not show much oscillation and settles quickly. 

According to the LQG approach, a system, described by Equation (1) and the optimal control forces u t) should minimize the performance index (2). An additional assumption is that the optimal feedbackis afimction ofthe measurement vector, containing the noised floor accelerations... 

It is evident that most studies reported to date have used number density, average size or weight per cent as control variables. Often these variables are inferred from other measurements, including density, solution supersaturation, refractive index etc. Inferential techniques have been shown to be particularly suitable for industrial scale applications where laser scattering devices for on-line size distribution measurement are not yet practical for industrial control purposes, although substantial progress is being made to that end. Even when usable, however, these measurement devices are often characterized by noise and require operation at very low solids concentration. 

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