Steady-state Criteria Research Articles

Integration of bioaccumulation in an environmental risk assessment

The potential of a substance to bioaccumulate in the aquatic environment into an organism depends particularly on its lipophilicity and its metabolism within the organism. Bioaccumulation is an exposure-related parameter and will determine the body burden but it is not an ‘effect’ or hazard in itself. Therefore, when appropriate, bioaccumulation should be included as an exposure related parameter in the environmental risk assessment of substances.The “direct” toxicity assessment should consider “time to steady state” in evaluating PNEC values. Depending on the steady-state criteria used, the duration of a 96h acute fish LC50 test may be insufficient for substances with a log Kow above the range of 3.8–4.5. If exposure and uptake are possible, the BCF is estimated in an iterative approach in order to address the potential for bioaccumulation and resultant “indirect” (dietary) toxicity. When the BCF value is above 1000, a PEC/PNEC assessment for predators is made. A step-wise approach is recommended. In practice, substances which are widely dispersed in the environment, which potentially can be taken up by biota, which are persistent, lipophilic and exhibit negligible metabolism will be selected by this scheme for a more detailed evaluation.

INDUCTION MACHINE MODELS NEAR VOLTAGE COLLAPSE

ABSTRACT A steady-state stability criterion based on the concept of synchronizing torque coefficient is presented for induction machines and various first-order models of small and large induction machines are derived near voltage collapse. In large induction machines, dynamic structural changes occur in the vicinity of voltage collapse, leading to a stronger interaction between speed and voltage variables. Two modified first-order voltage models characterizing the dominant behavior of large induction machines are derived near voltage collapse.

Depolarization shifts the voltage dependence of cardiac sodium channel and calcium channel gating charge movements.

In cardiac ventricular myocytes, membrane depolarization leads to the inactivation of the Na channel and Ca channel ionic currents. The inactivation of the ionic currents has been associated with a reduction of the gating charge movement ("immobilization") which governs the activation of Na channels and Ca channels. The nature of the apparent "immobilization" of the charge movement following depolarization was explored in embryonic chick ventricular myocytes using voltage protocols applied from depolarized holding potentials. It was found that although all of the charge was mobile following inactivation, the voltage dependence of its movement was shifted to more negative potentials. In addition, the shift in the distribution of the Na channel charge could be differentiated from that of the Ca channel charge on the basis of kinetic as well as steady-state criteria. These results suggest that the voltage-dependent activation of Na channel and Ca channel charge movements leads to conformational changes and charge rearrangements that differentially bias the movements of these voltage sensors, and concomitantly produce channel inactivation.

A hybrid multi-level switching converter for ring-magnet power supplies

Conventionally, ring-magnet power supplies (RMPS) are designed using phase-controlled rectifiers or resonant networks with DC-bias power supplies. These power supply configurations satisfy the steady-state performance criteria using large reactive components in addition to the magnet load. They have limited dynamic response, and often rely on corrector-magnet power supplies or other auxiliary power supply networks for the fast dynamic compensation required for output regulation and reference tracking. This paper proposes a hybrid multilevel switching converter as a RMPS which has fast dynamic response and meets the stringent output ripple content specifications. The proposed ring-magnet power supply also has the advantage of current waveshape programmability. The operating features of the hybrid multilevel switching converter has been described. The development of the RMPS with the hybrid multilevel switching converter as an unit has been explained. The current programmability of the proposed power supply configuration has been illustrated with computer simulations. Experimental results to substantiate the computer simulations are presented.

Transverse mode coupling and supermode establishment in a free-electron laser oscillator

A three-dimensional study of transverse mode evolution in a free-electron laser (FEL) oscillator is presented. The total electromagnetic field circulating in the resonator is represented as a superposition of transverse modes of the cavity. Coupled-mode theory is employed to derive a generalized 3-D steady-state oscillation criterion. from which the oscillator superrnode can be found analytically in the linear gain approximation. The oscillator supermode. which is the eigenmode solution of the oscillator at steady-state, keeps its transverse profile and polarization after each round-trip. Relations between the oscillator super-mode and the umplijier supermode are discussed. It is shown that they are identical only when the feedback process is entirely non-dispersive and non-discriminating. We employ a 3-D nonlinear simulation code based on the same transverse mode expansion to demonstrate the evolvement of transverse modes in the oscillator towards formation of a supermode in a simple example. The simulation shows that the steady-state result of the oscillation buildup simulation is identical to the supermode predicted by the analytical approach.

Analysis of mass and heat transfer in transient experiments over heterogeneous catalysts

This article discusses the effects of extra- and intraparticle mass and heat transfer in transient experiments over heterogeneous catalysts. To this purpose, the time-dependent transport in a catalyst particle when introducing a reactant in an inert gas phase has been analyzed. The analysis has led to one criterion assuring the absence of both extra- and intraparticle mass transfer limitations during steady-state experiments. Furthermore, criteria for the absence of mass and heat transport limitations during transient experiments have been developed by extending the criteria for the absence of mass and heat transport limitations during steady-state experiments. As will be shown, transient kinetics are not affected by heat transfer as long as the steady-state criteria for the absence of mass and heat transport limitations are fulfilled.

Cardiac channel gating charge movements: recovery from inactivation.

The nonlinear charge movements which occur during membrane depolarization of cardiac ventricular myocytes (QON) have been previously identified and separated, by kinetic and steady-state criteria, into constituent components arising from the gating of Na channels and Ca channels. In contrast, the nature and time course of the OFF charge movements (QOFF), which follow membrane repolarization have not been as clearly established. In order to address this question cardiac QOFF was studied using small-diameter, 17-day-old embryonic chick ventricular myocytes that can be rapidly and uniformly voltage-clamped. The application of brief (5.4 ms) depolarizing steps were employed to produce Na channel inactivation but little Ca channel inactivation. Following the return of the membrane potential to -100 mV QOFF, measured as the gating current termed IgOFF, displayed two kinetic components. Double exponential fits to IgOFF yielded time constants of a few tenths of a millisecond for the fast component (IgOFFfast) and of 1-2 ms for the slower component (IgOFFslow). The time course and voltage dependence for the slower component suggested that it might be linked to the inactivation, and the recovery from inactivation, of Na channels. In order to identify these kinetic components double-pulse protocols were employed in which the duration of the prepulse and the interval separating the prepulse and test pulse were varied. The time course for the decay of IgOFFslow following a brief inactivating prepulse was similar to the time course for the recovery of the Na channel QON (QNaRecov). Both IgOFFslow and QNaRecov preceded the recovery of the Na channel (ionic) current. The recovery from inactivation of both the Na current and QNa displayed a similar voltage dependence. These experiments have helped to identify the two components of cardiac IgOFF and, therefore, will facilitate the interpretation of further biophysical and pharmacological studies concerning cardiac Na channel and Ca channel gating charge movements.

Multilayer adaptive control of continuous bioprocesses using optimising control technique. Case study: Bakers' yeast culture

High costs associated with many fermentation processes in an increasingly competitive industry make any prompt application of modern control techniques to industrial bioprocesses very desirable. However, this is often hampered by the lack of adequate mathematical models, on the one hand, and by the absence of continuous, on-line measurement of the most relevant process variables, on the other hand. This paper addresses these problems and offers a new strategy to control continuous bioprocesses using a hierarchical structure such that neither structured process models nor continuous measurement of all relevant variables have to be available. The control system consists of two layers. The lower layer represents a dynamic adaptive follow-up control of a continuously measured output — in our case dissolved oxygen concentration. This variable is supposed to be strongly correlated with the key output variable — in our case cellular concentration which is not continuously available for measurement. The higher layer is then designed to maintain a desired profile of the process key output using a set-point optimising control technique. The Integrated System Optimisation and Parameter Estimation method used operates on an appropriately chosen steady-state performance criterion. A prerequisite for successful application of the proposed approach is an approximate steady-state model, describing the relationship between the measured output and the process key output variable. Furthermore, occasional in situ, off-line or laboratory measurement values of the key output variable are needed. Promising simulation results of the biomass concentration control, by manipulating the air flow-rate in the continuous bakers' yeast culture are presented.

Do Fluoroquinolones Alter the Effects of Warfarin Therapy?

<h3>To the Editor. —</h3> We read the article by Jolson et al¹with interest, and we are concerned over the lack of objective, balanced information presented. We are aware of at least two published clinical trials^2,3that did not validate the potential warfarin-fluoroquinolone interaction. The information provided by Jolson et al, as well as the published literature that has generated interest and concern about this potential interaction, has been derived from individual case reports. One cannot evaluate the predictability of an interaction from case reports alone, without appropriate clinical trials to support the interaction. Jolson et al state that the five patients had steady-state baseline prothrombin time (PT) ratios between 1.5 and 2 times the control ratio. They failed to mention their criteria for determining steady state. Additionally, reporting PT ratios rather than international normalized ratios (INRs) makes it difficult for the reader to judge steady-state criteria or

A continuous-time average-cost flexible manufacturing and operator scheduling model solved by deconvexification over time

A continuous-time flexible manufacturing and operator scheduling problem is introduced and solved. The principle concern is with scheduling operators over time to various activities of a manufacturing system with the purpose of optimizing some steady-state criterion. In mathematical terms the problem is modeled as a deterministic, infinite-horizon, continuous-time discrete dynamic program. Our solution procedure is to convexify the problem to obtain a linear program and then to deconvexify the solution of the linear program over time to arrive at an optimal solution which is periodic and piecewise constant. Apparent loss in object value due to the deconvexifications is circumvented with buffer inventories. The procedure can be reduced to solving a sequence of linear programs, and the complexity can be stated in these terms.

A Discrete Time Average Cost Flexible Manufacturing and Operator Scheduling Model Solved by Deconvexification Over Time

A flexible manufacturing and operator scheduling problem is introduced and solved. The principal concern is to schedule operators over time to various activities of a manufacturing system for the purpose of optimizing some steady-state criteria. In mathematical terms, the problem is modeled as a deterministic, infinite horizon, discrete dynamic program. Our solution procedure is to convexify the problem to obtain a linear program, and then to deconvexify the solution of the linear program over time to arrive at an optimal solution. Apparent loss in objective value due to the deconvexifications is circumvented with buffer inventories. The procedure is reduced to solving a sequence of linear programs and the complexity is stated in these terms.

A Vector-Controlled Cycloconverter Drive for an Icebreaker

A high-power high-performance variable-speed drive system using a cycloconverter-fed synchronous motor is described. The control uses the field-orientation principle to give the drive system good steady-state and dynamic performance criteria. The accuracy of the field orientation is improved with the use of current regulators operating in the airgap flux reference frame. The system was manufactured to deliver 16 000 hp to the twin propeller shafts of a Canadian Coast Guard icebreaker. The electric components of the system were tested as a unit in the factory and the desired system performance was achieved.

FEEDBACK EFFECTS ON SEQUENTIAL ORDERING IN HUMANS

Under various feedback conditions, 38 college undergraduates were asked to rearrange abstract graphic characters on a computer screen, placing them in arbitrarily designated "correct" sequences. Two sets of seven horizontally arrayed stimuli were used. In Experiment 1, subjects in Group 1 learned to arrange the first set under Selection Feedback in which a "+" appeared above each character after it was selected in the correct order and to arrange the second set under Order Feedback in which a correct response produced a copy of the character in its correct ordinal position at the top of the screen. For Group 2 the order of these conditions was reversed. In Experiment 2, for subjects in Group 3, correct responses produced neither of these types of feedback. Subjects in Group 4 received Order Feedback only until the first set was correctly ordered once. Order Feedback was more effective than Selection Feedback during initial acquisition of the first set but not during maintenance; no differences were found for the second set. Only 2 of 9 subjects successfully put the characters in correct sequential order under the No Feedback condition. When, in Experiment 2, Order Feedback was eliminated after the first correctly arranged sequence, the steady-state criteria were met more slowly than in Experiment 1.

Hot Box Operating Techniques and Procedures: A Survey

Abstract A questionnaire dealing with operating techniques and procedures of guarded and calibrated hot boxes was mailed to 14 hot box operators in the United States and Canada. Questions pertaining to testing capabilities, instrumentation, calibration specimens, steady-state criteria, etc., were asked. Responses from the 13 laboratories returning the survey are summarized in the paper. This information should be of interest to hot box operators, as well as prove helpful in future revisions of the guarded and calibrated hot box test methods.

Steady-state rf magnetron discharges

A study of the steady-state discharges and the spatial distributions of plasma parameters vertical to the magnetic field was made in an rf magnetron with oval cross section. Langmuir probes were used for the major diagnostics. The device generates low-ion energy, high-ion flux stable plasma (Ei∼102 eV, Γi∼5 mA/cm2 at 1 W/cm2 rf power). Ion transport toward the positive sheath is enhanced by the rf induced dc electric field in the uniform glow. The state of the glow is independent of rf power except ion density ni∝ (rf power)α. The voltage drop across the sheath follows V∝(rf power) β with 0&amp;lt;α&amp;lt;β&amp;lt;1. The discharge strongly depends on magnetic field strength and system pressure, and weakly depends on the gas chemistry. Generally, the state of the discharge is determined by charged particle generation, transport and loss processes, and in turn controlled by the system operating parameters. The experimental results obey the conservation laws and the steady-state criterion. The correlations between plasma parameters and system parameters are investigated and discussed.

Effects of hyperoxic gas mixtures on energy metabolism during prolonged work.

These experiments were designed to study selected respiratory and metabolic responses to exercise in hyperoxia. Four subjects were examined during 30-min bicycle ergometer rides at both 40% and 80% of their aerobic maximum. The VO2 was significantly increased at both work levels breathing 60% O2 versus 21% O2, while VCO2 showed no significant change during the 40% exercise tests but was significantly decreased during the 80% intensity rides. The average increase in the volume of O2 taken up during 30 min of hyperoxic exercise, compared with normoxia, was 3.3 liters at the 40% exercise level and 5.6 liters at the 80% level. Neither the magnitude of the O2 nor the CO2 storage calculated for the exercise bouts could explain these increases. Steady-state criteria for the gas stores were established by the stable values of PETCO2, VO2, VCO2, and VI from minute 6 through 30 at both work levels. R values decreased during the hyperoxic tests suggesting the possibility of a shift toward increased fatty acid metabolism.

Discussion on “Stability of reluctance machines” and “Steady-state stability criterion for induction motors”

Discussion on “Stability of reluctance machines” and “Steady-state stability criterion for induction motors”

Sufficient conditions for uniqueness and local asymptotic stability for a class of distributed parameter systems: non-adiabatic plug-flow tubular reactors with recycle and feedback control

An analysis of the stability of a non-adiabatic tubular reactor with feedback control, modelled by a plug-flow reactor in which axial dispersion of heat and mass is accounted for by recycle, is presented, A sufficient condition for local stability is developed in terms of system parameters and steady-state profiles. This condition is extended to a condition which is in terms of system parameters and outlet temperature only. Also, a unique stable steady-state criterion, which is in terms of system parameters only, is obtained. These conditions show that under certain assumptions the stability can always be proved for a suitable choice of controller settings.

Steady-state stability criterion for induction motors

The instability of induction motors has been widely observed when operating on variable-frequency supplies, particularly on light loads at low speeds. This phenomenon is analysed for steady-state conditions using the single-phase equivalent circuit of the motor and considering a stable system to have constant-power balance. The analysis confirms the low frequency, low-slip instability and predicts a high-slip limit of stable operation. These limits are both confirmed by measurements on a motor/inverter drive. The amplitude and frequency of oscillations are not investigated.

Steady-state vertical turbulent diffusion of radon

Hourly average radon-gas concentrations have been measured at elevations of 6.1, 61, 174, and 271 meters on a continuous basis for over 1 year. A search through 5 months of data revealed 383 hours of data that satisfy a steady-state criteria. The steady-state radon profiles correlate with atmospheric stability through a stability ratio and are naturally divided into four categories. The turbulent transfer is parameterized in terms of a radon diffusivity K that is related to general meteorological conditions. An independent specification of K is made based on dimensional reasoning and empirical turbulence statistics in an extension of work by Hanna (1968). A two-layer domain for diffusivity is used in a computational model in which the upper-layer K is independent of height but varies with meteorological conditions; and in the surface layer below 100 meters K is given as K(Z) = K(Z0) (Z/Z0)α.