Ja Johannes Poulis

Physics lecturing with audience paced feedback

For some time we have experimented at the Eindhoven University of Technology with a system which provides students with an electronic feedback path to the lecturer, audience paced feedback (APF). In this paper we describe this APF system, and give indications of its effectiveness.

Disturbances in weighing - Part I A survey of work presented at the preceding VMT conferences

Abstract A survey is given on disturbances which determine the accuracy of weighing in vacuum. The nature of the disturbances covers the range from fundamental to purely practical. Apart from the physical description of the disturbing phenomena, numerical estimates for beam-type balances are presented. Part I covers the following phenomena: • Brownian motion • Knudsen forces • Cavity forces • Unequal thermal expansion of balance arms • Magnetostatic effects • Radiation pressure • Vibrations of the building • Other effects Additional effects will be discussed in Part II during the 21st Conference, 1985 at Dijon.

A Jäntti Approach for Quick Calculations of Sorption Equilibria

Jäntti introduced a method to calculate the adsorption equilibrium by measuring the actual adsorbed amount three times after a change to the gas pressure. He successfully applied the method to gas/solid systems in which simple adsorption processes occur. In the present paper we discuss a more general approach for different modes of measurement (adsorption and desorption), including sorption processes obeying second order differential equations.

Criticism on Jäntti's Three Point Method on curtailing gas adsorption measurements

Jäntti introduced a method to reduce the time required for the stepwise measurement of adsorption isotherms (Jäntti et al., Progress in Vacuum Microbalance Techniques, Vol. 1, Heyden, London, pp. 345–353, 1972). After a pressure change he measured the adsorbed mass three times and calculated its equilibrium value for the new pressure. In the present paper we discuss the applicability of this method in a broader scope (without starting from a single combination of gas and solid adsorbent) and also discuss the influence of measurement inaccuracies. It is shown that the method can serve as an early warning system, which decides whether an improvement of the adsorption model is necessary.

Extension of the Applicability of Jäntti's Method to the Fast Calculation of Desorption Data

The time for sorption measurements may be reduced substantially by measuring several values at short time intervals at the beginning of a kinetic curve and extrapolating them to the equilibrium value. The method used by Jäntti for simple adsorption processes has been extended to complicated processes described by a second-order differential equation. A simulated example is provided.

Fast Measurements of Adsorption on Porous Materials Using Jäntti's Method

In 1972, Jäntti et al. formulated a method to shorten the time taken for adsorption measurements. We suggested at the Kiev International Conference on Vacuum Microbalance Techniques (1999) that the applicability of this method could be widened by considering a number of parallel adsorptions. There is, however, a more complicated but perhaps more interesting variant possible when extra mass transport is considered to occur in series. Such an application could involve adsorption within a porous structure where diffusion must be considered as a process occurring in series with adsorption on the surface.

Automated balances of the second generation

Abstract In automatic weighing a Lorentz-force is commonly used to compensate for the force to be measured. In this procedure the compensating force is adjusted such that it brings back the balance beam to its equilibrium position whenever the force to be measured varies. To avoid instabilities of the feedback system, the high frequencies are usually blocked In the feedback system applying low pass filtering. This Involves loss of Information present in the high frequency part of the (compensating) force signal. On the basis of the equation of motion of the balance, an analysis is given which provides a correction procedure Improving the high frequency performance of automated balances.

The static capacitor method of measuring the effective dipole moment of surfactant molecules

Abstract On the basis of a general theory the electrical phenomena above a dipolar monolayer on a water surface are described. This theory leads to an equation that permits discussion of the “ionizing gap method” as well as of the “vibrating plate method.” Both methods are commonly used for measuring the effective dipole moment of surfactant molecules. Using the same equation, a new method is described, which is referred to as the “static capacitor method.” Its limitations and possibilities are briefly discussed.

Disturbances in weighing - part II

Abstract Continuing the survey of disturbances which affect the accuracy of weighing in vacuum or in controlled atmosphere (published In Thermochimica Acta 82 (1984) 1, 42-50), the present paper deals with the following phenomena: - convectlve currents In the balance case - buoyancy - sorption effects and contamination - electrostatic effects.

Jäntti’'s method for the fast measurement of adsorption combined with diffusion

Abstract On the basis of a molecular model for adsorption kinetics Jäntti introduced a method to calculate equilibria immediately after a change of the pressure of the sorptive gas. In the present paper we show the usefulness of the Jäntti-approach to multilayer adsorption combined with diffusion of the adsorbate into the solid.