RU2199106C2 - Procedure determining coefficient of moisture conductivity of sheet capillary-porous material - Google Patents

Abstract

FIELD: construction, food, paper- making and other branches of industry where coefficient of moisture conductivity of capillary-porous materials should be taken into account. SUBSTANCE: procedure determining coefficient of moisture conductivity includes formation of uniform initial moisture content. Surface of sample is put in pulse contact with moisture source with arrangement of electrodes of galvanic converter on surface of tested sample over concentric circumference relative to point of contact of moisture source with sample surface and coefficient of moisture conductivity is computed with measurement of change of emf of galvanic converter in time. EFFECT: increased speed of experiment and provision for nondestructive control over coefficient of moisture conductivity. 2 dwg

Description

 The proposed technical solution relates to measuring equipment and can be used in the field of analytical instrumentation in paper, construction, food and other industries.

A known method of determining the coefficient of mass conductivity and potential conductivity of mass transfer (A.S. 174005, class G 01 k N 421, 9 ₅₁ , 1965), which consists in pulsed wetting of the material layer and measuring at a given distance from this layer the moisture content of the material over time. The mass conductivity coefficient is calculated according to the established dependence.

 The disadvantage of this method is the implementation of destructive testing of the prototype when placing the sensors in the inner layers of the test body, the high complexity of the method in preparing the samples, eliminating the possibility of controlling the mass conductivity of sheet and film materials, the need for individual calibration of the sensors for each material.

 The closest is a method for determining the coefficient of moisture conductivity (DRYING TECHNOLOGY, 15 (2), 265-294 (1997)), which consists in moistening a flat sample in the form of a circle (i.e., creating a uniform initial distribution of moisture in the sheet sample being studied), organizing intensive moisture loss at the edges of the sample by bringing it into contact with a medium having a moisture content different from the sample, made in the form of a ring, measuring (scanning) the moisture distribution profiles inside the free part of the circle of the sample from the center to iferii using infrared (IR) sensor, based on the desired coefficient set based on the data dependencies preliminary calibration IR sensor for the test material.

 The disadvantage of this method is the great complexity of the method due to the need for individual graduation of the system for each material studied and the use of reference samples; the use in the experiment of samples of only a certain shape and size, a significant dependence of the data on impurities in the material, its color and surface condition, distance and angle of measurement.

 The technical problem of the proposed technical solution involves increasing the efficiency of the experiment and providing the possibility of non-destructive testing of the coefficient of moisture conductivity of sheet materials.

 The technical problem is achieved in that in a method for determining the moisture conductivity coefficient of sheet capillary-porous materials (for example, paper), which includes creating a uniform initial moisture content, bringing the surface of the sample in contact with a medium with excellent moisture content from the sample, and removing the signal from the galvanic sensor (EMF) placed on the sample at a fixed distance from the mass transfer region of the sample with the mass source and the calculation of the coefficient of moisture conductivity. Unlike the prototype (DRYING TECHNOLOGY, 15 (2), 265-294 (1997)), they carry out a pulsed point contact of the moisture source with the surface of the sample, place the electrodes of the galvanic converter on the surface of the controlled sample in a concentric circle relative to the point of contact of the mass source with the surface, measure the change in the EMF of the galvanic converter over time, calculate the coefficient of moisture conductivity of the test material without using calibration dependencies according to the established dependence spine, which increases the efficiency of determining the coefficient of moisture permeability.

 The essence of the proposed method is as follows: the investigated sheet material with a uniform initial distribution of moisture (including zero) is placed on a flat substrate of a material not wettable by water, for example, fluoroplastic.

 A probe is pressed against the surface of the sample with a pulsed point source of mass and electrodes of a galvanic converter (GP) located on concentric circles from the source. After applying a moisture impulse (instantaneous wetting of a point on the surface of the sample), the change in the EMF of the GP in time is recorded.

при -∞<x<+∞; -∞<y<+∞; τ≥0;
где
U - концентрация распределенного в твердой фазе вещества, кг/кг;
q - количество жидкой фазы, наносимое в течение импульса, кг;
х, у - пространственные координаты, м;
α_m - влагопроводность, м²/c;
τ∈[0,∞] - время, с;
δ(x,y,τ) - дельта-функция Дирака;
при начальных и граничных условиях:
U(x,y,0)=0;

U --> 0 при |x|,|y| _→ ∞; (- условие симметрии),
аналогичной распространению влаги в неограниченной среде при нанесении импульсного воздействия от линейного источника влаги.The process of moisture propagation in a flat sheet material after applying such a pulse, provided that the measuring electrodes are at a distance r ₀ > 10h, where h is the thickness of the studied sheet material, is described by the boundary-value problem:

at -∞ <x <+ ∞; -∞ <y <+ ∞; τ≥0;
Where
U is the concentration of the substance distributed in the solid phase, kg / kg;
q is the amount of liquid phase applied during the pulse, kg;
x, y — spatial coordinates, m;
α _m - moisture conductivity, m ² / s;
τ∈ [0, ∞] - time, s;
δ (x, y, τ) is the Dirac delta function;
under initial and boundary conditions:
U (x, y, 0) = 0;

U -> 0 as | x |, | y | _ → ∞; (is the condition of symmetry),
similar to the spread of moisture in an unlimited environment when applying a pulse effect from a linear source of moisture.

In this case, the change in moisture content in the source area is described by the function:
U (r, τ) = q / (4πτexp [-r ² / 4α _m τ]).

The moisture conductivity coefficient can be found by the well-known formula:
α _m = r $\genfrac{}{}{0ex}{}{\text{2}}{\text{0}}$ / (4πτ _m ),
where τ _m is the time corresponding to the maximum on the curve of moisture content at a distance r ₀ from the source U (r ₀ , τ);
r is the radial coordinate measured from the source axis, m

To fix τ _m, it is necessary to continuously monitor the change in U (r ₀ , τ), and the measurements should be carried out strictly at a distance r ₀ from the mass source, which is extremely difficult when using known moisture converters (conductometric, dielcometric, radioisotope, etc.). As a result of this, the determination of the maximum of U (r ₀ , τ) is associated with significant errors. In addition, well-known types of moisture converters need individual calibration for each material, which significantly reduces the efficiency of control.

In the proposed technical solution for fixing the maximum moisture content at a distance of r ₀ from the source, miniature electrodes of the GP were used, which were located around a circle of radius r ₀ . The EMF of such a converter is determined by the binding energy of moisture with the material in contact with the surfaces of its electrodes, which is ultimately uniquely associated with the moisture content of the material precisely on a concentric circle of radius r ₀ centered at the point of wetting of the material.

 In FIG. 1 as an example, shows the static characteristic of GP on pulp samples (filter paper).

Since the static characteristic of GPs is monotonous, at the moment when the moisture content U (r ₀ , τ) reaches its maximum value, the EMF of the GP also reaches its maximum. Figure 2 shows examples of changes in the EMF of the GP in different sections when studying the moisture conductivity of filter paper at different values of r ₀ = r ₁ , r ₂ ..., r ₅ .

Claims (1)

The method of determining the coefficient of moisture conductivity in sheet materials, which consists in creating a uniform initial moisture content in the test sample, bringing the surface of the sample into contact with a medium with a different moisture content, measuring the time variation of the sensor signal and calculating the coefficient of moisture conductivity, characterized in that they produce pulsed point contact the test sample with a moisture source, the electrodes of the galvanic converter are arranged in a concentric circle ty relative to the wetting point, measure the time variation of the EMF of the galvanic converter and calculate the desired coefficient by the formula
α _m = r $\genfrac{}{}{0ex}{}{\text{2}}{\text{o}}$ / 4πτ _m ,
where τ _m is the time to reach the maximum on the EMF curve of the galvanic converter;
r _{o is the} distance between the electrodes and the source of moisture

Images