A) Ostwald; b) Ubbelode: 1 - ball measuring vessel volume flowing through capillary fluid; 2 - capillary; 3 - ball vessel for collecting fluid

General conditions methodology works on all devices are layout of the stand and strict horizontal or vertical installation, the device; padding dry thoroughly washed device investigational product and its thermostatting at the chosen temperature, measurement of time after a certain amount, which is determined by the viscosity and the difference in heights or hydrostatic pressures. Before working measuring device tonirovat by reference liquid - distilled water, sugar solution or castor oil. The absolute items not requiring calibration constants determined by the size of the working bodies in accordance with equation with Poiseuille flow:

where: p - hydrostatic pressure, PA;

d - internal diameter of the capillary, m;

l - capillary length, m;

ETA - dynamic viscosity, PA•s;

w - liquid velocity, m/s

It should be borne in mind that the geometric measurements should be made with extremely high precision. Therefore preferable capillary viscometers to be calibrated by reference liquid.

Viscometer Ostwald used as a relative unit. Calculation formula is:

where: CT water-constant of the instrument at a temperature measurement, m2/S2;

p is the density of the liquid at a temperature of filling, kg/m3;

t - time with.

Viscometer Ubbelode you can use both relative and absolute. In the first case his graduate by reference liquid; the second, in accordance with equation with Poiseuille flow define constants; they can be defined as a complex, based on data from the prior calibration and direct geometric measurements. To calculate the strength using the formula:

where index «refers to the data obtained from the calibration of an instrument on water or any other liquid.

Changing the pressure of the expiration of the RV you can build calibration rogramme that allow you to calculate the constants for working measurements. Similar graphs are built for measurement of the investigated liquid. Vital graphics PT = f(R)that for each temperature in the scope of the law Poiseuille flow give a horizontal line. When begins turbulization and the law Poiseuille flow stops objectively reflect the process lines are deflected upward. If liquid has anomalous viscosity, direct can be partially curvature or not to go out of origin, cutting off the x axis segment, proportional to the marginal shear stress θ0.

In conclusion, the conditions necessary to achieve the required accuracy of measurements on the capillary viscometers:

a) ensure the laminar flow regime of the flow;

b) to exclude the effect of end effects, for example, when using both capillaries of different length;

C) check the absence of the effect of wall skidding, for example, by coincidence of flow curves in grease variables to capillaries of different diameter

g) with measuring the weight method you have to remember that many of food materials have properties of condensed under pressure. This issue requires special research material in conditions of all-round compression.

Capillary viscosimetry because of its features is very limited and rarely used in industrial conditions.

Ball viscometer Ball viscometer, refer to the inhomogeneous research methods are widely used when working with homogeneous Newtonian or semi-structured (neonatologie) fluids. The viscosity is determined by the time of passing the ball of the test section of specified length. The devices are constructed on two main schemes (Fig. 2.7):

a) the ball freely falling in a vertical cylinder;

b) a ball rolling down an inclined cylinder.

The drawback scheme a) is that when dropping the ball in the semi-structured liquid possible deviation from the rectilinear motion, i.e. manifested so-called «wool» ball, which can lead to the distortion of the results of measurements.

Therefore, a more widespread viscometers a rolling ball. The scheme of such example is a viscometer of Keppler is shown in Fig. 2.8.

Two ball moves in the inclined pipe 1, forming a narrow Crescent-shaped crack with her wall. The structured liquid passing through the crack of the structural bonds are destroyed, so an exact reproduction of the results in two consecutive measurements may not be. The instrument is calibrated for measuring viscosity Newtonian fluids in the range from 3•10-4 up to 60 PA * S. passport measurement error Newtonian fluids does not exceed 0.5% for laboratory model. The instrument is not applicable for systems with the ultimate shear stress.

To calculate the strength using a formula derived from the Stokes ' law:

η = К(ρ_Ш – ρ)t,

where K is a constant of the instrument, m2/S2;

ρШ - density of the ball material, kg/m3;

p is the density of the liquid at a temperature measurement, kg/m³;

t - time move the ball on the section h, p.

The difficulty of using ball viscometer for rheological studies structured liquids, is that to get curved flow necessary to conduct a series of measurements with the use balls of various diameters, as in the study of thixotropic liquids possible large errors due to the uneven speed of the ball the length of the test section.

One of the challenges of implementing the ball viscometer is that under the impact of falling ball on the product rheological properties of last change, therefore the result of each subsequent observation differs from the previous one. If there are strict requirements of GOST about finishing the measurement judged on the achievement of the difference between two successive observations of not more than 1C. For example, for condensed milk is achieved after 10 - 15 observations, indicating the duration of the test.

Due to these shortcomings ball viscometer is not widely used in the food industry.

List of literature

1. Machikhin Waerloga food products : a manual / Y.A Machikhin, J.K. Berman ; Yu. A. Machikhin, Yu. K. Berman. - M : MGUPP. - 1999, P.1. - 250 C. Instances: total:10 - XP1(1), B2(2), A1(7)

2. Slash EAST of Engineering rheology biotechnology environments : a manual for students of higher educational institutions on "Technology of raw materials and products of animal origin / C. D. Slash, Ya. I. Vinogradov, A. D. Malyshev. - SPb. : Giord, 2005. - 648 S. : Il. Instances: total:10 - items(1), HR(1), B2(1), C(1), A1(2), A4(4)

Lecture 13

Date: 2015-12-24; view: 831