Practica n° 2 Reómetro Brookfield

I. INTRODUCTION

El Reómetro Brookfield DV III ultra programable, mide parámetros de fluido esfuerzo de corte, Viscosidad y de velocidad de corte. La viscosidad es una medida de la resistencia de un fluido para fluir. Usted encontrará una descripción detallada de los modelos matemáticos de viscosidad en la publicación de Brookfield "Más Soluciones a Problemas complicados” cuya copia fue incluida con su DV III ultra.

The principle of operation of the DV-III Ultra is to drive a spindle (which is immersed in the test fluid) through a calibrated spring. The viscous drag of the fluid against the spindle is measured by the spring deflection. Spring deflection is measured with a rotary transducer. The viscosity measurement range of the DV-III Ultra (in centipoise or cP) is determined by the rotational speed of the spindle, the size and shape of the spindle, the container the spindle is rotating in, and the full scale torque of the calibrated spring.

The DV-III Ultra can also measure yield stress (in Pascals or Pa). See Section I.3 for more information on yield stress.

1.1. Torque Measurement

There are four basic spring torque models offered by Brookfield:

Spring Torque.

Model dyne*cm mN*m

LVDV-III Ultra 673.7 0.0673

RVDV-III Ultra 7,187.0 0.7187

HADV-III Ultra 14,374.0 1.4374

HBDV-III Ultra 57,496.0 5.7496

The higher the torque calibration, the higher the viscosity measurement range. The viscosity measurement range for each spring torque model may be found in Appendix B.

1.2. Viscosity Units of Measurement

All units of measurement are displayed according to either the CGS system or the SI system.

(1) Viscosity appears in units of centipoise (shown as "cP") or milliPascal-seconds (shown as mPa*s).

(2) Shear Stress appears in units of dynes/square centimeter ("D/cm2") or Newtons/square meter ("N/m2").

(3) Shear Rate appears in units of reciprocal seconds ("1/SEC").

(4) Torque appears in units of dyne-centimeters or Newton-meters (shown as percent "%" in both cases).

The equivalent units of measurement in the SI system are calculated using the following conversions:

SI CGS

Viscosity: 1 mPa*s = 1 cP

Shear Stress: 1 Newton/m2 = 10 dyne/cm2

Torque: 1 N*m = 107 dyne*cm

References to viscosity throughout this manual are done in CGS units. 

1.3. Yield Stress Measurement

Another feature now available in the DV-III Ultra Rheometer is the yield stress test.

The yield point is the point at which a material begins to flow. The associated properties are the yield stress and yield strain. The yield stress is the critical shear stress, applied to the sample, at which the material begins to flow as a liquid. The yield strain is the deformation in the material, resulting from the applied stress prior to the start of flow.

Many materials are designed to have a yield point, so that the behavior of their products satisfies various customer needs. Foods often have yield points. Ketchup in particular must flow out of a bottle when shaken or squeezed, but then solidify on the targeted food such as french fries. Shaking or squeezing the bottle stresses the ketchup so that it flows; after the ketchup settles on the fries, its structure rebuilds so the ketchup "sits" in place rather than flowing off the fries like water. Puddings have yield points, as well. The "body" of the pudding appeals to consumers - it is solid at rest, yet it's easily spooned out of its cup and is easy to eat. Thus, the yield behavior of many foods contributes to the food texture that we like.

Many paints have low yield stresses. Many latex house paints, for example, are easily stirred or poured. Brushing or spraying provides enough stress so that the paint flows easily and smoothly over a painted wall. However, a thin layer of applied paint (if a good one!), allowed to rest undisturbed on the surface, regains its structure quickly so that there is very little unsightly "dripping" afterwards. The smooth appearance of the painted surface is very appealing to the homeowner.

The operating principle is to drive a vane spindle through the calibrated spiral spring connected to a motor drive shaft (see Figure I-1). The vane spindle is immersed in the test material. The resistance of the material to movement is measured by observing increasing torque values as the DV-III Ultra motor rotates. The amount of shaft rotation is measured by the deflection of the calibrated spiral spring inside the instrument. Spring deflection is measured with a rotary transducer.

If the vane spindle did not move at all, the data would look like the graph in Figure I-2. The data often looks like the graph in Figure I-3 because there is usually some deformation of the test material due to the increasing force imparted by the vane spindle. The maximum torque value is the yield point. The straight line in Figure I-3 is a repeat of what was shown in Figure I-2. An algorithm in the firmware converts the maximum torque value into a yield stress value.

The shear stress measurement range of the DV-III Ultra (in Pascals) is determined by the size and shape of the vane spindle and the full scale torque range of the calibrated spring

I. INTRODUCTION 4

1.1 Torque Measurement 4

1.2 Viscosity Units of Measurement 4

1.3 Yield Stress Measurement 5

1.4 Components 6

1.5 Dimensional Information 7

1.6 Utilities 8

1.7 Specifications 8

1.8 Safety Symbols and Precautions 9

1.9 Data Retention 9

1.10 Set-Up 10

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