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Calibration of the Measuring System Factor for the Starch Cell Rotor

In this topic
Overview
Operating Parameters
Performing the Calibration

Overview

Calibration of measuring system is required to obtain the factor used in the calculation of the sample viscosity from instrument parameters. Typically the cell has already been calibrated by TA Instruments' engineers and it is unlikely that the factor for any other unit will differ substantially from this. Therefore, you should not find it necessary to recalibrate. However, a suggested procedure is provided here for your convenience.

Operating Parameters

The operating parameters of rotational rheometers are the torque, M, which in the S.I. system has units of newton meters, N.m, and the angular velocity, Ω, which has S.I. units of radians per second, rad s^-1, although revolutions per minute, rpm, are also often used (1 rpm = π / 30 rad s^-1).

To obtain the sample viscosity, a measuring system factor, F_g is used, which depends on the form and dimensions of the measuring geometry used. Then the viscosity, η, is given by:

η = F_g M / Ω

The S.I. units of viscosity are pascal seconds, Pa.s, and those of F_g are therefore m^-3.

For standard measuring systems such as the cone and plate or concentric cylinders, which produce well defined (viscometric) flow regimes, F_g can be derived analytically, and there is no need for calibration. However, for measuring systems such as the Starch Cell impeller, which produce non-viscometric flow regimes, F_g cannot be derived analytically, and calibration is required.

Performing the Calibration

Sample Type

For the calibration, at least one Newtonian liquid is required. Newtonian liquids are those for which the viscosity is independent of the applied shear stress or shear rate. A lower cost alternative is to use materials that are known to show Newtonian behavior, such as sugar solutions or lubricating oils, the viscosity of which can be independently determined.

Recommended are standard liquids that are close in viscosity to those expected for typical starch samples (about 1 Pa.s), at a temperature close to ambient (for example 20°C).

Procedure

Install the Starch Cell as described in the DHR Series/AR Series Starch Pasting Cell Getting Started Guide.
Remove the cup and add the volume of standard liquid as would be used for a starch sample (normally about 28 cm³). It may be necessary to perform a preliminary inquiry to establish the appropriate level in the cup.
Replace the cup and cap, and send the instrument to the required geometry gap.
Using the instrument status page, set the required angular velocity, which should be the same as the value used for the starch testing, normally 160 rpm (16.76 rad s^-1). Record the reading for the torque when this reaches a steady value.

The measuring system factor is then given by:

F_g = η W / Ω

The default value of 10800 m^-3 was obtained as the average of several runs for three standard liquids, by TA Instruments' engineers. This can be replaced by the value you determine, if prefer.

NOTE: Starch pastes are non-Newtonian liquids and their viscosities will depend on the shear rate or shear stress imposed. For non-viscometric flows, such as that produced by the Starch Cell impeller, the shear stress and, therefore, the sample viscosity will not be uniform within the sample. The value reported by the SPR will be the Newtonian equivalent viscosity, which is often sufficient for practical purposes.