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The multi-curve referral method

This is an indirect (calculation) ratio-method that employs between 2 and 10 calibrated reference curves against which to compare the measured viscosity and temperature from a single viscometer.

Key features:

Applications:

The multi-curve method is well suited to those applications where there is a wide variation in product quality or where a variety of products are to be measured.

To be successful the products must show viscosity as a proportional relationship from one product to another.

Typical applications include:

How it works:

The process viscometer reports the temperature and viscosity of the process sample stream.

The viscosity computer then determines the viscosity in each of the stored reference curves at that temperature.

The ratio of the measured viscosity to the viscosities from the reference curves is then determined.

The viscosities of the reference curves at the reference temperature are then determined. (the reference temperature may be fixed or variable).

The ratios determined at the process temperature are then applied to the reference curve viscosities at the reference temperature in order to determine the reference temperature viscosity for the process stream.

Limitations:

The method is based on certain assumptions about the continuing accuracy of the reference curves in representing the characteristics of the products.

If product qualities can change, for whatever reasons, the method may be unsuitable or may come to require periodic re-calibration of the reference curves. This is not necessarily onerous.

However, it is not generally suitable for products which do not show a proportional relationship.

In specific applications there are ways to overcome this limitation.

Lubricants are a particular example because they are designed with particular regard to the Viscosity Index (VI).

This may mean, for example, that two lubricants may have the same viscosity at one temperature and yet have different viscosities at other temperatures.

The usual recourse for such fluids is the dual viscometer method.

The exception is in packaging plants where interface detection and product identification is required and where this is a very cost effective alternative to a dual viscometer system which would otherwise be necessary.

It should not be used where the process temperature is significantly different to the reference temperatures. The complex nature of the temperature viscosity relationship may be such that the errors involved become unacceptable.

The test of acceptability depends on the tolerance of the process to error. i.e. In some applications a 2% error is acceptable and in others a 5% error.

Viscosity: Indirect Measurement, Multi-Curve:

Multi-curve systems can be relatively simple.

This example is for fuel oil blending trim control aboard a fuel barge.

ASTM.ppt

This Powerpoint provides a description of the Multi-curve method.

Select view slide show to see the animations.