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The Direct Method:
The Direct Method is a means to determine the viscosity at a reference temperature independent of any temperature viscosity relationships.
Key Features (of digital viscometer based systems):
How direct Measurement works:
A continuous sample stream from the main process passes through a system containing the viscometer where the temperature of the stream is controlled to the reference temperature.
The viscometer thus measures the viscosity at the reference temperature without the need for any calculations nor needing any knowledge of the viscosity temperature behaviour of the fluid.
Key to the success of this method is the ability to control temperature within very tight limits irrespective temperature changes in the inlet fluid.
Typically if the reference temperature is 100C, then the viscometer will be operating
at 100C (<+/-
In the oil industry, the standard method for measuring viscosity has been the Process Capillary Viscosity Analyser which has been unrivalled for over 40 years until the development of the digital viscometer.
The capillary is immersed in a temperature bath, set to maintain a constant temperature. This use of the capillary and a temperature bath is a happy combination as both have similar operational requirements; the need for very low flow rates and the need for a very low mass of the sensor.
There are, however, many limitations of the capillary viscometer and its use has been severely limited.
In principle, this approach ought to suit any number of existing technologies that have limited performance, but this has proven difficult due to the process conditions associated with using the temperature bath effectively.
Other than the capillary there are no other technologies that are truly compatible with a temperature bath.
The alternative is to use heat exchangers.
Using Heat Exchangers:
Heat exchangers represent another problem; the difficulty of maintaining a constant temperature due to the slow response of the heat exchanger.
How the direct method works for the digital viscometer using heat exchangers:
The problem of maintaining a constant temperature using heat exchangers was solved
using a special temperature control set-
The solution is based on dynamic fluid mixing.
Part of the constant-
Downstream of the heat exchanger the two flows, at different temperatures, are recombined in a static mixer.
As temperature of the re-
Because there are no intervening heat exchanger surfaces the response to a change is very rapid allowing very precise control of the temperature.
If temperature change is due to a trend in the sample stream then it is necessary to modify the heating which is done using a second control valve in the heating service line.
Though the advantage is that no knowledge of the temperature viscosity relationship is required, a disadvantage is that only one reference temperature can be used.
To determine the viscosity at other reference temperatures complete additional systems are required, one for each reference temperature.
In some applications it is necessary to know the viscosity at more than one reference temperature e.g. To determine Viscosity Index.
The “mixer” circuit is relatively easy to control and to maintain a stable temperature.