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Behavioural Viscosity measurement is where all that is required is the viscosity at the process temperature.

This is a comparatively simple measurement. It is the most common process viscosity measurement and currently, the most successful even with very simple viscometers.

In these applications it is the behaviour of the fluid that is important. The viscosity of a fluid will  affect how it flows in pipes or systems, the way in which it flows over a surface and the thickness of the film produced, or the size and behaviour of droplets in atomisation processes such as in spray painting or burner fuel atomisation.  

It may also be a “perceived” quality factor e.g. of two shampoos with identical cleaning power, the one that is more viscous is thought to be more “rich”.

Measuring the viscosity allows for very effective control of these behaviours by modulating solvent addition in inks or paints, or heater control to exploit the temperature dependent viscous behaviour.

In these applications even quite coarse viscosity measurement can be highly effective.

Typical behavioural applications include:

Example: Heavy Fuel Oil:

Heaavy fuel oil is a useful fluid to consider for the simple reason that in its production, analytical viscosity measurement is required for blending control, while for fuel heating when the fuel is burnt it is a behavioural viscosity measurement and the comparison illustrates extremely well the differences between the technologies.

For heavy fuel oil atomisation control there have been a number of different technologies successfully used in engine applications for many years.

In burner applications where the fuel is to be atomised through an burner nozzle or in engine applications where the fuel is atomised by an injector, it is critical to obtain the optimum spray pattern and droplet size for efficient combustion.

All other things being equal, the spray pattern and droplet size are affected by the oil viscosity.

If it is too viscous the drops are large, they take extra time to burn and do not mix well with the air flow so incomplete combustion will result.

Also, because of the size the droplets tend to project  through the optimum combustion region and, in steam plant, can collect on the firewall tubes and cause a burn through or in engines, on the liners.

If the viscosity is too low a soft non-dispersing spray is created. This does not mix effectively with the air either, but in this case combustion will take place in the region of the injector or nozzle.

The viscosity of the oil is modified by heating the fuel to a greater or lesser extent. The heater is controlled by the viscometer.

The photo below illustrates a typical fuel heater module for a large diesel engine.

Most behavioural viscometers only indicate the dynamic viscosity.

For over 40 years the marine industry standard has been a  variation on the capillary viscometer. While the process capillary viscometer required a temperature bath together with heat exchangers, pumps, filters etc the Leslie Twin Capillary consists of just the capillary tube a small pump and the dP transmitter in a flow- through pipe arrangement.

The two capillary devices are not interchangeable. A limitation of capillaries is the need for cleanliness in the fluid and while this is not possible with heavy fuel oils and results in a need for good maintenance, it is worth noting that the twin capillary has had little success on the same fuels in burner applications. Why? Because the fuel to engines is settled, filtered and centrifuged to clean it up while that to burners is not.

The twin capillary is giving way to newer technologies which will handle dirty fluids such as the 7829 digital viscosity transmitter which also indicates the density, dynamic and kinematic viscosity and calculates the ignition index.


Viscosity: Behavioural Measurement:

A 7829 ViscoMaster™ Digital Viscometer during installation in a fuel heater module for a large diesel engine.

The viscosity of a  heavy fuel oil may vary from 380cSt at 50°C to 15cst at 140ºC.  

This rate of change of viscosity is significant and to control the viscosity to 10cSt, the accuracy of the sensor can be 2-5% fsd and still be very effective since +/-0.5cst in a range to 380cst is relatively good.