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[JMW]

Bitumen is presenting me with a problem.
I need to measure the EVT value.
That is, the temperature at which the viscosity is a defined value. If this value is 125cst, then as the quality changes, the temperature at which the viscosity is 125cst will also change. The EVT value will be used to control the production process.

In bitumen blending it is quite often the viscosity at a reference temperature that is used to control the blend, not the EVT value and I'd like to know if this is an equivalent measurement or not and if not, to find who makes an EVT analyser for process measurement.

I am told that the EVT can be measured using a process capillary viscometer. I have tried to find a manufacturer without success so if anyone can assist I would appreciate it.

However, I am more familiar with the capillary viscometer where the temperature bath maintains a constant temperature and the viscosity at this temperature varies with quality.
If the Viscosity Index of the bitumen does not vary then i don't see that it makes much difference whether you control the process with either the EVT value or the viscosity at a reference temperature. However, as resid or bitumen quality changes, does the viscosity index change significantly? in other words, could i have two different bitumens each with the same viscosity at 200degC but where the EVT temperature for a given viscosity differs significantly?

Why is the EVT value so important?
I can appreciate that where quality varies significantly from one grade to another, if measuring viscosity at a reference temperature you would need a bank of capillary viscometers each with a different size capillary to measure the viscosity while if you measure the EVT you need only one viscometer but with the ability to control to a wide range of temperatures. But is this a consequence or an objective? i.e. what is the significance of the EVT value does it just simplify measurement or is there a strong reason for this way of describing bitumen quality?

It often is the case that some measurements are made the way they are for historical reasons.
In the case of fuel oils, the viscosity at 50degC is the preferred measure of quality while the EVT has an obvious value as it determines the temperature of the fuel as it reaches the injectors or nozzles of an engine or burner but quality is controlled relative to the 50degC viscosity (ISO 8217 2005E) So the EVT is an important, but not controlling parameter for fuels. Why is it different for Bitumen?