They must have planned on not running the engine! I'd think with several trips through the
cooling system (especially a Volvo system) it would be VERY well mixed, especially if
the heater is in the circuit.

Having said that, I worked in the plastics industry for a while and they had density
colums that they made up with water and various alcohols which they mixed as they
poured, so they had a density gradient from the bottom to the top. They calibrated
them with glass balls of known density which they dropped into the columns.
(The columns were transparent glass tubes about 2½" in diameter)
For high density polyethylene they were most concerned with densities of 0.94 to 0.96
and they had that spread out over a height of about 4 or 5 inches so they could see small
differences in density. They would cut a small piece of plastic into a specific shape
(so they could tell which was which) and carefully place it in the column.
It would sink to where the liquid was exactly the same density as the plastic.
Since they had calibrated their charts on the exact heights of the calibration balls,
they could quickly read the density from the height off their charts.
When the column got so many samples in it that it was confusing they had a strainer
basket that sat in the bottom of the column and they would slowly lift it all out,
then put the calibration balls back in. As long as they were careful the column would
stay at its density gradient through several cleanings. Then it would slowly
recalibrate itself toward the middle of range from both ends

--
George Downs Bartlesville, Oklahoma