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Spirit Indication ABV - Inaccurate for sweet wine

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I am trying to determine the alcohol content in some batches of blueberry wine using an Alla France -2 to 10 baume hydrometer and the spirit indication method.

It works fine for dry wine, but is wildly inaccurate for sweet wine/port.

Home made dry blueberry wine
-2.1 baume start, 21.1C (0.98572SG)
0.8 baume end, 22.4C (1.00485)
My result is 15.5%, which looks right.

I have tried my method with two commercial wines. One 'dry red' generic wine is labeled 10.5%, and I got 11%. All good.
Another 13.5% labeled shiraz came in at 12.9%. Great. I know I can get the spirit indication test right.

But here is where things fall apart:
Cheap tawny port. 17.5% on the label.

6 baume start, 19.9C (1.04317SG)
7.7 baume end, 21.5C (1.05608SG)

Calculator says 10%. Clearly wrong.

I tried my blueberry port.
3.7 baume start, 18.8C (1.02619SG)
4.8 baume end, 22.2C (1.03424SG)

Calculator says 6.2%. Clearly wrong.

What am I doing wrong? The calculator says: "Note: This procedure, if performed carefully, will provide accurate results in wines regardless of residual sugar."

The spirit indication test is failing badly for wines with high residual sugar. I am boiling the wine over a medium gas flame in a small pot. 250ml volume, and replenishing it with Brita filtered water (not distilled) back to 250ml. That works fine for dry wines but doesn't for sweet fortifieds. Is the heat maybe modifying the sugar?

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