My alcoholmeter reads 84% but the spirit is at 30°C. What is the real strength?

Lower than the display shows. Warm spirit is expanded and less dense, the alcoholmeter sinks deeper, and the scale reads a few points high. The correction therefore goes downward, and the direction trips people up constantly, so here it is done properly.

Alcoholmeters are calibrated at 20°C. At 30°C the reading of 84% corresponds, via the OIML alcoholometric tables, to a true strength of roughly 80.5 to 81% ABV at 20°C: the warm reading overstates the alcohol because the whole liquid expanded, not just the water.

The practical rule: near spirit strength, each degree above 20°C inflates the apparent reading by roughly a third of a percentage point. Ten degrees warm means roughly three points too high. For anything that matters, bottling, dilution arithmetic, records, cool the sample toward 20°C or use the temperature correction tool rather than a rule of thumb.