I have the Gen 1 BM20 with 3 center rod markings - 15l, 20l, and 25l. I used both a 5 liter Erlenmeyer flask and my brewing scale. I check my scale regularly vs. 2 other scales in the house, so it isn't really calibrated, but all 3 would have to be drifting by the same amount if they are going out of cal. Turns out the Erlenmeyer wasn't very well calibrated. 5 liters in the flask was 5.2-5.3 liters when weighed. Since I don't really want to weigh my water every brew day, I was calibrating the depth vs. the volume. I measure depth to the bottom of the kettle (and NOT the raised section the center rod is connected to). Here are my results...

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`Volume (liters) Depth (cm)`

15 16.5

20 21.9

25 27.3

30 32.7

35 38.1

40 43.5

The 3 markings are 5.4cm apart (with a micrometer, just a shade under 5.4cm). And each of the markings was dead on to the volume expected. So each liter equals 1.08cm; I suppose I'll round to 1.1cm/liter. I didn't measure 5 or 10 liters depth, I'm sure the heating coil makes the liter to cm relationship a little different for the first 5 liters.

At 40 liters, the level was at the absolute top of the kettle - 1 more liter would have overflowed the kettle. So to the earlier questions we had about maximum pre-boil volume... I'd be pretty nervous about 38 or 39 liters as shown in the Grain and Grape method. You'd have to make your own call and certainly use foam control, but I think my max will be 35 liters... remember, this is measuring with 18C water, boiling wort will be 4% greater volume.