OK The results are in.
- I started with 17lt (17kg weighed) of tap water in the BM at 47C ex the HWS
- Jacket on the BM
- Dickos Hood on
- Set mash schedule to all zeros and boil schedule to 60 mins at 100C so BM would function on auto
- During boil phase heating element cycled ON/OFF at 81% ON - 18% OFF ratio. This was checked/taken over 6 min time period by counting the "BM Flashing Timer" and noting when the Heater light was ON and when it was OFF
- At the end of the boil time (60mins) the remaining water was reweighed at 14.651kg (14.651lts)
- Evaporation rate = 17 minus 14.651 = 2.349kg (lts) or 13.8%
So the next question to understand is are Coalminers "Boil Off" rates above the result of setting the boil temperature to 102C?
Next question is what/how much is an acceptable target "Boil Off" rate. The Institute of Brewing publication "The Function of Boiling Wort" (unable to attach the PDF link here??) talks about traditional boils lengths being of 90 mins and 10% minimum evaporation and then goes on to say that modern kettle designs operate with 60 min boils and evaporation rates of between 5 and 9%
From my observation of the BM during this test was that to achieve a boil off (evaporation rate) of between 5 and 9% it would be a very very gentle boil with minimal movement of the surface of the wort
I guess the above Institute of Brewing publication raises the age old question about the need or otherwise of a "vigorous/aggressive boil"
In view of this it would appear that it would be acceptable to set the BM temperature to 100C and achieve boil off rates of around the 12 to 14%
Open for discussion
Wobbly