I don't use the IP to ferment dosa batter. What works for me every time (and I make dosa batter every week) is to place the pot with the dosa batter beside an air vent overnight. The warm air coming from the vent ferments the batter perfectly. You could also wrap the pot in a towel or something, if you like. I use a stainless steel stockpot to hold the dosa batter, and if you use my method, use a metal container rather than glass or plastic.