Can i use fever thermometer instead of bbt for ovulation chart?how much does fever thermometer & bbt range before and after ovulation?

No: although both will give the same result, a BBT thermometer allows smaller changes in temperature to be noted as is needed. I think most ob/gyns and infertility specialists have abandoned BBT testing, using only ovulation predictors kits.

Digital thermometer: Any digital thermometer should be fine provided it is calibrated to 1/10 of a degree.