A "doubler" will certainly have your 30ml container filled only half means, enabling you to fill up the rest with the nicotine mix of your picking. If you are covering up, then you would normally be utilizing a PG base with a high level of pure nicotine such as 100mg/ml. On a global scale, this base is likewise not so typical.
http://jeffreyshdt130.theglensecret.com/just-how-to-make-e-liquid