Non toxic smoke fluid
I like that this type of fluid in safe to use in doors. Not toxic like some of the other smoke systems I looked at. I bought the refill fluid at the same time I purchased the Smoke Pencil Pro Kit. Since I had never used the smoke pencil before, I wasn't sure how much fluid it would take to detect where my air leak was on the product I was trying to find a leak. It takes very little fluid to operate the smoke machine. Therefore I have enough fluid to last me a life time. If you're not going to be using your smoke machine everyday, I wouldn't bother buying an extra refill. My smoke kit came with 1 bottle of fluid, which I knew, so I didn't really need more.