Here are some frequently Asked Questions about compression fittings.

Q1."Should I use PTFE thread sealing tape or pipe joint compound on my compression threads?" 
A. No, you definitely should not. PTFE tape and pipe joint compound (also known as "pipe dope") should only be used on "IPS" ("iron pipe size") threads, which are normal pipe threads. IPS threads make their seal along the threads themselves; when a piece of IPS-threaded pipe is screwed into a fitting, PTFE tape and/or pipe dope is used to assist that seal on the threads. Compression fittings, on the other hand, seal between the beveled end of the ferrule/sleeve and the beveled female part of the fitting, and so using pipe dope or tape on compression threads could actually prevent the fittings from making an adequate seal

Q. "What is the difference between 'nominal' dimensions and 'outside diameter (OD)' dimensions?" 
A. Nominal is the term used to describe most pipe and fitting measurements in the plumbing industry. Nominal refers to the inside diameter measurement of the pipe. The actual inside pipe measurements will vary slightly due to the wall thickness of the pipe, so it is referred to as "nominal" meaning a small variance of size. The outside diameter (OD) dimensions refer to the actual outside diameter of the pipe. The outside diameter dimensions of pipe are a constant size which allow pipe fittings to be used with the same type of piping with multiple grades of wall thickness.