 "Should I use PTFE thread sealing tape or pipe joint compound on my compression threads?"
 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.
"How will I know which size compression fittings to use for my copper tubing?"
You will need to measure the outside diameter of your copper tubing. Whatever the outside diameter of your copper tubing is will be the size of the compression fitting you will need. For instance, if the outside diameter of your copper tubing is 1/4" then the compression fitting will need to be 1/4". Compression fittings are not sized like copper sweat fittings.
"I have clear vinyl tubing, can I use these compression fittings with it?"
It really depends on the outside and the inside diameter of the clear vinyl tubing. When using plastic tubing, the "plastic sleeve" must be used and the "brass insert" must be used. The plastic sleeve is used to seal against the outside of the plastic tubing and the tapered end will seal into the compression fitting. Plastic sleeves are used because the brass compression sleeves can more easily cut into the plastic tubing, potentially causing a leak. The brass insert is used to keep the inside of the plastic tubing from collapsing as the compression fitting is tightened. The size of the plastic sleeve and the brass insert must be chosen according to the outside dimensions of the clear vinyl tubing to be used. The brass insert must be able to fit snugly inside the clear vinyl tubing and will generally work if the clear vinyl tubing has 1/16" wall thickness.
"What is the difference between 'nominal' dimensions and 'outside diameter (OD)' dimensions?"
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.
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