Your units file (xxx.FIL) has a switch to use or not use nominal pipe sizes and the default setting is to use nominals. You type in the nominal OD and CAESAR II will replace your value with the actual OD.
But tubing actual OD, oftentimes, matches this entered nominal and will convert your data. It appears, then, that you cannot properly specify these tubes.
To fix this, you would create a new units file where you turn nominals "OFF".