Ink Sac Basics: Selecting the Correct Sac Size 

It is a widespread misconception that the correct size of ink sac can be determined by measuring the end of the section where the sac is attached (the sac nipple or peg). In fact, sac size must be determined by reference to the internal dimensions of the pen barrel. This video demonstrates why one cannot rely upon measuring the sac nipple to determine the correct sac size. In some cases the proper size sac is larger than the nipple, while in others, it is markedly smaller.