Pendemonium Sac Assortment - 10 of the most popular sizes includes: 1 each of #14, #20, 2 each of #15, #16, #17, #18 latex pen sacs and basic instructions on sac replacement excerpted from Frank Dubiel's book, Complete Guide to Fountain Pen Repair and Restoration, published by Pendemonium.. We carry most sizes of pen sacs and diaphragms. If you are unsure as to what size sac to order, feel free to email us for assistance. Pen sac diameters are measured in 64ths of an inch. For example, a #14 size pen sac is 14/64 inch. All straight sacs are made long and can be easily trimmed with scissors to proper length. All pen sacs sold by Pendemonium are made exclusively by The Pen Sac Company