Easy to use...they don't clog, skip or blot, and write equally well at temperatures from -20° to 140°F. Almost 2000 feet of recording from one pen. Eliminate the chore- and the mess - of inking. The line quality is excellent. Use disposable cartridges to fit your circular chart recorder pens. Measure the length from bottom of arm to chart. Nib lengths are .14", .37", .60" or .83". Fits a wide variety of flat pen arms. Ask about retrofitting your original pen arm to accept felt-tips. 6/pack.