The Straight 304 Stainless Steel Reusable Dispensing Needle is used with all ramé-hart goniometers. Needles are 2" long. 22 gauge is the standard size. The smaller the gauge, the larger the diameter. 30 gauge is the smallest while 10 gauge is the largest. All sizes from 10g to 30g are kept in stock. If you are using this needle with our Automated Dispensing System, you will require the optional needle fixture (listed under Automated Dispensing System options). For a detailed list of needle dimensions and sizes with part numbers, please go to: http://www.ramehart.com/pdf/needles.pdf Note that the p/n for this item is 100-10-12-XX where XX refers to the gauge, between 10 and 30.