Spiritis all of the answers are good ones.I tend to go up a bit if the diameter of the hair calls for it, but for brows I do still usually find an F3 or F4 is large enough. I cant see too many brow hairs calling for an f5 but it could happen.You’ll quickly get a feel when you’ve gone too large insertions will be more difficult and uncomfortable but in general with any kind of thermolysis a larger needle gives more even heat patterns and usually a better treatment overall.
If in doubt, contact Jennifer at Texas Electrolysis Supply and she may be able to ship you a few of all sizes so you can try out different sizes and get a feel for both inserting them, and what size is called for.If you decide togo this way just as a test, any old probe will do sterex,insulate/noninsulated. If t’s just to test a few hairs and see what works best it doesnt matter much. Sterex would be perfect