I bought the 3 set because the cap can only be off a short time to avoid drying out. I had a large number of $100 bills to check and the bank was closed. Bank had suggested I buy the pen. I checked all the bills rotating pens. Out of the large stack one bill did not pass. The out of town person paying me with the bills replaced that bill with one that passed. All passed the next day at the bank. I would highly recommend using these pens for peace of mind when you must deal with large sums of money when bank is closed. The negative is that the pens cannot be used long at a time. Very easy to use.
I used to carry cash and now everything looks suspect in terms of cash. Protect yourself one step further with these ever-so-subtle pens with the word "Counterfeit" in large letters.