Thanks for answering that Steve and IamHolland, I've been busy with a spawning in my pond. It's one big stinky mess, so I've had to focus my attention on that. Yes, doing the salt method will give you a definitive volume, plus, the meter is handy for if you have to medicate with salt (in my case, the NitrIte level is so high, I have to add salt, and I need to reach the minimum amount of 0.1% to protect the fish. I can calculate the amount I need based on my volume, but I like to check to make sure that I've reached the 0.1%, just in case.

Sorry you sold your expensive pen, but a new one will give many years of use. You can always use a refractometer also, although for me, they're hard to read.