I found this site on the web, apparently it is more accurate the the salt test method for calibrating your hydrometer. I haven't used it though, has anyone tried this yet?
I'll echo what kuzi said and add the fact that that test kit costs me 5 bucks plus shipping and the salt test costs me a pinch of salt and a few drops of distilled water from a gallon jug which cost me a grand total of 1$.
I would like to point out that I've tried the salt test on more than one occasion, and every time seem to get varied results. Must be my fault, but the kit can be had for a few bucks, and when ordering a hygro sometimes comes free. And it lasts for 3 months after opening. I like using them, worked fantastic with my new hygro, and will be used for another that's coming to me shortly.
I got one free with my hygrometer. It works nicely. I actually played with the salt test and this method to see how consistent they were. Both methods seem to be equally reliable, provided you set up your salt test correctly. If you are having trouble getting consistent results with the salt test, you may want to grab one of these to take the worry away. Otherwise, the salt test is simple and free.
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