Steve,

Sure I have a gallon in my back pocket. I think there is a DIY standard somewhere on RC or elsewhere. Let me take a look in the AM.

Another option is to use tap water as a standard. SAWS publishes thier TDS data on their website. It may not be a super accurate way, but good enough for a qualitative yes/no answer on whether your meter is working properly. SAWS says the TDS range is from 283-358 with an average of 321 ppm.

Gary