ppm at Ec 500 or ppm at Ec 700 which one?

Which of these should I us for my readings in the United States? My wand has 2 listing.It lights up and then to the left the ppms are shown in EC 500 and to the right ppm at EC 700?? They differ as much as 350 ppm so I have been going to the lower reading for saftey but wanna know if anyone knows US standard of ec so I can get correct ppm

If any of my pm's help would you please post them?

for us, go by the ec X 500 PPM.

Ok so here where I am now on this.There are 3 different Ec ppm readings.Ec 500, Ec 650, and EC 700.The US uses 500 or 650.Now hers where you gotta do homework.Each meter maker uses different ones so the actuall ppm is an estimate from the manufactures conversion built in.So what I did is take the EC reading on my wand I then ( since I use FoxFarms) went to the nutrients website got thier reccomended hydro feeding chart for thier nutes.They have a scheduled with a reccomended EC reading which then has a ppm count that should go with the EC that is desired.I looked at the EC lets say seedlings and cutting they say EC of 1 under that its has ppm of 700-840.Now I look at my truncheon meter .I see EC 1.0 I followed it to the ec 700 side ppm is way different I follwed over to the ppm for EC 500 and Im right in that range so That tells me the units I want to watch for my meter with Fox Fram nutrients is the ppm at EC 500. Now I know

that is correct, watson

I am a friggin idiot for not figuring this out before now. I thank you 2 contributors of my ever growing studentship..

thanks for answering my question too