PPM has been way confusing to me, with 2 or 3 different scales in use. I got my $20 ppm meter off eBay (
https://www.ebay.com/itm/370309496899). I didn't even know which scale it was using until I compared the reading to the Greenleaf feed calculator (they show results for 500 and 700 ppm scales, or EC). When this meter gives up the ghost I'll get an EC meter.