Hydrion Pro Ph Strips (Ph 0.0 To 6.0).  Item 9200.  100 Strips.

I use these exact pH strips to test my Star San.  See: Testing Star San Effectiveness

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Micro Essential Labs Hydrion Pro Ph Strips (Ph 0.0 To 6.0) Item 9200 100 Strips

Also: Star San- 32 oz | Star San Tips, Tricks and Guidelines