Is there a difference in the definition of set pressure between air & liquid applications?

Yes. Liquid Applications: Liquids tend to be incompressible, meaning they cannot be compressed like air. Liquids can be under pressure but as soon as the volume changes they immediately lose all pressure (pressure goes to zero). There are three accepted definition in the industry for liquid applications. They are: start to leak, first steady stream and full flow. For Liquid applications – Kingston defines “Set Pressure” as the first steady stream of flow out of the valve.