If the inside pressure is 300 Pa and the outside pressure is 200 Pa then the difference is 100 Pa and you want that directed outward from the tube.
Pick the tube surface and assign 100 Pa and look at the arrow. If it points outward, you are done, if it points inward, change the value to -100 Pa and it will point outward.
The surface normal might point inward or outward depending on which CAD system created the surface.
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