The Issue will introduce:

- Xu's sealing law and theorem of fluids,
- new physical quantities: fluid current (fluid leakage current), fluid leak resistance (tightness), fluid resistance, fluid reactance, fluid impedance and pressure's sustainability,
- new concepts: self-sealing mechanism for material, self-sealing mechanism for O-rings, flowing & leaking mechanisms of fluids, and sealing difficulty factor of seals,
- new seal-designing methods and leakage-measuring methods/devices,
- new explanations of common applications and demonstrations of changes in fluid pressure with velocity that exist in the textbooks and on websites.

As far as flow is concerned, electricity is a fluid. Electrical flow in various electrical circuits at a fixed voltage is determined by their electric (leak) resistance and reactance. Of course, fluid flow in various fluid circuits such as in pipes, on objects and in joints at a fixed pressure is determined by their fluid (leak) resistance and reactance. Both the prior hydromechanics and the prior relevant international standards of seals were developed on the basis of not knowing such concepts and theories, and must be problems.