1. Atmospheric pressure: the pressure caused by gravity on the surface of the air column on the earth's surface. It is related to its altitude, latitude and meteorological conditions.

2, differential pressure (differential pressure): the relative difference between two pressures.

3. Absolute pressure: all pressures in the space where the medium (liquid, gas or steam) is located.

 Absolute pressure is a pressure relative to zero pressure.

4, surface pressure (relative pressure): if the difference between the absolute pressure and atmospheric pressure is a positive value, then this positive value is the surface pressure, that is, the surface pressure = absolute pressure-atmospheric pressure> 0.

5, negative pressure (vacuum gauge pressure): corresponding to the "gauge pressure", if the difference between absolute pressure and atmospheric pressure is a negative value, then this negative value is negative pressure, that is, negative pressure = absolute pressure-atmospheric pressure <0.

6, static pressure: generally understood as "the pressure that does not change with time, or the pressure that changes slowly with time, that is, the surface pressure value measured in the fluid that is not affected by the flow rate".

7, dynamic pressure: corresponding to the "static pressure", " the pressure changes rapidly with time, that is, the dynamic pressure refers to the kinetic energy of a unit volume of the fluid."Usually calculated with 1 / 2 ρν 2. In ρ - -fluid density; v- - -speed of fluid motion.”

 What are the similarities and differences between the HART protocol and the fieldbus technology?

Both HART and fieldbus technology can realize remote access to the status and parameters of the field equipment. At the same time, both technologies support connecting multiple devices on one bus. Both HART and fieldbus use equipment description to realize the interoperation and comprehensive application of equipment. So, there are some similarities between them.

There are four differences between them:

1) The fieldbus adopts real full digital communication, while HART is superimposed on the original 4 ~ 20 mA analog signal by FSK, so it can be directly connected to the existing DCS system without reconfiguration;

2) The field bus mostly adopts multi-point connection, and the HART protocol generally adopts multi-point connection only when monitoring and using;

3) In the control system composed of fieldbus, the equipment can communicate directly without the intervention of the host;

4) Field bus equipment can provide more diagnostic information than HART equipment.

 Therefore, the fieldbus equipment is suitable for the high-speed network control system, while the superiority of the HART equipment is reflected in the compatibility with the existing simulation system.