## 1.4 Some basic concepts of pneumatic technology

### 1.4.1 Common pressure unit

In the pneumatic system, the problem of pressure unit conversion is often encountered. In particular, the unit system adopted by some imported equipment often confuses operators and users. Table 1-3 solves this problem. Middle return home: break the game and go straight in five bedrooms.

Table 1-3 Conversion of Common Pressure Units

 (Pa)N/m2 (bar)105N/m2 lbf/in2 psi (Torr)mmHg 1 10-5 1.45×10-4 75×10-4 9.81×10-4 0.981 14.22 736 1.013×105 1.013 14.68 760 105 1 14.5 750 133 1.33×10-3 1.92×10-2 1 9.81 9.81×10-5 1.42×10-3 7.36×10-2 69×103 6.9×10-5 1 51.72

### 1.4.2 Standard state and standard volume

The volume of gas is comparable only when its temperature and pressure are the same. Therefore, a unified standard state is defined, so that the state of gas can be converted into a unified standard state by using the gas state equation under normal state.
Common standard states are as follows.

① Physical standard state (also called reference state):

P=1.013bar, T=273K (0C), R=287Nm/(kgK), air, 0% air relative humidity

② Engineering standard status:
P=1bar, T=293K (20 ℃), R=288Nm/(kg • K), moist air, 65% air relative humidity.
“(ANR)” can be marked after the units in the engineering standard state.

To describe the standard flow, the following conditions for measuring flow must be observed. As the standard state of air, the symbol of physical standard state is used. The volume filled with dry air is marked as VN. The unit of standard volume is cubic meter (m3). Nm3 (standard cubic meter) is often used when describing the standard volume. From ideal gas and constant mass flow, we can get:

ṁ=const=pQ/RT=pNQN/RNTN

In this way, the standard volume flow QN can be calculated by the following formula:

QN+QpTN/pNT