What Is a Vacuum Excavation?

A vacuum is any gaseous space with a pressure lower than normal pressure. A space without any substance is called an "absolute vacuum." Even under extremely high so-called "ultra-high vacuum" conditions, gas molecules per cubic centimeter are There are more than 3 million. Therefore, there are still a large number of gas molecules in the vacuum (but much thinner than ordinary air), and the molecules are constantly moving and functioning. The process of generating a vacuum is called vacuuming and exhausting. Air or pumping. The tools commonly used to generate vacuum are called vacuum pumps. Water pumps, mechanical pumps and oil diffusion pumps are commonly used. In addition, a variety of special getters and condensation traps are also used.

Vacuum does not mean nothing, but refers to a state below atmospheric pressure. The pressure of the high and low vacuum gas is expressed in units of Pa (Pa) according to the International System of Units (SI). The conversion relationship between some units used in the past (now abolished) and Pa is: 1mmHg = 1/760 atm = 133. 322 Pa.

According to the pressure, the vacuum can be divided into:

Rough vacuum 1.013 x10 ⁵ ~ 1.3 x10 ³ Pa

Low vacuum 1.33x10 ³ ~ 1.3x10 ^-1 Pa

High vacuum 1.33x10 ^{-2 ~} 1.33x10 ^-5 Pa

Ultra-high vacuum 1.33x10 ^{-6 ~} 1.33x10 ^-9 Pa

Very high vacuum <1. 33 x10 ^-9 Pa

It can be seen that the so-called high vacuum refers to the low pressure of the system. It can be known from calculations that even under the ultra-high vacuum of 1.33 × 10 ^-6 Pa, there are still about 3 × 10 ⁸ gas molecules per cubic meter, per second The number of times the bell collided per unit area was 3.84x10 ¹² times, but it was much thinner than the gas at normal pressure. In this case, the behavior of the gas is importantly different from atmospheric pressure, causing vacuum technology to

1. Air can slow down certain physical and chemical effects. For example, a vacuum (10 ^-1 ~ 10 ^-2 mm Hg) is needed in the double wall of the thermos, because it can reduce the conduction and convection of the gas and make it difficult to radiate heat. , So as to achieve the role of insulation. The thermometer needs a vacuum (10 ^-2 ~ 10 ^-3 mm Hg), which can reduce the pressure change caused by the volumetric compression of the remaining gas in the glass tube and affect the measurement height. The electric bulb needs a vacuum (10 ^-1 ~ 10 ^-3 mm Hg) to prevent the tungsten wire from being incinerated by the action of oxygen in the air at high temperatures.

In the case of high vacuum, the chemical effects of residual gas and certain materials can be ignored. For example, titanium can easily absorb oxygen, nitrogen, silicon, carbon and other elements at high temperatures, and pure titanium can be obtained by vacuum smelting. The substrate of semiconductors, germanium or silicon, can destroy their characteristics as long as there are 100% or 1 / 1000th impurities. Except for smelting in a vacuum (10 ^-5 ~ 10 ^-7mm mercury height), there is no other way to Make them purified.

2. Vacuum can lower the vaporization point. Taking water as an example, the boiling point of water is:

twater = 100 + 0.0367 (P-760)

In the formula, the unit of P is millimeter mercury high, so in the mountains, when P <760 millimeter mercury high, the water will open at less than 100 .

3. The insulation of the vacuum is strong. Between two electrodes under high vacuum, it can withstand a very high potential difference without leakage or breakdown, so there is a vacuum capacitor (10 ^-4 ~ 10 ^-5 mm High mercury). ^[2]

The process of creating a vacuum is called evacuation or exhaust. Tools that use a physical method to generate a vacuum are commonly known as vacuum pumps. Some pumps can reduce the gas pressure from 1 atmosphere, while others can only pump lower pressure to lower pressure. It requires the former pump Reduce the gas pressure from a 760 mm Hg level to a specified value and start working. The former is called a "pre-stage pump" and the latter is called a "secondary pump". The air pressure required for the secondary pump to start is called "prepared vacuum".

The volume of gas extracted per unit time at a specified pressure is called the pumping rate. The minimum pressure that a pump can reach after sufficient time is called the ultimate vacuum.

There are many types of vacuum equipment, from the simplest laboratory jet water pump to the high-vacuum steam diffusion pump. There are many types of vacuum equipment, but the principle and structure of the vacuum equipment are often similar to the compressor. This is because the compressor outlet generates compressed gas while For its inlet, it is essentially a suction and vacuum process. Vacuum pumps are generally divided into two categories, namely mechanical pumps and diffusion pumps. ^[2]