1. Environmental impact
The pressure of the atmosphere is caused by the collision of air molecules, so the greater the density of air, the more air molecules per unit volume, so the greater the pressure.
The greater the air humidity, the greater the air density and the greater the atmospheric pressure.
The higher the altitude, the thinner the air and the lower the pressure.
The higher the latitude, the thinner the air and the greater the pressure.
The higher the temperature, the faster the air molecules move, the more molecules per unit volume, the greater the pressure.
It can also be seen from the thermodynamic formula PV = nRT that P is the gas pressure, V is the gas volume, n is the number of gas molecules, R is the thermodynamic constant, and T is the ambient temperature. When V, n and R are unchanged, the higher the temperature, the greater the pressure. However, considering that the laboratory generally maintains a constant temperature, it is not necessary to consider the influence of temperature within the range of 3 Â° C.
In addition, atmospheric pressure will also be affected by the acceleration of gravity (gravity of the earth). The altitude and latitude are different. The pressure will change for this reason. In addition, the gravity of the sun and the moon, especially the gravity of the moon, will reduce the value of gravity acceleration, The atmospheric pressure sooner or later is lower than the atmospheric pressure at noon (regardless of the temperature).
When the air circulates, the air will flow from the place where the pressure is high to the place where the pressure is low, causing a local pressure gradient.
2. Influence of the instrument
Consider first from the instrument itself, a prerequisite for this method is to maintain a constant intake pressure. The parameters of the instrument itself are basically fixed and can be ignored. To provide stable pressure, there are the following aspects
a) Fischer pump: to provide stable air pressure to the instrument;
b) Regulator tube: control the inlet pressure of the sample tube to 50cmH2O;
c) Pressure gauge: keep the inlet pressure of the sample tube at 50cmH2O;
Note: The pressure of the pressure regulator tube is maintained by the height of the liquid level of the inner tube. The pressure of the pressure gauge is increased by 25cmH2O by the front tube and 25cmH2O by the rear tube to maintain 50cmH2O, so the diameter of the front tube and the rear tube should be as much as possible. Consistent (factory requirements), when measuring, you can use a ruler to measure whether the water surface of the front pipe can rise 25cm when the pipe is empty.
3. Influence of standard tube and sample tube
The standard tube is mainly used to prevent contamination, and it is calibrated before leaving the factory. It does not need to be recalibrated. It can be washed regularly with absolute ethanol.
Since the sample tube often rubs against the sample, filter paper, and copper plug, it will wear out after a period of use and will cause a larger diameter, so the cross-sectional area A will increase, and the sample height L will become smaller. If the surface of the sample tube is worn and not smooth, some samples will stick to the tube wall, causing the sample height L to become smaller. If the copper plug, filter paper and tube wall are not tightly connected, causing the sample to leak out, it will also cause the sample height L to become smaller.
Filter paper: Different filter papers, Fischer will also have a big difference.
4. Impact of operation
Mainly is the determination of the best porosity and reading habits.
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