How is pressure balance achieved for the small holes in the waveguide?
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Is pressure balance required only when the density of the external liquid is high? If the density is low, is it not needed? Or do we need it regardless of density?
Uh, you mean it has nothing to do with density? So under what circumstances does this hole come into play?
The small hole in the waveguide is used to balance the pressure inside and outside the tube, serving as a kind of communicating vessel. Without balance holes, as the liquid level rises, the gas inside the guided wave tube gets compressed, creating a certain pressure; as a result, there is a discrepancy between the liquid level inside the tube and that in the tank.
It’s like when you place a cup upside down in a basin of water; do you remember an experiment from your childhood that showed that with the cup placed in that way, there is no water inside the cup, but there is plenty of water in the basin. If there are holes in the cup, water can flow into it. If the waveguide is not perforated, it will result in the water level inside the waveguide being at a different height from that inside the tank. Usually, it is high inside the tank and low inside the waveguide. An opening position that is too low in the waveguide can also lead to the same consequences. Therefore, the waveguide not only needs to have openings, but there are also requirements regarding the spacing between these openings as well as their size; the openings must be evenly distributed along the entire tube. Most importantly, there must be an opening above the highest liquid level. For example, if the tank is 12 meters high and the actual maximum water level reaches 10 meters, then there must be a hole above the wave guide at 10 meters. Otherwise, the error may be very large at high liquid levels.