When replacing the working liquid in the liquid level meter, the positive and negative pressure inlet valves should be closed, and the equal pressure valve between the positive and negative pressure chambers should be opened so that the pressure difference between the two sides is zero, then the inlet cover should be opened to replace the liquid. Otherwise the working liquid inside the tube may be ejected under the action of the differential pressure. The type of working fluid contained inside should not be mistaken.
The liquid level gauge should be opened before the unit is started with the upper and lower valves. If used in operation, the upper valve should be opened first, followed by the lower valve, and the speed of opening the valve should be slow. When cryogenic liquid air, liquid oxygen and liquid nitrogen are led from a fractionating tower or storage tank by a conduit, they are bound to vaporise and the liquid level cannot be measured with an ordinary glass level meter, but must be measured with a special structured level meter, the principle of which is shown in Figure 116. It also uses the principle of hydrostatic pressure of the liquid.
