The diaphragm box level meter is used to measure the level of the open tank. The box contains a large amount of air, which is kept within a flexible diaphragm. A tube connects the diaphragm box to a pressure gauge.
Construction and Working:
The liquid level meter of the diaphragm box consists of two flanges in the middle of which is contained a rubber or oil resistant diaphragm element synthetic composition. The box is immersed in the liquid to be measured and filled with air capillary extends from it to the instrument.
The deflection of the diaphragm produced by the head of the liquid causes a compression of the air in the connection capillary. The pressure exerted by the liquid against the volume of air within the box represents the fluid pressure at that level
A secondary instrument is connected to the other end of the capillary, the pressure gauge at the secondary instrument measures the air pressure and relates the value to the fluid level. The capillary The connecting tube, which enters the upper flange, extends into the diaphragm case and is bent to one side, to prevent it from being sealed by the diaphragm.
There are two common types of diaphragm boxes – open and closed. The open diaphragm box is immersed in the fluid within the vessel. The closed diaphragm box is mounted externally from the vessel and is connected by a short length of piping.
- Relatively simple
- Suitable for various materials and very accurate
- The box can be installed in a well outside the tank and the well is communicated to the tank with an impulse pipe. The impulse pipe and the well are filled with an inert liquid.
- Requires more mechanical equipment, particularly with pressure vessels
- The head developed is not sufficient to meet up the line losses as well as for a satisfactory indication. Hence ranges are quite limited.