Displacer level gauge - If a metal cylinder is hung on a spring balance, the weight of the cylinder is measured. This effect can be explained by Archi
If a metal cylinder is hung on a spring balance, the weight of the cylinder is measured (see figure). If the cylinder is fully submerged in water, the spring balance will show a lower weight. If the cylinder is halfway immersed, the spring balance will show a weight exactly halfway between the other two weights.
This effect can be explained by Archimedes' principle, which states:
"A body submerged in a liquid experiences an upward force equal to the weight of the liquid displaced".
The spring balance in figure shows the apparent weight, which is equal to the actual weight minus the displaced liquid weight. Therefore the apparent weight, which acts as tension on the spring, depends on:
- The volume of the cylinder immersed in the liquid (which represents the liquid level)
- The density of the liquid.
The spring holding the displacement cylinder may be connected to a transmitter, where the tension on the spring can be converted to an output signal representing the liquid level inside the vessel. This signal can be transmitted for remote indication or to level control loops.
In a tank or a boiler under high pressure, it is difficult to transmit the movement of the displacer to the outside, which must be done without friction and without leakage.
The torsion tube is found to be particularly suitable for this and is therefore commonly used. Figure shows a cross section of the torsion tube and next figure is a diagram showing how the displacer and the torsion tube are attached to each other.
The functions of the torsion tube are:
- To supply resilience, making a spring unnecessary.
- To provide pressure tight and low friction transmission of the movement of the displacer from an area under pressure to the outside.
As shown in the figure, the displacer is hung on a lever. This lever is attached to the torsion tube. As the level in the tank changes, the apparent weight of the displacer changes, and as a result of this, the indicator outside the tank rotates moving the pointer up or down the scale.
The displacer is usually fitted in a “level body” or “level Troll” and the complete level gauge can be connected to the vessel in which the level has to be measured, as shown in figure above.
The Masoneilan level leg, assembly operates on the displacer principle, as illustrated beside.