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Float and buoy level gauges. Device, principle of operation, calculation, types and types of float and buoy level gauges.
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A float level gauge is a level gauge based on measuring the position of a float partially immersed in a liquid, and the degree of immersion of the float (sediment) at a constant density of the liquid does not depend on the controlled level. The float moves vertically along with the liquid level, and therefore the level value can be determined by its position. In static mode, the float is affected by: gravity G and the buoyant forces of the liquid and the gas medium. When moving the float, there is also a resistance force in the moving elements of the level gauge. If we neglect the resistance force of the kinematics and the buoyant force of the gas phase, then the forces acting on the float are related by the equation
G = V_zhρ _jg, where V_f is the volume of the submerged part of the float, ρ _g is the density of the liquid.
The volume of V_zh uniquely determines the draft (depth of immersion) of the float. When the density of the controlled liquid changes to Δρ _zh, the volume of the immersed part changes to ΔV_g, which leads to a change in the sediment, i.e. to the appearance of an additional error. The expression for ΔV_f can be obtained as
ΔV_w = (δV_w / δρ _f) Δρ _f = -V_w (Δρ _w / ρ _w)
Thus, the volume of the submerged part V_g, and hence the sediment of the float, is the parameter determining the additional error caused by the change in the density of the controlled liquid. To reduce this error, it is advisable to reduce the draft of the float, which can be achieved either by increasing the cross-sectional area of the float or by lightening the float.
In the simplest case, the float is connected to the pointer by means of a flexible mechanical connection. The dimensions of the float are limited by the dimensions of the level gauge, the mass of the float cannot be greatly reduced due to the need to ensure the required tension of the flexible element and overcome the frictional forces. The resistance force is determined by the choice of the coupling scheme of the float with the measuring circuit of the level gauge. This design has a large measuring range, but does not provide good sealing of the tank, so it is used only with a small overpressure or dilution and low temperatures of the controlled medium. An example of such a level gauge is the level gauges of the UDU type. They are designed to measure the level of petroleum products with a temperature of (-50...50) ° C, in the range of overpressures (-1.5...3) kPa. Measuring ranges 0...12 or 0...20 m, the main error ±4 mm.
At higher values of temperature and pressure of the medium, float level gauges with magnetic transducers are used. An example of such devices are magnetic level meters of the PMP type (Fig. 1) of NPP "SENSOR".

Rice. 1. Scheme of the PMP float level gauge:

1 — hull; 2 — cable output; 3 — umbrella; 4 — retaining ring; 5 — permanent magnet; 6 — float; 7 — guide tube; 8 — reed relay
A float 6 with a permanent magnet 5 moves along the guide pipe 7 under the influence of a change in the liquid level. Inside the tube 7 along its entire length are reed relays that are triggered by the magnetic field of the float. The retaining ring 4 restricts the movement of the float upwards, and the umbrella 3 protects it from droplets of condensation that may form on the inner walls of the tank. With a measuring range of 0.5 to 6 m, the level height is measured with a discreteness of 5 mm. When determining the mass, temperature is measured to take into account the change in the density of the liquid in the transducer. These converters can have as an output value a change in resistance, a current signal of 4...20 mA or digital.
Magnetic floats are part of ultrasonic level meters.