The metal tube rotor flowmeter, as a classic variable area flow meter, relies on a sophisticated mechanical balance system for its stable and reliable core. It does not directly measure the flow rate, but indirectly reflects the flow rate by measuring the displacement of the float. The entire process is a combination of fluid mechanics and mechanical mechanics.

Core mechanics model: force balance equation

The core of its work lies in the force balance of the float in the conical measuring tube. The float is mainly subjected to three forces:

Float's own gravity (G): vertically downward, it is a constant force.

The buoyancy of the fluid on the float (F float): vertically upward, equal to the weight of the fluid displaced by the float.

The pressure differential resistance (F-resistance) of the fluid to the float: vertically upward, this is the key variable. When the fluid flows through the annular gap between the float and the pipe wall, due to the throttling effect, a pressure difference Δ P is generated at the upper and lower ends of the float (the lower end pressure is higher than the upper end), thereby generating an upward force.

When the flow rate is stable, the float will stay at a certain position, and at this time, the three reach force balance: G=F float+F resistance.

The mapping relationship between displacement and flow rate

When the flow rate increases, the fluid velocity accelerates, and the pressure difference resistance F acting on the float increases accordingly, instantly disrupting the original balance. Due to F>G-F floating, the resultant force rises and the float rises accordingly.

As the float rises, the annular area between it and the conical tube wall gradually increases, causing the fluid velocity at that location to begin to decrease, resulting in a corresponding decrease in pressure resistance F. Until F decreases to satisfy G=F float+F resistance again, the float reaches a new equilibrium at a new, higher position.

On the contrary, when the flow rate decreases, the process is reversed and the float descends.

Therefore, each height position of the float corresponds to a specific annular flow area, which in turn corresponds to a specific flow rate value. By using magnetic coupling and other methods to transmit the displacement of the float to the indicator, the instantaneous flow rate can be intuitively displayed on the dial.

Conclusion

The essence of a metal tube rotor flowmeter is a mechanical feedback system based on force balance. It utilizes a conical tube structure to convert changes in flow rate into float displacement, which changes the flow area and ultimately achieves a new balance between pressure resistance and flow rate, thereby achieving accurate measurement of flow rate.