Basic knowledge

The gas mass flow meters (referred to as MFCs in the industry) widely used in fields such as environmental protection, analytical instruments, vacuum coating, semiconductors, photovoltaics, and biomedicine are mostly based on the thermal principle and laminar differential pressure principle. The YIDU flowmeter adopts advanced laminar differential pressure measurement principle, and mainly measures flow through laminar flow components combined with differential pressure sensors internally. So what exactly is differential pressure? Why can measuring differential pressure determine flow rate?

differential pressure

differential pressure is the difference in pressure between different positions. When gas flows in a pipeline, due to the viscous force of the pipe wall, there will be resistance to the gas flow, resulting in pressuer drop and the formation of a differential pressure along the flow direction. Differential pressure can be measured through a differential pressure sensor.

The relationship between differential pressure and flow rate

According to the expression of the law of conservation of energy in fluid mechanics (known as the Bernoulli equation):

Kinetic energy+gravitational potential energy+pressure potential energy=constant

The value of the constant on the right side of the equal sign does not have much significance, so the pressure potential energy can be simply understood as pressure, and the above equation can be expressed as:

1/2ρv²+ρgh+P=C

Considering a horizontal straight pipe, the gas gravitational potential energy ρ gh remains constant, with only two variables in the equation: flow velocity v and pressure P. As the flow velocity v increases, the pressure P inevitably decreases, meaning that the higher the flow velocity, the lower the pressure. The flow rate is directly related to the flow rate, and the flow rate (ml/s) is obtained by multiplying the flow rate (cm/s) by the cross-sectional area of the pipeline (cm ²).

Q: Which scenarios in daily life apply the relationship between differential pressure and flow rate?

A: There are many cases that reveal this pattern. For example, the cross-section of an airplane's wing is convex at the top and flat at the bottom. When the airplane is flying at high speed, the airflow above the wing passes through a longer distance, faster velocity, and lower pressure than the airflow below, resulting in lower pressure above the wing and thus obtaining upward lift.

   For example, there is a white line on railway platforms that prohibits tourists from crossing. This is because when the train passes by, it drives the air around the train to flow rapidly, resulting in a decrease in pressure. If a person is too close to the train, the differential pressure between the air in front and behind the body will push them towards the train, causing danger.

The above deduction and real-life examples reveal the qualitative relationship between differential pressure and flow rate. As long as the relationship between differential pressure and flow rate is accurately known, flow rate can be calculated by measuring differential pressure.

In conclusion

Fortunately, the two laws of absolute equilibrium in the universe, namely the law of conservation of mass and the law of conservation of energy, have specific descriptions in fluid mechanics, namely the continuity equation and the Bernoulli equation. Through these two equations, the exact relationship between differential pressure and flow rate can be derived. The derivation process will not be repeated here. Interested students are recommended to read the following content.