Current meter - flow meter

A general term for instruments that measure the speed of flow of fluids (gas, liquid, steam). They are broadly classified into the following six types based on the measurement principle.

(1) Method of calculating average speed from time and distance. This method involves inserting an indicator object such as a float into the current and calculating the flow speed from its movement. Another method involves temporarily injecting an easily detectable substance such as salt or a radioactive material and measuring the time it takes for it to reach a point a certain distance downstream.

(2) Methods that use dynamic pressure A typical example is the Pitot tube used to measure aircraft airspeed, wind speed, etc. Dynamic pressure is calculated as the difference between the pressure measured with the nozzle facing upstream and the pressure measured with the nozzle facing to the side of the flow, and the flow speed is calculated using Bernoulli's theorem. There are also many current meters that detect dynamic pressure using an impeller.

(3) Methods that use wave propagation A typical example is the ultrasonic flowmeter, which uses the Doppler effect to measure the difference in the speed of sound waves in a fluid depending on whether they are traveling in the same direction as the flow or not. The laser Doppler flowmeter, which measures the flow speed based on the Doppler effect of light reflected from suspended objects in the fluid, is also based on a similar principle.

(4) Electromagnetic Induction Method This method is used to measure the flow rate of conductive fluids such as water. Using Faraday's law of electromagnetic induction, which applies between an electric field, a magnetic field, and the motion of a conductor, the flow rate can be calculated from the magnetic field placed perpendicular to the flow and the generated voltage.

(5) Methods that utilize heat dissipation. A typical example is a hot wire anemometer, which utilizes the fact that the heat dissipation from a heated object has a specific relationship with the flow velocity.

(6) Methods that utilize the periodic motion of a fluid, such as the generation of vortices A typical example is a flow meter that uses Karman vortices. Based on the principle that the frequency of vortex generation is proportional to the flow velocity, an object that generates vortices and a vortex detector are used to measure the frequency and determine the flow velocity. In addition to Karman vortices, there are other periodic phenomena that occur due to the interaction between objects and flows, and these are also used to measure flow velocity.

[Mitsui Kiyoto]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

流体（気体、液体、蒸気）の流れの速さを測定する計器の総称。測定の原理から次の6種類に大別される。

(1)時間と距離から平均速度を求める方式　流れの中に浮きなどの指標物体を入れてその運動から流速を求めるものと、食塩、放射性物質など検出しやすい物質を一時的に注入して、下流の一定距離の地点まで達する時間を測定するものがある。

(2)動圧を利用する方式　航空機の対気速度、風速などの測定に用いるピトー管が代表的で、測定口を流れの上流に向けて測った圧力と側面に向けて測った圧力との差として動圧を求め、ベルヌーイの定理によって流速を算出する。また、動圧を羽根車を使って検出する流速計も多い。

(3)波動の伝播(でんぱ)を利用する方式　ドップラー効果の応用で、流体中の音波の速度が流れの向きと逆向きとで流速の分だけ異なることを利用した超音波流速計が代表的である。流体中の浮遊物などからの反射光のドップラー効果から流速を測るレーザードップラー流速計も、類似の原理に基づいている。

(4)電磁誘導を利用する方式　水などの導電性流体の流速測定に用いられる。電場、磁場、導体運動の間に成り立つファラデーの電磁誘導の法則によって、流れに直角に設置された磁場と発生電圧から流速を求める。

(5)熱の放散を利用する方式　熱線風速計が代表的で、加熱された物体からの熱の放散が、流速と特定の関係にあることを利用している。

(6)渦の発生など流体の周期運動を利用する方式　カルマン渦を利用した流速計が代表的で、渦発生の周波数が流速に比例する原理により、渦を発生させるための物体と渦検出器を用いて周波数を測定して流速を求める。カルマン渦のほかにも物体と流れの相互作用によっておこる周期現象があり、流速の計測に利用されている。

［三井清人］

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