Tub de Pitot

Diagrama d'un sistema Pitot estàtic, que inclou un tub Pitot, els instruments Pitot estàtics i les preses de pressió estàtica i dinàmica

Un tub Pitot és un instrument de mesura de la pressió que serveix per calcular la pressió total, també anomenada pressió d'estancament, pressió romanent o pressió de recés (suma de la pressió estàtica i de la pressió dinàmica).

Va ser inventat per l'enginyer Henri Pitot el 1732 ^[1] i va ser modificat al segle xix per Henry Darcy.^[2] Es fa servir molt per mesurar la velocitat del vent en aparells aeris i per a mesurar les velocitats d'aire i gasos en aplicacions industrials. Els tubs pitot mesuren la velocitat en un punt donat del corrent de flux i no pas la mitjana de la velocitat del fluid mesurat.^[3]

Teoria d'operació

En el punt (1) de l'esquema, embocadura del tub, es forma un punt d'estancament, la velocitat allà (v ₁) és nul·la, i la pressió segons l'equació de Bernoulli augmenta fins:

{P_{1} \over \rho }={P_{t} \over \rho }={P_{0} \over \rho }+{v_{0}^{2} \over 2}:

per tant:

P_{t}=P_{0}+\rho .{v_{0}^{2} \over 2}

Sent:

V ₀ ip ₀ = pressió i velocitat del corrent inalterats.

P _t = pressió total o estancament.

Aplicant la mateixa equació entre les seccions (1) i (2), considerant que v ₁ = v ₂ = 0, es té:

Y_{1}+{p_{1} \over \rho g}=y_{2}+{P_{2} \over \rho g}

Sent:

I ₂ - i ₁ = L (lectura en el tub piezomètric)

després:

$\P _{t}=\rho .g.L$

Aquesta és anomenada l'expressió de Pitot.

Referències

Mecànica de Fluids i Màquines Hidràuliques . Claudio Mataix Plana, 2009.

↑ Pitot, Henri «Description d'une machine pour mesurer la vitesse des eaux courantes et le sillage des vaisseaux» (PDF). Histoire de l'Académie royale des sciences avec les mémoires de mathématique et de physique tirés des registres de cette Académie, 1732, pàg. 363–376 [Consulta: 19 juny 2009].
↑ Darcy, Henry «Note relative à quelques modifications à introduire dans le tube de Pitot» (PDF). Annales des Ponts et Chaussées, 1858, pàg. 351–359 [Consulta: 31 juliol 2009].
↑ Geankoplis, C.J.. Transport processes and separation process principles (includes unit operations). 4th ed. New Jersey: Prentice Hall, 2003.

Vegeu també

Enllaços externs

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Què és el tub de Monsieur Pitot? Arxivat 2014-03-18 a Wayback Machine. Assaig a nivell divulgació sobre el tub de Pitot

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