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Speaker 1: Welcome to Brainstuff from house Stuff Works dot com where

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Speaker 1: smart happens. Hi Marshall Brain with today's question, how does

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Speaker 1: the spedometer in an airplane work? Air speed is a

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Speaker 1: measurement of the plane speed relative to the air around it,

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Speaker 1: not to the ground, but to the body of air

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Speaker 1: that the plane is flying through. The pet tube is

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Speaker 1: an ingenious device used by airplanes and votes for that matter,

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Speaker 1: for measuring forward speed through a medium. The device is

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Speaker 1: really a differential pressure gauge and was invented by Henry

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Speaker 1: Peto in seventy two. An example of an air pressure

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Speaker 1: gauge is a tire pressure gauge. It measures the pressure

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Speaker 1: inside the tire. A Peto tube is doing that same

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Speaker 1: kind of thing, measuring the air that's flowing path us

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Speaker 1: the airplane. The open end of the pet tube, usually

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Speaker 1: mounted on a wing or with a little angle bracket

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Speaker 1: on the fuselage, faces towards the flow of air or water.

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Speaker 1: The air speed indicator actually measures the difference between a

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Speaker 1: static sensor not in the air stream and a sensor

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Speaker 1: the p DO tube in the air stream when the

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Speaker 1: airplane is standing still. The pressure in both these places

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Speaker 1: is equal. In the air speed equal zero. The rush

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Speaker 1: of air in flight causes a pressure difference between the

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Speaker 1: static tube and the p DO tube that's in the

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Speaker 1: air stream. That pressure differential makes the pointer on the

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Speaker 1: air speed indicator move. An increase in forward speed raises

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Speaker 1: the pressure at the end of the pet tube. In turn,

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Speaker 1: the air pressure pushes against a flexible diagraphragm that moves

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Speaker 1: a connected mechanical pointer on the face of the indicator.

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Speaker 1: The indicator is calibrated to compensate for winds in the

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Speaker 1: air or the speed of the opposing current in the water.

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Speaker 1: In airplanes, electronics also compensate for altitude and air temperature

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Speaker 1: to make the air speed measurement accurate. It's possible to

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Speaker 1: take this basic pressure difference idea and make it really

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Speaker 1: complicated with digital readouts and digital sensing and all kinds

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Speaker 1: of computer stuff. But it's also possible to take it

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Speaker 1: in the other direction and make it incredibly simple. A

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Speaker 1: little mechanical device that you might hang off a hang

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Speaker 1: glider or stick outside a car, or use on a boat.

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Speaker 1: It could be just a little slider inside a tube.

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Speaker 1: Such a device is known as a direct reading airspeed indicator.

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Speaker 1: The instrument is for paragliding or ultralight aircraft, or any

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Speaker 1: kind of simple plane like that that needs a lightweight,

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Speaker 1: inexpensive airspeed indicator. The difference between the static pressure and

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Speaker 1: the pressure of the air rushing into the opening of

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Speaker 1: the bottom of the tube causes a red indicator disc

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Speaker 1: to rise or fall, since the upward pressure in the

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Speaker 1: tube is greater than the weight of that disc. If

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Speaker 1: you look at one of these, you'll notice that there's

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Speaker 1: a slight taper in the design of the vertical tube.

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Speaker 1: The red disc has a hole in the center of

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Speaker 1: it for the slighter rod to keep the disc level.

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Speaker 1: The airspeed indicator fastens to a strut of the hang glider.

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Speaker 1: Peedo tube devices and minometers are also used in commercial

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Speaker 1: heating and air conditioning systems for duct air flow measurements.

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Speaker 1: You just stick the device in the duct and you

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Speaker 1: can read off the speed of the air flowing through

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Speaker 1: that duct. Be sure to check out our new video podcast,

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