The Dissection: Air Pressure Gauge

How a simple mechanical system knows when your tires are low

By Vin Marshall Posted 11.09.2009 at 10:33 am 2 Comments

Air Pressure Gauge A tired, but as yet un-dissected air pressure gauge. Vin Marshall

You've almost certainly seen a pressure gauge somewhere: on an air compressor, a steam boiler or perhaps an automotive vacuum gauge. Have you thought much about how that gauge works? Magic? Elves? We'll rip one open to find out.

Anyone who has been reading my posts will have surmised that I like clever mechanisms. The Bourdon tube, which is the source of the pressure gauge's magic, certainly qualifies.

Bourdon Tube: The Bourdon Tube (pictured here in black wrapped around the circumference of the gauge) is what makes the magic happen. As the inside of the tube is pressurized, it straightens out slightly, pulling on the mechanical linkage that moves the needle on the face of the gauge. Vin Marshall

The operation is simple. One end of the curled metal tube is sealed and the other is connected to the source of the pressure to be measured. As that pressure increases, the tube expands and straightens very slightly, as if being inflated, which, essentially, it is. As it straightens, the sealed end moves and pulls on the attached mechanical linkage of gears and levers.

Air Pressure Gauge Linkage: As the Bourdon tube extends, it pulls on this mechanical linkage, which rotates the needle on the face of the gauge. Vin Marshall

Through that gear train, the slight movement of the tube is translated into rotation of the indicator needle. The tube, gear train and gauge face are sized and calibrated such that the amount of tube movement for a given applied pressure will move the needle to indicate that same pressure on the face of the gauge.

Simple and clever. Almost like magic.

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2 Comments

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animemaster

11/09/09 at 5:55 pm

I saw a great "How It's Made" episode on these. My only concern was metal fatigue generating errors over time as copper tends to stiffen significantly when it is bent.

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vinmarshall

11/10/09 at 12:03 am

@animemaster That's a great point. While these tubes only move very small amounts in operation, slight work hardening will still occur over time. That probably explains at least part of why these gauges need to be calibrated periodically. The one photographed here was last calibrated sometime around 1991.