Thursday, June 18, 2009

How Your Pump's Pressure Gauge Works

Recently, my 5 year old Park Tool floor pump started bleeding from its ears. The gauge hose ruptured and there was a major air leak, rendering the pump useless unless I replaced that hose (this is one of the most common failures that could happen to any pump, so if you don't want to be irritated, consider keeping a replacement hose with you for safe measure).
Since the pump was old anyway, I decided to get a new one instead.
Anyway, just for fun, I opened up the pressure gauge of the old one to show you what is contained inside of it. I'm sure some of you may have wondered...'hmm, how on earth does my pump detect the pressure inside my tube?'

So what is the pressure gauge anyway? It is a pressure sensor, right? And what does it do? It does the following 3 things :

1) It senses the pressure to be measured.
2) Part of the instrument responds physically to that pressure by stretching, bending or changing positions.
3) The instrument then converts this response to a pressure signal which, in our case, is in the form of the needle moving along the dials of the scale.

Park Tool's pressure gauge here is a dry instrument (no liquids), and looks to be a C-shaped Bourdon tube, named after Eugene Bourdon, a French scientist who invented it in the 1840's. This is basically a bent tube in the form of a C that actually straightens out as the pressure in it rises. So consider that it acts like a spring that stretches when pressure is applied.

One end of the tube is sealed shut, while the other is open to process pressure. This straightening out is converted to a signal the human can read through a geared linkage connected to the pointer or needle and a pressure scale. As the tip of the tube moves, it rotates a sector which turns a pinion attached to the dial pointer.
See the diagram below and you'll go : "Wow, that is clever!"

Now say that you use a pump for over 5 years. Think about the number of times this tube stretches and unstretches. Isn't it amazing that this elastic material can undergo so many cycles of flexing without fatigue or hysteresis? I'm not sure of what this metal is, but it maybe a phosphor bronze, or a beryllium copper. Maybe even monel?

More later. Take care now.

P.S : Say hi to my buddy's new puppy. Her name is Olive. Hey Olive, you're on the web!!!

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