## Thursday, June 18, 2009

### 3How 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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1. Thanks, I had wondered some of what it was using. Isn't phosphorous one of the metals used for a bi-metal temperature gauge?

I know when I am pumping a tire from flat the aluminum shaft gets fairly warm. Does temperature not have a significant difference on these? What about pressure increased inside the tube from heating up?

Cute dog, I need a dog sometime soon.

2. Hey, I just discovered this blog... cool post on the pump! I'm off to read more...

3. Philip : The temperature is the typical piston pump rises and falls pretty quickly (first law os thermodynamics). You can try it for yourself by placing a temperature probe in the pump.

It does not affect the gauge because the pumping is adiabatic, that is, heat does not enter or leave the system. In a typical pumping action such as you pumping your tire, the air is compressed so quickly that there is little time for heat transfer to take place. I believe that the gauge is isolated from the cylinder and piston assembly and hence does not see any heat rise. Feel free to correct me.

Thank you. I read every single comment.