How Do Diaphragm Pressure Gauges Work?

A diaphragm pressure gauge is a device that uses a diaphragm with a known pressure to measure pressure in a fluid. It has many different uses, such as monitoring the pressure of a canister of gas, measuring atmospheric pressure, or recording the strength of the vacuum in a vacuum pump.

...
Image Courtesy of: http://www.efunda.com/DesignStandards/sensors/diaphragm/diaphragm_intro.cfm

Purpose

Basic Mechanics

The diaphragm has a flexible membrane with two sides. On one side is an enclosed capsule containing air or some other fluid at a predetermined pressure. The other side can be left open to the air or screwed in to whatever system the gauge is meant to measure. The diaphragm also attaches to some sort of meter, which shows how high the pressure is.

Detecting Pressure

A fluid in contact with a flexible membrane pushes on that membrane, bending it. The pressure is a measure of how hard it pushes. When the outside preference is low, the reference pressure bends the membrane out. As the outside pressure increases, it pushes back on the membrane, bending it back the other way. By measuring how far the membrane bends, the gauge can detect the outside pressure.

Measuring the Pressure

There are many different ways to measure the pressure from a dynamic pressure gauge. One of the simplest ones is to attach a needle to the gauge. When the pressure increases, it pushes on the needle, moving it up and down along a dial which shows the pressure. Another way is to use an electric resistance strain gauge. An electric resistance strain gauge uses a long strip of an electric resistor--a device that resists the flow of electricity. The resistor is attached to the diaphragm. As the diaphragm bends, it stretches out the resistor, increasing the resistance. The resistor has an electric current running through it. The more the diaphragm bends and increases the resistance, the more the current drops. By measuring the electric current, the gauge can determine how far the diaphragm has bent, and thus, how much pressure the outside air is creating.