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Gas Laws

The ‘Gas Laws’ is an umbrella term for 4 principles that sought to explain the relationship of gases to volume, temperature and pressure.
They’re more than 200-years old so it may seem a curious topic for a technical article in 2017, but I often find it’s useful to revisit ‘First Principles’ as they are of such importance, and find application in fields as broad as gauges, munitions, diving and countless other areas.

Charles’ Law

The volume of a gas is proportional to the absolute temperature
  Where V1 is the initial volume and T1 is the initial temperature and V2 is the volume after heating and T2 is the temperature of heat applied.

Boyles Law

The volume of a gas is proportional to the pressure

Gay-Lussacs Law

The pressure exerted on the walls of a vessel are proportional to the temperature

Avagadro’s law

At a given temperature and volume, all gases contain the same number of molecules.

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The General Gas Law

Each of these laws interact with each other; they don’t work in isolation.  The General Gas law encompasses all of the above

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This “Before” and “After” photo shows why one shouldn’t dispose of used aerosol cans in a fire. (Don’t try this at home!)
There’s a couple of Gas Laws at work here. Charles tells us that the pressure inside the can is proportional to the temperature.
And Gay-Lussac tells us that the pressure on the walls of the can is also proportional to temperature.

How much pressure? 
Let’s assume that the leftover Lynx was just about at atmospheric pressure (103 KPa). The fire was pretty hot; perhaps 900 °C. It was a warm summers day at perhaps 25 °C.
By substitution we can solve for P2. ((103 * 900)/25). So we can calculate that the walls of the cans were subject to 3.7 MPa! The thin walls of the aluminium can had no chance against almost 4-megapascals of pressure.
popcorn Pop-corn is another, far-safer example. The fleshy endosperm of the popcorn contains about 15% water. Placed in hot oil or butter, it softens and expands, eventually vaporising the water contained and as described by Messrs Boyle and Gay-Lussac, the expanding gas and steam puts huge pressure onto the walls of the kernel until it can no longer withstand the pressure and bursts into the tasty snack we know and love.
Both Charles law and Gay-Lussac are at play here.

The sad, left-over, un-popped corn kernels probably had a porous skin that allowed moisture to escape and the pressure was not able to build.

Hopefully you’re aware that Homershams supply quality pressure gauges. These pressure gauges contain a mechanism based on a Bourdon tube.
The sealed Bourdon tube is forced to obey the laws of nature and expands when gas pressure is applied. By attaching a pointer to the end of the tube we can create an accurate pressure measuring device, based again on the work of our old friend Boyle.

The demise of the Comet aircraft is another good practical reminder that we must not ignore the impact of the gas laws. Gay-Lussac is at play here.
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The de-Havilland Comet was a triumph in engineering. Jet speed transport and the comfort of a pressurised cabin, ushered in a new age of air travel. That is until they started falling from the sky. Its failures were a mystery until it was found that the continual pressurising and depressurising of the cabin cause a stress fracture in the planes body (centred on rivet holes).

One might perhaps forgive de-Havilland; this was the world’s first jet-liner and first pressurised cabin and engineers were dealing with forces they little understood and had no experience of. Once the 2 failure modes were understood (ie stress concentration around a crack or hole and continual forces of expansion and contraction due to the pressurised hull ie Gay-Lussac) the Comet was redesigned to deal or eliminate these issues. The new Comet 4 continued service until 1997.

The Internal Combustion Engine and the cartridge of a bullet are other demonstrations of the gas laws.
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A  bullet is propelled by the expanding gases from the burning powder/propellant, expanding rapidly to force the projectile out with enormous force and speed.

Thoughts on Calibration

As we’ve learned above, temperature and pressure are intractably linked because of this we control our laboratory temperature within ±1 °C. Testing pressure onsite outside of the laboratory requires added procedures to ensure a calibration is done correctly.

For example the extra-low-pressure pump that we use onsite has isothermal insulation to help us test Magnehelics accurately on site. The low pressure pump (left) is shown compared to a ‘standard’ model.  Note the large isothermal insulation around the low pressure bellow vessel.
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Incidentally if you’d like to learn more about our IANZ accreditation, have a look at Homershams scope of accreditation. 

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