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  • Last Updated: 08 December 2021

The Great 9kg vs 8.5kg Gas Bottle Puzzlement

Many people wonder why some BBQ gas bottles are labelled as “9kg” while others are labelled as “8.5kg” when they look the same?
The fact is that they are one in the same.
Then, the obvious question is “Why are they labelled differently?”

9kg Gas Bottle

A 9kg gas bottle is considered the standard size BBQ gas bottle and is roughly equivalent to a 20 lb propane tank.  9kg represents the nominal weight of the gas contents. A 9kg gas bottle weighs approximately 17.5 kg when full, so it is very portable.

Variation is the Norm – No Standard Sizing

The filling of nominal “9kg” bottles with 8.5kg of gas was a safety initiative of the Australian LPG industry.

The same is true of “4kg” bottles being filled with 3.7kg of gas.

But why?

Unfortunately, there are many LPG bottles that are labelled as 9kg or 4kg that are actually manufactured undersize, with less than the labelled capacity.

This is a result of the manufacturing and design variances amongst the many manufacturers of gas bottles that are imported, into Australia, from many countries around the world.

Surprisingly, Australia has no mandatory standard sizing, so they are all approved for sale.

20% Oversized for Expansion

LPG bottles are designed to be full while allowing 20% of the space, called “ullage”, for the natural expansion of the LPG.

Filling undersized bottles to a full 9kg or 4kg, thus leaving less than 20% ullage, would create a safety issue.

Overfilling can result in gas venting from the pressure relief valves.

This is especially true in the warm Australian climate.

Between a Rock & a Hard Place

However, filling undersized gas bottles correctly and safely meant that the Australian suppliers were misrepresenting the contents as 9kg or 4kg when it was actually less.

This put the industry in the untenable position of being either unsafe or misrepresenting the quantity of gas provided.

Neither of these options was acceptable.

The Solution

The solution was to take into account the cylinder size variations and standardise on 8.5kg and 3.7kg refills.

This allows for the size variances between bottles and prevents any unsafe overfilling.

To make sure that consumers are fully informed, the bottles are labelled accordingly.

What About Refilling Instead of Swap?

Unlike SWAP’n’GO® bottles, which are filled by weight on digital scales, refilling is done by decanting.

The operator fills the bottle until some starts leaking out of the bleed screw.

Refilling can result in either overfilling or under filling, as well as correct filling, depending on the gas cylinder and the operator filling it.

Dip Tube Variations

In addition to the cylinder size variances, there can be variances in the dip tube lengths.

Dip tubes can even be missing or have fallen out of the bottom of the valve.

The accompanying picture shows some examples.

Dip tubes lengths are important because they are connected to the bleed screws.

The refill operator opens the bleed screw and fills the cylinder until the level of LPG reaches the bottom of the dip tube, at which point liquid LPG will start to bleed out.

This is his signal that the cylinder is full.

If the operator is slow to stop the flow, the result can be dangerous overfilling.

Long dip tubes can result in under filling.

Short or missing dip tubes will cause overfilling.

A missing dip tube is the worst case scenario, as the entire ullage space would be filled, allowing no room for expansion.

This is one of the reasons that swap is a much safer alternative, as dip tube variances have no effect when filling by weight.

10 Reasons Why Swap is Better than Refilling

Why should you use swap instead of refilling?
The simple answer is that you are getting much more than just the gas…

Here are some examples of the extra services* provided to swap customers:

1. The gas bottles are collected from your local dealer and transported back to the processing plant.

The gas bottles are then washed to remove any dirt or BBQ grease.

2. The gas bottles are refurbished to ‘near new’ condition.

With the largest supplier, over 50% of the gas bottles are repainted before refilling.

Gas bottles with heavy rust are shot blasted down to the bare metal and then primed and repainted.

3. Most accept out-of-date gas bottles at no extra charge.

They check the inspection date on every gas bottle before filling.

They only refill those with valid test dates.

4. If a gas bottle has reached its expiry, they perform all of the required inspections, refurbish it, put in brand new valve and then apply a new date stamp.

For more information on gas bottle testing, please see Gas Bottle-Cylinder Testing Facts.

5. They recycle thousands of gas bottles every year that do not meet their quality and safety standards.

They also make them ‘safe’ for recycling by removing any traces of gas so that our recycling friends can stay safe, too.

Please also see:

Safe Disposal & Recycling of Gas Bottles

6. They uses digital scales to ensure proper and safe filling based on weight.

They set the scales for each individual gas bottle, based on its empty (tare) weight.

This avoids dangerous overfilling or lost value through under filling.

With swap, you know you are always getting what you pay for.

7. They typically perform two leak checks on each gas bottle.

The largest supplier has a safety record that is second to none.

Nothing is more important to them than the safety of their customers, employees and the community.

8. They cover the gas bottles with protective netting to help prevent scrapes and scratches during transport.

The largest supplier applies a yellow quality assurance seal around the gas bottle collar so you know that the gas bottle was filled and safety checked.

9. The gas bottles are transported to and from their refurbishment and filling plants so that you get a ‘near new’, fully inspected, accurately filled and leak checked gas bottle ready to go, with no waiting.

10. Swap is available Australia wide, from about 5,000 locations, so you should never have far to drive.



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The information in this article is derived from various sources and is believed to be correct at the time of publication. However, the information may not be error free and may not be applicable in all circumstances.