Introduction: What is Primus Winter Gas?

Primus Winter Gas claims superior performance in cold weather over their Power Gas and Summer Gas blends (at least with upright stoves). Primus’ claim is thus:

(diagram by Primus)

Let’s start learning about this claim by watching the Primus’ promo video:

YouTube video

With this release Primus offers several “engineering” explanations for how it works:

The main innovation behind Primus Winter Gas™  cold weather performance is the Vapour Mesh™ a highly absorbent paper inside the cartridge that will keep the pressure higher making the stove run better. The VaporMesh increases the surface on which the liquid fuel can evaporate into gas state. The increased surface makes the change from liquid to gas faster and the speed of this process is what determines the pressure inside the cartridge. Compared to conventional cartridges this makes it work in conditions when it was previously hard to use gas fuels. [Italics added by Author]

(Primus illustration)

Surface Area: The emptier the cartridge, the larger the exposed surface of the Vapor Mesh is. That is why Winter Gas cartridges get better and better relative to conventional cartridges during the life-span of the canister.

After using a cartridge for 60 minutes of cooking, a Winter Gas cartridge is approximately 9% more powerful than one without VaporMesh. After 120 minutes of use it delivers about 15% more power.

Inter-molecular forces: As said before, it is not the gas blend that makes Winter Gas perform better in cold temperature.

Source: (retrieved December 14, 2015)

Primus’ use of the phrase “inter-molecular forces” is not explained, and seems to be invoked solely for the purpose of attractive “science-sounding” marketing copy. As you will learn, the reason for the higher performance of Winter Gas has little to do with “inter-molecular forces.”

The text on the EpiGas website is in Japanese, but using Google Translate should allow you to read it. The paper absorbs liquid fuel and the liquid fuel can migrate through the paper; this increases the evaporative surface area from the liquid to the vapor state inside the canister. But how much does this design influence cold weather effectiveness? Source: EpiGas (

The idea of using absorbent paper liners is not new. The screw-thread canister was invented decades ago by a UK firm called EpiGas (now owned by a Japanese company), and they have been selling the 8 oz (230 g) and 16 oz (450 g) canisters with absorbent paper liners for some time.

I wonder why EpiGas did not get the awards a few years ago that Primus recently received? (But then again, I wonder whether any of the judges were even aware that using these liners is not a new idea.)

For reference, the new Primus line-up of canisters is thus:

Summer Gas20% propane, 80% n-butaneGreen
Power Gas20% propane, 80% iso-butaneRed
Winter Gas20% propane, 80% iso-butaneBlack
Old Power Gas25% propane, 25% iso-butane, 50% n-butaneGrey

Let’s dive into a few issues as we address the validity of Primus’ claims, leading to the inevitable conclusion that these claims are based on a very shaky foundation.

We’ll end with an assessment of whether or not the marginally effective performance realized in the real world is worth the added cost premium for Primus Winter Gas.

Problem #1 – Primus’ Claim Violates a Law of Physics

In our forums, several science and engineering types (some of them professionals) have examined the claim that the increased surface area of the blotting paper will increase the internal pressure. The consensus: the claim seems to be false because the fundamental laws of physics that govern vapour pressure behavior are violated.

Vapour Pressure

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