Sea to Summit bags (like most quality sleeping bags) are tested in certified laboratories for temperature rating according to the European Norm EN 13537. Below, you will find an excerpt from an information sheet on the subject.
Before we get to the details of the testing, one continual source of confusion is the difference between the Lower limit and the Comfort Rating for any given sleeping bag.
The Lower limit of a sleeping bag should be used as a guide for male sleepers
The Comfort Rating of a sleeping bag should be used as a guide for female sleepers
In the case of the Spark Sp III (often referred to as the "Spark 18")
the Lower limit would be 18°F - which is what a male sleeper should use for orientation
the Comfort Rating would be 29°F - which is what a female sleeper should use for orientation
(So - a woman sleeper purchasing the Spark Sp III should really think of it as a "Spark 29")
Because of misperception, some women purchase the Spark over the Flame (or the Ascent over the Altitude) believing them to be warmer. This is not the case.
For the Spark Sp III, the women's-appropriate rating would be (as described above) 29°F
The Flame Fm III has a women's-appropriate rating of 25°F - so it is a warmer bag.
Here are details about the test protocol:
All Sea to Summit Unisex Sleeping Bags are listed using the LOWER limit, which is appropriate for male sleepers. Women end-users should use the COMFORT rating of a Unisex bag.
All Sea to Summit Women's Specific Sleeping Bags are listed using the COMFORT rating.
Please note: Both ratings would require the sleeper to be dressed in Expedition Weight base layers and have adequate insulation underneath the sleeping bag in the form of a sleeping pad or sleeping mat. The EN test protocol uses a sleeping pad with an R-Value of at least 4.
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