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Deciphering Temperature Ratings
Arguably the most critical factor when choosing your sleeping bag is its warmth, and how much it will provide. Obviously you want to be comfortable when the temperature starts dropping, but you also don't want to overheat if conditions are mild. The key to this is matching the temperature rating of your sleeping bag with the temperature range that you're likely to encounter when you're asleep.
What Is The Temperature Rating?
The temperature rating is a measure of a sleeping bag's performance under certain temperatures. To determine the temperature rating, all Mountain Designs sleeping bags are tested using either EN ISO 23537-1 or EN 13537, the most widely recognised international standards for measuring the warmth of sleeping bags. The testing is conducted in a laboratory to standardised user body specifications, and outlines three different values - T Comfort, T Limit and T Extreme.
'T Comfort' value refers to the lowest temperature at which a standard user can expect to sleep comfortably in a relaxed body position, such as lying on their back (related to a standard woman and in standard conditions of use). This is a suitable guide for the coldest temperature a 'cold sleeper' would want to use the sleeping bag for.
'T Limit' value refers to the lowest temperature at which a standard user can expect to sleep in a curled body position and remain asleep (related to a standard man and in standard conditions of use - note that the temperature is lower because the metabolic rate used in the equation for a man is higher than that used in the equations based on a woman's physiology). This is a suitable guide for the coldest temperature a 'warm sleeper' would want to use the sleeping bag for.
'T Extreme' value refers to the lowest temperature at which a standard user can survive in when using the sleeping bag (related to a standard woman with a curled body position in a situation of high cold stress with shivering). This is a survival rating only, and at this extreme temperature the user can expect to feel very cold with a real risk of health damage by hypothermia. Use is not advised at this level.
Comparing Different Styles & Brands
Mountain Designs recommends using the temperature rating as a reference point for comparing other EN ISO 23537-1 tested bags and EN 13537 tested bags within the Mountain Designs range or across other brands. Keep in mind, however, it should only be considered a guide as everybody has different body types and coping mechanisms. As a general rule, you should look at the T Limit as a starting point for comparison.