This week's Monday Kit Tip is something of a shopping aid if you're looking for a sleeping bag – what do all those temperature ratings actually mean?
It used to be that sleeping bags came with season ratings – one-season, which was cold and for summer, two-season, which was slightly less cold, right up to five-season, which was presumably so warm that it needed its own season inventing.
These days though, sleeping bags come complete with not one, but three temperature ratings based on laboratory tests performed on metal dummies laden with sensors. And very confusing they are too.
So what are they? First, and the highest one is 'Comfort' – and it means pretty much what it says, this is the temperature a normal sort of person will find effortlessly comfortable in that bag. Think of it as a general sort of 'this is about what it's aimed at' rating.
Next is 'Limit of Comfort' – the best way to think of this is that it's the lowest temperature where an average sort of person is going to feel snug and happy in the bag. Beyond that, you enter a downward spiral where the bag becomes progressively less comfortable until you reach...
... the 'Extreme' rating. This is arguably the most confusing one of all. It's not intended to represent the lowest temperature the bag should be used at, in fact it's the lowest temperature the bag will likely keep you alive at. Not comfortable or uninjured or happy, just alive. It's an extreme survival figure. So don't go assuming that you'll be even close to comfortable at the Extreme rating temperature.
Of course it's not all that straightforward. Different people sleep at different temperatures and you can also improve performance by following our Max Your Bag tips by optimising your sleeping preparing and using your bag as efficiently as possible.
Interestingly, Mountain Equipment chooses to give a Sleepzone figure for each of their bags to minimise confusion – to put things in perspective, their lightweight Helium 400 has a Seepzone of +20 to -3˚C, while the lab ratings are Extreme, -16˚C; Comfort Limit, +1˚C; Comfort, +4˚C.
Overall, ratings are good way of getting some comparison between bags from different brands, but they are just a starting point albeit a useful one. Happy shopping...