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TIP: The online Bushwalk Inventory System can help bushwalkers with a variety of bushwalk planning tasks, including: Manage which items they take bushwalking so that they do not forget anything they might need, plan meals for their walks, and automatically compile food/fuel shopping lists (lists of consumables) required to make and cook the meals for each walk. It is particularly useful for planning for groups who share food or other items, but is also useful for individual walkers.Calculating double bag rating
Wed 22 Jul, 2015 6:23 pm
From a link to BPL
X -(70 - Y)/2 = Z
In Fahrenheit but it works in a Celsius conversion also
Where X is the Outer/warmer bag and Y in the Inner/cooler bag
I've just used it for my combination and I think it gives a very conservative rating so suitable for cold sleepers such as myself
I used it to work out the limit for my own combination of the half bag and parka inside my Ultra Flouro and got a rating of -20C for my legs and -30C for my torso
Which is what I have been assuming for ages as the top half but I was a little surprised at the leg rating as it "feels" a lot warmer than that
X -(70 - Y)/2 = Z
In Fahrenheit but it works in a Celsius conversion also
Where X is the Outer/warmer bag and Y in the Inner/cooler bag
I've just used it for my combination and I think it gives a very conservative rating so suitable for cold sleepers such as myself
I used it to work out the limit for my own combination of the half bag and parka inside my Ultra Flouro and got a rating of -20C for my legs and -30C for my torso
Which is what I have been assuming for ages as the top half but I was a little surprised at the leg rating as it "feels" a lot warmer than that
Re: Calculating double bag rating
Wed 22 Jul, 2015 7:17 pm
I suspect you get a better result than the formula suggests because you probably have a lot less warm air escaping from that inner bag when moving than if you were using the inner bag alone in its intended temperature range.
When I use my Ultralite at -5/7c something I am warm but I notice more of a temperature change when I roll inside that one (just with an underlayer) than when I do the same inside the Summerlite with puffy clothing.
possibly just less air space does the trick (the Ultralite and Summerlite have about the same inner dimensions)
BTW, I have suggested many times that theory and practice don't always correspond.
When I use my Ultralite at -5/7c something I am warm but I notice more of a temperature change when I roll inside that one (just with an underlayer) than when I do the same inside the Summerlite with puffy clothing.
possibly just less air space does the trick (the Ultralite and Summerlite have about the same inner dimensions)
BTW, I have suggested many times that theory and practice don't always correspond.
Re: Calculating double bag rating
Wed 22 Jul, 2015 9:47 pm
Just adding thickness of insulation would indicate a somewhat lower temperature rating as total loft exceeds 200mm in the legs and close to 270mm in the torso
Re: Calculating double bag rating
Sun 26 Jul, 2015 5:36 pm
PHD reckon their -6C rated Hispar Combi bag with their -9C Hispar 400 is good to -33C, and the Combi+Hispar 500 is rated to -39. This formula gives -21C and -24C. Pretty big difference, although PHD's ratings are suspect. Also not sure why the outer bag has to be the warmer one, or why it would make a big difference. If you do it the other way around you get -22.5 and -28.5 which is closer.
Specifically for me a -7 and -10 EE Revelation this would give a value of -23.5C (or -26C). Which seems a bit high given the amounts of down involved (about 975g of 800 in the Revelations) but in the right ball park (a WM Lynx has 965g of 850, but a lot of that is on the bottom).
I'd also take this formula with a huge grain of salt, but it provides another data point at least.
Specifically for me a -7 and -10 EE Revelation this would give a value of -23.5C (or -26C). Which seems a bit high given the amounts of down involved (about 975g of 800 in the Revelations) but in the right ball park (a WM Lynx has 965g of 850, but a lot of that is on the bottom).
I'd also take this formula with a huge grain of salt, but it provides another data point at least.
Last edited by Joomy on Sun 26 Jul, 2015 8:23 pm, edited 2 times in total.
Re: Calculating double bag rating
Sun 26 Jul, 2015 6:51 pm
Well PHD will not put any of their bags to the EU test so I have always assumed it's a "Best case" rating and not at all a comfort rating
It is a formula to give us a ball park figure tho.
It is a formula to give us a ball park figure tho.
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