Sleeping bag maths

[Dave Barter]

My Yeti bag is rated comfort +13degrees C
My PHD Minimus is rated comfort +5 degrees C

So if I put my Yeti in the Minimus what is the comfort now?

[Scattamah]

Can one of the bags be half/three-quarter/fully unzipped? If so, it's now a throw/quilt perhaps with a footbox and can be used accordingly. You will most likely get a better loft on the bag closest to you for your troubles.

Re: maths - not my forté.

Greetz

S.

[whitestone]

This is for quilts but the general principles hold https://support.enlightenedequipment.co ... t-Layering

I've a PHD bag rated to +5C and a Cumulus 150 quilt rated to +4C, together they are fine to -6C, well that's the coldest I've been in the combination.

[Dave Barter]

Can one of the bags be half/three-quarter/fully unzipped? If so, it's now a throw/quilt perhaps with a footbox and can be used accordingly. You will most likely get a better loft on the bag closest to you for your troubles.

Re: maths - not my forté.

Greetz

S.

Yes the Yeti has a zip. I've also got a Cocoon Mummy Liner as well so hoping that the combination of the three can cope with a few harsh nights.

[Bearlegged]

This is for quilts but the general principles hold https://support.enlightenedequipment.co ... t-Layering

I love the comments: Really helpful! Utterly wrong!

[Bearlegged]

I did a bit of thinking about this, and I'm not certain that the science holds, but how about...?

We have a two duvets, 4.5 and 9 TOG rated. These can be poppered together to give a duvet with a rating of 13.5.
If this is a valid way of working such things out, then if we can work out some way to convert comfort ratings to TOG ratings, one can add the values together, and thus extrapolate the combined comfort rating. With me so far?

OK, so a cursory google didn't throw up any simple conversion mechanism, but Alpkit list many of their bags with both TOG and EN13537 comfort ratings (I've suffixed them with C = comfort, L = limit, and E = extreme):
TOG...Leeds comfort...EN13537C...EN13537L...EN13537E
13.5...-21..................-10.5...........-18.1.........-39.1
11......-13..................-4.2.............-10.8.........-29.5
9.5.....-5.5.................-0.5.............-6.5...........-23
6........4.....................8.3..............3.8.............-10
5........7.....................10.8............6.8.............-6.6
4.8.....7.....................11...............7.4.............-5.8

Still with me? I hope someone is, because this is where I get stuck. I'm hoping that someone with better and fresher maths skills than I can pick this up and run with it. Work out a consistent conversion formula between degrees C and TOG, and also extrapolate these values to give us a wider range of values we can work within...

[whitestone]

The Wikipedia page on Tog has a chart showing the range of comfort rating temperatures for duvets with different Tog values. I think this is as close as you are going to get. The Tog rating is closely related to the R-value of thermal resistance used for sleeping mats, it's just ten times larger so an R-value of 3 = a Tog rating of 30.

There's this page on Alpkit's site https://www.alpkit.com/spotlight/choosi ... eeping-bag with this table

Season ....................... Night Time Temperature .... Thermal insulation
Summer ........................15ºC to 8ºC .........................3 to 5 TOG
Spring/Autumn ...............10ºC to 0ºC ......................... 5 to 8 TOG
Winter ......................... 3ºC to -10ºC ..................... 7 to 10 TOG
Mountain ........................ -5ºC to -20ºC ..................... 9 to 12 TOG
Polar/High Mountain -15ºC to -40ºC ................... 11 to 16+ TOG

There's isn't a conversion between EN13537 and Tog ratings because they are measuring different things. The Tog is measuring thermal resistance of some material between two points, whether that's inside/outside or under/on top of, whereas EN13537 is measuring how that material when used in a sleeping bag affects comfort and the ability to keep warm. You could have two bags using the same material and fill (so same Tog value) but they get different EN13537 derived values because they are constructed differently: rectangular vs mummy shaped; small vs large; etc.

[Zippy]

I would total up the weight of the fill (with weighted average for the fill type), then assume this to be one sleeping bag - and look up comfort ratings for bags of that equivalent spec. + add your pinch of salt and experience as your starter for 10.

[whitestone]

I would total up the weight of the fill (with weighted average for the fill type), then assume this to be one sleeping bag - and look up comfort ratings for bags of that equivalent spec. + add your pinch of salt and experience as your starter for 10.

Shouldn't be far off that Chris. You can assume that down fill powers within 50 of each other are the same, 600fp is roughly half as insulating as 900fp so you would have to adjust in that or similar cases.

[psling]

My Yeti bag is rated comfort +13degrees C
My PHD Minimus is rated comfort +5 degrees C

So if I put my Yeti in the Minimus what is the comfort now?

See, I wouldn't be bothered with all the maths, whether the Yeti would be compressed inside the Minimus, etc., etc., etc..

I'd get out in the garden in your bivi bag on a cold, dry, frosty night with a thermometer alongside and see how comfortable the combination is. If it gets too cold for comfort, pop back in the house, make a hot drink and browse on line for warmer bags/liners/overbags/quilts/battery powered heating elements

[Dave Barter]

I agree Pete but it's not been cold enough down here recently and I need to know before I head off to colder climes next week

[Bearbonesnorm]

As Zippy says, what's the combined amount of down?

[simon72]

I have tried just this type of experiment. I’ve got a PD600 and a Rab infinity down jacket. The Rab has as much down as my summer bag, it is a monster.

I would often get really cold legs in winter despite using the PD600 and Rab in combination, until... a very cold night outside the star inn when I decided to take the jacket off and shove it down and over my legs, using it as an over quilt. I’ve never looked back, it really works. And when I get up the “quilt” converts magically into a jacket!

[ianfitz]

I don’t know the answer but Phd make overbags and filler bags to be used in the way you intend.

https://www.phdesigns.co.uk/combi-and-f ... e1b3oidfa3

A bit of geeking at that page may well give some answers.

Or if you’d rather I have a cumulus 250 and/or a jacks r better winter quilt you’d be welcome to borrow for your trip.

Pm or email if you want to do that.

[Alpinum]

This is the basic maths behind quilts:

17.5-3.5×loftheight in cm

For sleepingbags loftheight in cm is divided by two. (As you lie on roughly half of the insulation)

If you have two bags use the wider/larger one on the outside. Then measure the (empty) height of both bags and calculate the estimate temp rating with above formula.

The rating you get works for sleeping without or hardly any garments obviously.

[Dave Barter]

I don’t know the answer but Phd make overbags and filler bags to be used in the way you intend.

https://www.phdesigns.co.uk/combi-and-f ... e1b3oidfa3

A bit of geeking at that page may well give some answers.

Or if you’d rather I have a cumulus 250 and/or a jacks r better winter quilt you’d be welcome to borrow for your trip.

Pm or email if you want to do that.

Very kind of you Ian. Based on all of this I reckon I am sorted with my Yeti as a quilt and the PHD bag with the added insurance of a down jacket. Main thing is that I won't die

[Bearbonesnorm]

Where are you going Dave?

[Dave Barter]

Scotland hopefully

[Asposium]

This thread got me pondering a new sleeping bag.

Any thoughts on this one?

http://www.mountain-equipment.co.uk/firelite
Does seem very light for its temperature rating.

[ScotRoutes]

That weight seems about right for that rating of bag. For comparison, my Phantom 32 is rated 2/-2/-17 and weighs 653g. That's what I use for 9 months of the year in Scotland.

[Bearbonesnorm]

Does seem very light for its temperature rating.

I can't see the overall bag weight of that, just the fill weight ... personally (and we know it's a personal thing) but I wouldn't be warm at -9 in a bag containing less than 500g of even high quality down such as this.

[Asposium]

Does seem very light for its temperature rating.

I can't see the overall bag weight of that, just the fill weight ... personally (and we know it's a personal thing) but I wouldn't be warm at -9 in a bag containing less than 500g of even high quality down such as this.

Overall weight is 770g
Fill weight is 469g

Quite impressive how products have moved on.

[whitestone]

Does seem very light for its temperature rating.

I can't see the overall bag weight of that, just the fill weight ... personally (and we know it's a personal thing) but I wouldn't be warm at -9 in a bag containing less than 500g of even high quality down such as this.

Overall weight is 770g.

469g of 90-10 Russian Goose Down with a minimum fill power of 800. 90/10 seems at the lower end of quality