old moonstone down liner

[cupcake]

i have an old synthetic moonstone sleeping bag i bought about 10 yrs. ago. i don't remember its temp rating, but i did buy a down liner to extend its range. the liner fits inside the bag on the topside only --- it buttons into loops in the bag --- i haven't seen liners like this in years -- it seems that now the sleeping bag makers think about the sleep systems in terms of stuffing a summer wt. bag into another bag to extend range.
does anyone know how i can find out how many extra degrees of warmth this liner may add to a bag? it's mummy cut, and though i don't use the old moonstone bag anymore, i think the liner would be a great addition to my 32* bag and 0* bag -- all i'd have to do is sew in some loops.
thanks,
d

[peter_pan]

Denise,

Figure the value of down at number of inches thicknessx40 subtracted from 100. Example 1.5 x40=60 from 100 is 40 degree bag/quilt. then ask some common sense questions. Does it wrap the sides as well? If not deduct some comfort. Also key is what pad or hammock you are on/in to figure the bottom issues. and whether or not you are in a tent or cowboy/girl camping.

[Youngblood]

Denise,

Figure the value of down at number of inches thicknessx40 subtracted from 100. Example 1.5 x40=60 from 100 is 40 degree bag/quilt. then ask some common sense questions. Does it wrap the sides as well? If not deduct some comfort. Also key is what pad or hammock you are on/in to figure the bottom issues. and whether or not you are in a tent or cowboy/girl camping.

Peter Pan, I'm not trying to pick on you, I have seen that equation in Jardine's book and other references (that I can't recall) as well. I don't think that equation is valid, basically because it is referenced to 100 degrees and I don't think 2.5 inches of insulation will take you to zero degrees. (I have also seen Jordan's approximation and wasn't sold on it.) I think a guideline referenced to something around 70 degrees would be more valid. Something like the thickness x 20 subtracted from 70 seems more like it to me. Do you think you get 40 degrees of insulation per inch of down... or more like 20 degrees per inch?

Youngblood

[peter_pan]

Youngblood,

You may have a good point. I think the formula works well in the 20 to 70 degree range. I have spent several nights comfortably in the mid 20s in a 2.5 " quilt. The formula appears to be supported by many manufactures that claim 40 degree range on 1.5 inch thickness...they express it as 3" loft. You frequently see 4 " loft expressed as 32 Degree or 35 Degree. Many 5 " lofts are cited as 20 Degree bags. Some 6 and 7 " lofts normally are advertized as zero degree. These last two points suggest the formula may not be accurate beyound the range of 20-30 degrees.

I have used 1.5 " quilts both below and above hammocks consistently into the 30s suggesting the estimate of 40 degrees is ok for 1.5".

As alluded to in closing my last post, this is a subjective area. Factors such as clothes worn, hats used, wind exposure, use of tents, metabolic rate of each individual, what is eaten before tuning in, and even % of humidity all play a role in comfort.

That being said, I agree, use of Jardine's formula may be flawed beyond 20-30 degrees ( would add this covers most 3 season application though).
Your proposal to use," inches Times 20 subtracted from 70" has merit, especially in the colder range.

One issue though, at 70 degrees you would be a little cool with nothing.

All of this makes me believe that the issue is not linear but curvilinear and further subject to a number of variables. This sounds more like an optimization problem for an Operations Research Systems Analyst with a computer and access to a super test lab. Federal funding?????