It seems that sleeping bag ratings have no consistency. Temperature ratings are still determined entirely by the manufacturers of the bags. My 3-pound Sierra Designs bag, for example, was rated to 20 degrees. Honestly, it never kept me as warm as my 17-ounce Western Mountaineering sleeping bag, which is only rated down to 40 degrees. Isn't this a problem when you buy a bag? Maybe a 45-degree bag will keep you warmer than a 30-degree bag.
Consistent Sleeping Bag Ratings
No matter what temperature a bag is rated for, under any system of testing, it won't necessarily keep you warm to that temperature. We can't solve the problem of people having different metabolisms and bodies. A particular bag might be good for one person down to 20 degrees, while for another it is only good to 40 degrees. You generally can figure out if you are a cold or a warm sleeper, but that doesn't help if you don't know whether a bag is rated too high or too low.
You need to know that if a bag says 30 degrees it will keep you warmer than one that says 40 degrees. With that, even if you add or subtract 10 or 20 degrees for your personal tastes, you can still figure out which bag is the warmer one. How do we get this consistency?
Begin testing with any sleeping bag, by putting a bag of water in it that is human-sized, weighing perhaps 160 pounds. Have three standard sizes for small, regular and large sleeping bags. Always start with the water temperature at 100 degrees Fahrenheit, and measure how long before it drops to 90 degrees. External air temperature has to always be the same too, whther it is 60 degrees or 40.
The numbers are not crucial. What's important here is that once the standards are chosen, every bag is tested the same way, with the same conditions (even the temperature and material of the testing platform would have to be the same). This is what will give consistency to the sleeping bag ratings for warmth.
Now, if a bag rated to 40 degrees keeps the water above 90 for two hours, a bag rated for 30 would obviously have to keep it above 90 degrees for a longer time. Pegging heat-retention times to specific temperature ratings would be a bit tricky at first. However, once done, each new bag on the market could be submitted to the testing and quickly given a consistent rating. We would know that a lower rating would always mean a warmer bag, degree-by-degree. We could even have old bags tested to see if it is time to replace them.
Would manufacturers pay a private testing company to have their bags rated? Some, at first, because it would be a an advantage for those companies who are already conservative in their temperature ratings. They would have "proof" that the bags are even warmer than they were claiming. Then, eventually, all bag makers would feel some motivation to have their sleeping bags tested, because consumers would be wary about buying ones that weren't tested.
I hope someone will take this idea and run with it. An existing consumer rating company, like Consumer Reports, could do this on their own and report the results. Even if they listed the bags without temperature ratings, but in absolute order by which held the heat in the best, it would be very useful. One could look at the list and if their current bag kept them warm to 25 degrees, ythey would know that any bag higher on the list would be warmer. Isn't it time for consistent sleeping bag ratings?