Hi all, thanks for your comments. Here are some individual responses:
David: You're right, a better comparison would be with the WM MegaLite. I was thinking in terms of the lightest bags and overlooked the MegaLite as being a closer comparison. Sorry for that. Weight-wise, the MegaLite is the same as the Hydrogen.
Mark: Good observation on the curves crossing. I don't have an explanation and am still thinking about it. I ran the test twice and got similar results, so something is causing the difference in warm up. For my State of the Market report on UL 3-season down mummy bags, to be published this spring, I plan to run comparison tests using a regulated heating pad inside, and maybe that will work better. It will be a "poor man's mannequin test" to produce some Relative Warmth data, similar to Richard Nisely's tests.
Best regards and happy hiking,