Ok, so is it correct that you're getting the correct per-chip hashrate with my firmware?
I guess you misunderstood me. You're talking about building a precision thermal sensor whereas what I had in mind is to estimate junction temperature based on indirect off-junction temperature measurements. For instance, you can measure temperature drop at two points in a heat conductor. If thermal conductivity is known, this will give you an estimate on the amount of heat that passes through it. You can then extrapolate the temperature drop into the junction, if thermal conductivity between junction and your measurement points is known. Perhaps with additional measurements and calibration, you can do it pretty accurately, if you dare to perform everything that is needed. A MS degree in physics is recommended

But I don't think that it's really necessary. In my case, the chips work well (1.725GH/s per chip) and this can justify that they don't overheat. I do get the heatsink temperature reading that can be used to judge if the fan has failed or smth, so that the software could stop chip operation to prevent damage.
That's a sustained temperature reading. They work non-stop for several days in this mode.