Perhaps I'm reading this wrong but your original question appeared to be about offset. The answer to that is, yes there is an offset of several degrees. I don't know what is normal in your machine but in some it can be as much as 20f. If the pid is showing boiler temperature of 215 but water from the brewhead measures 202, don't worry about it! Offset is programmable in most pids but first you have to find the true temperature of the water hitting the puck. You then subtract that from the displayed temperature on the pid and set the offset to this amount. The pid is then displaying the correct brew temperature.

Well I suppose my original rambling idea was how much difference does 10 degrees F make to the taste? Because my quoted readings appear to be relative temperatures, (it would seem that either the thermocouple inside the boiler or the one in the portafilter is not calibrated) and given the described coffee tastes; smooth but weak at the +10 degree temp, and stronger (more defined) but sourer at the lower temperature, is it possible to surmise which temperature is likely to be closer to actual temperature?

Put another way; if the lower temperature was correct and I was actually brewing at close to 202F, if I then brewed at 212 would I expect to get a really bitter coffee?

I suppose what it seems like is that if the higher brew temperature tastes better; I should go with that.

Well I suppose my original rambling idea was how much difference does 10 degrees F make to the taste?

A huge difference. Depending on the coffee, one degree will make a big difference.

Stick all the gear in cabinet somewhere. Pull a shot, sour, up the temp 2 degrees and repeat until it goes bitter (or lower if you start bitter until it goes sour) then reduce 1 degree at a time till it hits the sweet spot. That is, provided the beans are good, grinder adequate, technique good, dose proper, etc... Do not worry about what the gauge or PID reads, worry about what you taste and adjust accordingly. Then you can start playing with dose and extraction rates/volumes to fine tune that particular coffee.

Hi Dave, that's exactly why I started this thread; that is what I thought I had done when I thought that my portafilter temp was similar to what the boiler PID was telling me.

I started at 200, and worked my way up to 206, and I found a sweet spot at 205, so I stuck with that. Then I built a Scace-type device and found that my portafilter temp seems to be way lower than the boiler PID, so I hiked the temp up by 10 degrees, and found a taste that was smoother but weaker.

From this very rough testing it seems that there are peaks and troughs along the temperature taste curve, not just a regular bell curve.

To pose the original question in another way; if you kept on rasing the brew temperature (above the so-called "ideal" temperature for the beans) for succesive shots, would they keep on getting bitterer and bitterer, or would you reach a plateau? Might the taste then start getting less bitter?