I have since made restrictor discs which fit in the outlet spout to vary the flow rates. (James- if you did not get these from Paul let me know and I will send you some). There is also another way to vary flow which I have yet to test fully.

The machine was calibrated using a mercury thermometer with a small range scale from 70-100.5C with a readable precision of 0.1C. This was located in a restricted Tee-Piece on the outlet spout so it was submerged. I found a temp drop of approx 0.2C between the tank and outlet which is taken into account, the readout temperature is not measured in the tank. The temp was tested with a newly calibrated digital thermometer in the actual flow at the top too.

The latest prototypes used in Atlanta have an external dial underneath to adjust the temperature readout offset which can be done live while powered up and flowing. i.e. if the actual temperature running through the pipes is 96C you could adjust the readout to say 94C if you so wished. Or readjust if it was out of calibration.

When water exits a sprayhead it will lose heat quite quickly, especially if measuring in a cup or french press which will draw out the heat. During SCAE gold cup testing I had up to 4C difference in my readings compared to other people on the same machine. Trying to measure fine sprayhead flows is very difficult, it should be deep in the flow and submerged and allow time to heat up.

I have always questioned what people really mean if they recommend brewing at a specific temperature. There will be heat loss all the way, the vertical drop from the outlet to the bottom of the french press, the french press itself drawing heat, even the grinds will absorb a little heat!, and then over the brewing time it is constantly losing heat. So if somebody recommends brewing with 96C water the coffee could really have been steeping in 93C water for most of the time. When testing water boiler output temperatures into cups I can get differences of ~2C from what other people would measure, but it is consistently different. The key is consistency & repeatability as mentioned in the video, if a particular coffee tastes best brewed into a cold french press “when the display reads 94C” then that is repeatable.

So, how much difference did the heated french press seem to make to the cup? Also if you lose temperature in a nonheated french press, what happens if you try to compensate with an initial higher temperature?