I think PT100 RTD probe is a better choice versus a type K thermocouple. When reading a thermocouple, the controller also need to read a cold junction temperature. Depending on the cold junction temp. sensor's location, it can be slightly off from the true temperature which will introduce a small error to the probe reading. With PT100 RTD sensor, you don't have to worry about this issue.

Chert wrote: although maybe this image helps determine yes or no. I mean, is the angle in the image the correct direction? VFA is no longer around but maybe the part is still made.

Thanks for the link. That is helpful. Cafeparts has a useful cross referencing database which shows similar parts. There are a couple of new candidates in there. As Gary points out, your part seems to be available from a number of vendors so it looks likely that it is still in production. I would still prefer something that I don't have to disassemble and send out for chroming though...

redbone wrote:I think PT100 RTD probe is a better choice comparing to type K thermocouple. When reading a thermocouple, the controller also need to read a cold junction temperature. Depending on the cold junction temp. sensor's location, it can be slightly off from the true temperature which will introduce a small error to the probe reading. With PT100 RTD sensor, you don't have to worry about this issue.

Thank you for the suggestion. I will look into getting one for testing. I was aware that the reading from the thermocouple has to be compared with a reference to the cold junction but I haven't spent any time yet looking into how this actually done. My thinking was that this problem could likely be dealt with by some careful calibration. It doesn't really matter what temperature the thermocouple "thinks" the boiler is at so long as it thinks it consistently. If the controller is inside the case close to the boiler, the ambient temperature is likely to be inconveniently high, but fairly stable (once everything has warmed up). If, as I am beginning to think, it can live under the drip tray, there may not be an issue at all.

I am also thinking about putting in another sensor to take readings from the group or possibly from the brew reservoir as these are ultimately more significant than the boiler temperature itself.

Yesterday I actually did boil water.

I am missing a few pictures. The first thing to do was to take the 1/4-20 screw off the TC and to put it into an adaptor which threads into the top of the tank. The probe is now in direct contact with the airspace above the water. The lag between the installed TC and the faster-reading TC from the multimeter is now down to about 7 degrees. I set the temperature controller to auto-tune to a set point of 70 degrees C and did something else for an hour and a half. Then I moved the set point to 95C. I don't have the data with me but the controller overshoots by 5 or 10% or so during a cold start. After a short period (roughly 15 minutes total) the installed probe catches up with the reading from the multimeter. If the set point is then increased slowly, the two readings move pretty much in lock-step. This means that with a little tweaking of the PID coefficients (likely the D), the controller could probably be made to hit the set point with little or no overshoot. I think however, that it may just be easier to divide this into two problems. The first is heating up from a cold or warm start. The second is maintaining temperature at the set point. With a micro controller replacing the temperature controller it will be easy to set a safe initial set point, wait for the heat to disperse through the system so that readings become reliable, and then move the set point to its final value.

The good news is that the controller is already very good at keeping the temperature stable once it gets there.

The two pictures I did take yesterday :

I would consider using a thermowell to allow for easy replacement of the sensor.


Pressure gauge on La Pavoni Europiccola?

redbone wrote:I would consider using a thermowell to allow for easy replacement of the sensor.

The brass adaptor that I used was essentially that, but was creating too much lag in the readings because of the thermal mass. A thin-walled thermowell may not be as much of an issue and I ultimately need to find something to hold the probe anyway as the adaptor I made is not a permanent solution.

This is such a remarkable project. Having only rebuilt a machine for one project and made my own PID in another, I really appreciate how you are doing this from the ground up. I wish I had the skills and the tools necessary! It would be a really great project.

Ha - thank you - I am probably a little to close to digging my own copper mine on this project

The last piece????

Ok, well not technically the last piece, but the only one standing between me and an entirely functional machine.

The check valve cap for the group is fairly straight forward. I had ordered a couple of carbide 60 degree threading inserts and a holder a few months ago to eliminate the guess-work. I had initially gone "old-school" and used a jig to grind a thread cutter from tool steel. It works, but it isn't perfect and it can't thread as close to a shoulder as the insert. Having turned down some hex stock to the major diameter of the M18x1 thread and added a relief groove at the shoulder I set up the lathe to cut the thread in 29 passes and left it to its own devices.



The hex stock is a short-cut - ideally, I would machine a smaller concentric hexagon on the top as there isn't enough room to get a socket driver on when it is installed in the group. It will do for now however.



... and that is it. A complete working machine! I threw together a temporary drip tray from some aluminum sheet, metal duct tape and copious use of a hammer - file under ain't pretty, but works.



Now, while I wait for sheet metal done by my betters, it is time for the somewhat drier (literally, not metaphorically) work of testing, coding and electronics.

Congratulations! You've done it. I look forward to phase II.