Choosing a thermocouple for a La Peppina

#1: Post by **ddr** » March 27th, 2009, 2:40 pm

Hi Folks,
I decided to play around with temperature control on my "new" La Peppina. I bought a Fujitsu PXR3 and SSR, and the dealer threw in a "K type thermocouple with a short lead wire". I was happy to have any TC, especially for free, so I can play around with this new gizmo while I figure out (ha, figure out; I have not done much figuring out of things for a while, it is easier to ask other people!) what TC to buy for the La Peppina's open boiler design.

My first question is: Should I get a submersible TC and drop it into the open boiler, or should I attach one somewhere else? I do not feel comfortable tapping a hole for a screw in probe.

My second question is: Once I know submersible or not, how do I decide what type (K, J, RTD, etc.) to use?

I have never used a PID before, although I think I understand the math behind it.

As always, thanks for your help,
Dan

#2: Post by **cafeIKE** » March 27th, 2009, 3:18 pm

T thermocouples are good for the espresso temperature range. The thinner the TC, the quicker the response.

A TC outside of the boiler has more hysteresis than one in the tank. How finely you want to control the boiler temperature dictates where you place the sensor. Any outside mounting point must have good thermal tracking of the boiler. If you mount to the boiler, ensure that the contact is robust and not likely to come loose.

#3: Post by **Bluecold** » March 27th, 2009, 3:52 pm

I've got a PT100 RTD because wikipedia said it was better. And it gives (gave

) me .1C readout on my auberins controller which TC's didn't.
Just drop it in the tank. Don't drill a hole, you'll probably crack the enamel.

#4: Post by **JB130** » March 27th, 2009, 4:33 pm

ddr wrote:Should I get a submersible TC and drop it into the open boiler, or should I attach one somewhere else? I do not feel comfortable tapping a hole for a screw in probe.

Definitely put the TC in the water, seems to me the only reason you would do otherwise is if the boiler were closed and you had no access.

#5: Post by **cafeIKE** » March 27th, 2009, 6:22 pm

Bluecold wrote:Just drop it in the tank.

Um, the probe should be in a fixed location. Water temperature varies throughout the boiler and if the probe is not always in the same place, tuning / performance could be a tad problematic.

An RTD sensor is faster and more accurate than a thermocouple, but can be less robust and often much more expensive. The PID only samples every so often. 1ms response when the PID isn't going to check for 100's of ms and it's driving a heater with an even slower response is a waste. For espresso, a T-TC is plenty fast.

#6: Post by **Bluecold** » March 27th, 2009, 7:16 pm

It's not like you'll be rocking the Peppina back and forth. The sensor will stay put if you just hang it in. If it doesn't, tape the wire to the outside of the kettle.

#7: Post by **cafeIKE** » March 27th, 2009, 7:28 pm

Ever drop a TC is a boiling pot? It tends to dance about...

Fast, Good or Cheap. Pick two

#8: Post by **another_jim** » March 27th, 2009, 7:35 pm

Keep the sensor submerged, right at the screen at the bottom. For a permanent mount, attach it to the screen. This is close to the heater and the lever intake. Close to the heater means it reduces the lag time from a change in heat to the sensor response. The shorter the lag, the better the control. Close to the lever intake means you are closer to the actual shot temperature. So in this case, the best position is a no brainer. Usually, the position for best control is not the same as the position for best process measurement, and you have to compromise.

#9: Post by **GVDub** » March 27th, 2009, 7:37 pm

If you're looking to control the average temperature of the kettle, wouldn't a little shifting of the probe be better than a situation in which you have precise control over one very small cell of water?

March 27th, 2009, 7:42 pm

Edit: Sorry, I forgot to say that the PXR3 I bought does not take an RTD, so that makes things simpler. Now I need to find a submersible T Type. More searching to go.

Thanks folks, I will have to take a closer look when I have La Peppina in hand. Since the boiler is up top and exposed I cannot insulate it, but perhaps there is a spot underneath that will do. I like the idea of mounting underneath as then there is no wire sticking out the top, but I suppose if I did have a wire I could run it down the back and not see it anyway. The nice thing for me is that I have no rush, this is a fun side project.

I like the idea of attaching it to the screen at the bottom, that seems like a good spot if I do use a submerged TC.

Regarding shifting to get an average temp, I would think at a small volume (half a liter or so) the water temperature should be pretty uniform as long as I am reading an inch or so away from the heat source. I am not sure if it really is necessary to have an average temp reading, or just a consistent reading as long as I know the difference between the temp in the PF and the temp displayed.

In the meantime I am reading through thermocouple and PID manuals for fun

Take Care,
Dan