Finding temperature offset from boiler to grouphead

#1: Post by **crwper** » August 12th, 2021, 7:27 pm

I recently installed a PID controller on my Gaggia Classic Pro, and wanted to determine the temperature offset from the boiler (where the thermocouple is installed) to the group head. I found this post in the forums where a simple technique is described:

Getting accurate shot temperature displays on PIDed double boilers without a thermometer

I tried it out, but it wasn't obvious exactly where boiling started, so I compiled a short video with two-second clips of each test as I slowly increased the PID temperature (current temperature shown at the bottom of the video).

I think this makes it pretty clear that flash boiling occurs around 104-106 °C as measured by the PID controller, but I'd love to hear your comments. I've given some additional details of my tests in the video description.

#2: Post by **pizzaman383** » August 13th, 2021, 6:10 pm

On the Gaggia the temperature on the outside of the boiler is not directly representative of the water temperature. The typical PID setpoint range for that machine is 210-215 degrees F. Adjust that to taste of the shots then pick an offset that feels comfortable. Or get/borrow a scace and measure

#3: Post by **Jeff** » August 13th, 2021, 8:13 pm

Repeatability is, for me, more important than the exact number. I want to go up or down 2°C from where I am, and be able to get back. I don't care if my machine's "number" is 88 or 94 or ..., as long as I know my "normal" is 88 (for my machine).

August 14th, 2021, 12:03 am

pizzaman383 wrote:On the Gaggia the temperature on the outside of the boiler is not directly representative of the water temperature. The typical PID setpoint range for that machine is 210-215 degrees F. Adjust that to taste of the shots then pick an offset that feels comfortable. Or get/borrow a scace and measure

Yep--the point of this experiment was to try to determine the temperature difference you've mentioned. 210-215 °F is about 99-102 °C. Looking at the video, when I have my PID controller set to 102 °C, the water coming out of the group head is well below boiling. Boiling point of water here is about 94 °C, so I would say with the PID set to 102 °C I'm getting about 92 °C at the group head.

This seems on the low side for espresso, but I'm curious to hear more about how you would dial in water temperature. What tastes are you looking for to tell you which way the temperature should go?

August 14th, 2021, 12:06 am

Jeff wrote:Repeatability is, for me, more important than the exact number. I want to go up or down 2°C from where I am, and be able to get back. I don't care if my machine's "number" is 88 or 94 or ..., as long as I know my "normal" is 88 (for my machine).

I agree that repeatability is more important than the actual number. I'm curious what taste cues you use to tell you if you need to go up/down in temperature? How do you decide whether to change the temperature or, e.g., grind, dose, or shot time?

#6: Post by **Jeff** » August 14th, 2021, 12:15 am

That's a complete topic in and of itself!

Jim's Espresso 101: How to Adjust Dose and Grind Setting by Taste is a great start.

From experience, I'll start a medium-dark roasted coffee 2-
4° C cooler than I would a medium-light. I'll dial in grind roughly first, then ratio, then back to grind (tiny tweak at this point). If I'm still not where I need to be, I'll typically bump temperature. Generally higher is more extraction and lower is less extraction. With typical "espresso" roasts, there's a balancing game where more extraction is good up to the point where you're getting unpleasant bitterness. With lighter roasts, the challenges are different.

August 14th, 2021, 9:32 am

Thanks, Jeff! I've spend a bit of time reading Jim's guide, and others like it, over the past couple of weeks. However, they usually seem to treat temperature as a constant, and describe adjustments to grind, dose, and shot time (or weight). Reading about your process, it seems like you treat all of these variables similarly when it comes to taste, i.e., they all adjust extraction level, so you're always going to be pushing along the spectrum from sour/acidic, through sweet, to bitter. It sounds like you get the other three parameters pretty much dialed in, then use a couple of degrees of temperature as a fine adjustment. Does that sound right?

#8: Post by **Jeff** » August 14th, 2021, 9:45 am

Ah, I wasn't clear!

Dose I generally don't mess with because I messed with it enough using my range of coffees and beans to know that 17 g is going to get me into the range I enjoy. Same for temperature, "88-90" on my machine's control is going to be close. You have to start somewhere, and that, for me, is a good point. Again, unlikely to be way off.

Primary is grind.

I can usually get that tasting good in two or three shots.

Ratio is another shot or two. It might be shorter for an "espresso" roast that has too much bitterness (and going coarser isn't better overall) or longer for light roasts that aren't sweet or are too sour (that going finer didn't result in a better balance). (Same grind, just more or less water. Weight in the cup is the controlled variable, time is an output.)

Everything after that is really at the "tweak" level. With around 14 shots in a 250 g bag, that's 10 little tweaks I can explore to see if there's an even better setting somewhere.

Edit: At least in my experience, 1° C is about the minimum, repeatably noticeable change. A "bump" to me is typically 2° C.

#9: Post by **pizzaman383** » August 14th, 2021, 9:53 am

To find the temperature setting range for your machine and palate adjust your dose and grind to get a reasonable flow rate. Then pull a number of shots using the same coffee and basket with a different PID temperature setting from say 205 to 220 in increments of a few degrees. The low temperature shots should taste noticeably sour - keep increasing until the sour is less noticeable and that is the bottom of the range. The hotter shots should start to taste noticeably bitter and that temperature is the top of your range.

August 14th, 2021, 3:05 pm

Jeff wrote:Dose I generally don't mess with because I messed with it enough using my range of coffees and beans to know that 17 g is going to get me into the range I enjoy. Same for temperature, "88-90" on my machine's control is going to be close. You have to start somewhere, and that, for me, is a good point. Again, unlikely to be way off.

Fantastic. That lines up pretty well with other advice I've seen, then.

pizzaman383 wrote:To find the temperature setting range for your machine and palate adjust your dose and grind to get a reasonable flow rate. Then pull a number of shots using the same coffee and basket with a different PID temperature setting from say 205 to 220 in increments of a few degrees. The low temperature shots should taste noticeably sour - keep increasing until the sour is less noticeable and that is the bottom of the range. The hotter shots should start to taste noticeably bitter and that temperature is the top of your range.

I will definitely give this a shot (or a few shots). Thanks!