Goal for part 2 of the Cremina temperature study (Part 1 here
) is to find out how boiler temperature affects shot temperature while keeping group-head temps constant and how group head temperature affects shots temperature while pulling shots at same boiler pressure.
Older Cremina's don't have boiler pressure gauge installed so it is hard to know what the boiler pressure is. Most of the machines cycle within about 0.2 bars so the temperature spread is about 3°C or ~5°F.
Now since I use analog gauge for the boiler pressure I really can't know for sure the exact water temperature in the boiler but I think that I am in ballpark.
To measure group-head temperature I attached K-Type thermocouple to group-head and connected that to Fluke 54-II. That looks like this:
For this first set of tests I have pulled shots while starting the pull when group head was at 180°F and at set boiler pressure points. For each shot I just cooled down the group-head by locking cold portafilter in until the group temperature dropped to desired temp. I find this consistent and repeatable way to cool down group-head.
I pulled shots when boiler was at 0.7, 0.8 and 0.9 bars. Here is round 1:
This round show that, as one might expect, higher boiler pressure results in higher shot temperature. This is really good since it means that we can rely on boiler pressure to influence the shot temperature as long as we know what group head temperature is and what boiler pressure is.
I repeated exactly the same procedure second time in round #2 and got these readings:
There is indeed variation but to me they are remarkably similar which means I was able to replicate with some variation the shot temperature profiles by simply starting shot at same group head temperatures and same boiler pressure.
Also what is interesting and encouraging about this is that if your machine cycles within 0.1 bars you can pull shot at any time while knowing group-head temperature and be within good shot temp range. And if you pay attention to time when your heater kicks in and out you can even tune shot temperature without having the boiler pressure gauge.
For the round 3 of this testing I decided to turn things around. I wanted to pull shots at same boiler pressure but I wanted to have different group-head starting temperatures. Goal of this would be to see how group head temperature affects shots temperature while trying to keep boiler temperature about same.
So here is what I've got, I did only one set of data since I was tired:
As expected cooler group-head results it colder shot while boiler pressure is about the same. SUMMARY
It is clear that we can control the shot temperature on Cremina fairly easily as long as we know the group head temperature. Knowing boiler pressure is very nice bonus but I do not think that is required to be able to get into the desired range: Cold, Medium and Hot. If your boiler cycles say between 0.7-0.8 bars, which I think is good range then:
For cold shot you get the group head down to about 165°F and there you go.
For medium shot you get group head to about 170°F and you'll be in medium range.
For hot shot you get group head to about +175°F and that is it.
I am still awaiting the temperature strip with different range to put on machine and to use instead of Fluke. EricS (thank you so much) also sent me his small digital thermometer which is cheap and can be non-permanently (reversible, you can take them off) attached to group head to get digital readout on temperature so I will try that as well and report how it works out. I think that digital read-out is better since there is no temperature range as it is on temp strip which means that these can be used on other home levers like Ponte Vecchio, Elektra, Pavoni etc.
Also please keep in mind that I really think that being exact here is not the goal at all. People (and in my experience as well) have been pulling killer shots on Cremina without all this information for long time. The goal is to gain consistency and even better shots
So please don't get hang up on exactness and trying to get exact shot brewing temperatures. That would be crazy
, if I say so myself. Rather aim for cold, medium and hot and more likely just single range
that you find the best for your taste...
Hope this helps.