Olympia Cremina is legendary machine, but it does take some time to learn how to get most out of it. One of the biggest challenges is temperature management. Since Cremina's group head is not that large it does not have mass to effectively dissipate heat very quickly. Also you cannot realistically expect to have exact or precise temperature management with machine like this. You can target normal and high temperature range but that is about it. Even then though you can make amazing espresso with Cremina so exact temperature is really not that important...
My goal was to find out how machine behaves temperature wise under normal usage conditions and to learn what can be done to control it and cool down machine effectively since it tends to overheat as you pull shots.
To do this I used Fluke 54-II and K-Type thermocouple calibrated using boiling water and adjusted for my altitude. I also used double Elektra basket with hole drilled in the middle for thermocouple. That looks like this:
All temperature tests were prepared using 15.5 grams of coffee ground into the basket and pressed with my fingers to tamp them down. The thermocouple head was visible on top:
Thermocouple height is such that it does not touch the group head screen.
For each measurement I have ground fresh round of coffee to pull. Interestingly enough I was able to pull shot of espresso with thermocouple installed like this and have normal pressure on the lever through the pull.
I think this is one of the best ways (not easiest though) to measure the temperature through the shot since it is closest approximation of conditions coffee puck experiences temperature wise.
My machine is cycling somewhere between 0.75 - 0.9 bars of pressure. I found that lowering pressure does not provide dramatic difference since group head heat sinking capability has much larger influence on shot temperature than small differences in boiler pressure. I did play with it though.
My first round of tests was to see how machine behaves when its up to pressure from totally cold state. I knew that machine is too cold to pull shots at that point, so I did 2 second warming flush by raising the lever for 2 seconds to let water out and that way heat the group head. I suspected that shot would be cold and it was.
My second test of that round was to heat up group until OE Thermal Strip
installed on the group head lights up first bar marked 90C in blue. I wanted to see what temperature of the shot will be. And here are the graphs:
As you can see 2 second flush on cold machine is not enough since shot temperature ended up at about 185°F which is just too low.
Second shot with the group head heated up and first bar on OE temp strip turning blue resulted in shot that is too hot, scorching 213°F.
Interesting thing from these graphs is that temperature through the shot is fairly stable on Cremina. For some reason I expected more variation.
Lessons learned, 2 second flush for cold machine is not enough. Also when OE Thermal Strip lights up first bar in blue (or at all for that matter) the shot will brew way too hot.
Next without turning off machine I waited for it to cool down to the point where first bar on OE temp strip was completely out and then I pulled the shot. Here is what I got:
Temperature was still too hot since group head did not cool down sufficently.
My next try was to let group cool down again but this time get it to cool down even more by locking in cold portafilter and waiting until group head just above the bottom bell shape was at 165°F temperature. I measured this by pressing thermocouple to group head. I then pulled the shot and this is what I got, much better, brew temperature was at 202°F:Round 2
For second round I turned off the machine and let it cool down completely. I wanted to find out how much flushing I need from cold start so I get normal around 198°F (+/- ~3°F) shot temperature. I also wanted to see what is temperature of the next shot I pull 5 minutes later without any cooling down procedure implemented. Same for 3rd shot pulled 8 minutes later.
I decided to do about 5 second half-flush. What is half-flush? Well, if you raise lever all the way up water rushes out violently and in 5 seconds you'd flush a lot of water. To mitigate water loss then, I raise lever so I get about half the flow of the fully raised lever. Its not trickle but its not fully open flush either.
I pulled 3 shots and here is what I got:
First shot was still bit cool, but acceptable at about 190°F. Cool but OK.
Second shot I pulled without any flushing or cooling down the machine was 5 minutes later and it was at about 201°F. Not bad at all.
Third shot was pulled also without any flush or cool down and was 8 minutes later. Temperature was about 203°F. Little hotter but not bad at all. Since I knew for third shot that group head will be warmer I pulled it at bottom of heat-cycle which is 0.7 bars on my machine.
After this I pulled and measured 4th shot knowing that group head is hot. I turned of machine and waited for 0.6 bars to pull shot. That shot was hot at 205.5°F.Round 3
For this round I also turned off machine and let it cool down for couple of hours. Then brought it back to pressure. Goal of this round was to get better temperature on first shot and see how subsequent shots behave. I also wanted to try some cooling down techniques and see what their effects are.
I decided I will half-flush this time about 7-8 seconds.
Here is what I've got for first 3 shots:
This time first shot from cool machine was at about 199°F. Right on target. I was worried how the second shot will come out and as feared it was hotter than second shot in 2nd Round. This run at about 205.5°F.
Seeing how group head is already hot I decided to lock in cold portafilter to cool it down. Procedure is basically that you get cold portafilter and lock it in completely. You wait until portafilter is heated up, i.e. it sunk heat from group then you take it out. With this done, 6 minutes later I pulled 3rd shot.
Cold portafilter did trick and I was at 206°F for 3rd shot. Hot but not scorching.
As 4th shot then I tried different cooling technique. I got wide cappuccino cup, filled it with cold water, immersed group head bottom in it and made a half pull with lever to suck water from the cup partially into the group. Hold it like that for couple of seconds.
I thought this should be effective cool-down technique but it was not. Shots I pulled 7 minutes after 3rd shot and with this cool down technique resulted at 208.5°F pull.
It seems that cold portafilter lock-in works best so I did that for 5th shot which was 6 minutes later. I also turned off machine to bring boiler pressure to 0.6 bar to see what the effect was. This pull was at 204.5°F which is much better than previous shot.
Since cold-portafilters work, and I saw that group head is hot, I decided lets do cold portafilter twice. For 6th shot that is exactly what I did and pulled it 10 minutes later. This is what I got:
About 198°F temp, very nice and worked like charm.
At this point there was only one more thing to test. I wanted to see how machine idles. I let it then stand under pressure as it is for more than 30 minutes and pulled the shot:
Temperature was about 201°F. Not bad at all. It means if machine was idle for 30 minutes or more you can just walk and pull the shot and its not at bad range at all. No flush needed, nothing. Just walk in and pull.Conclusions
This really helped me tune-up procedures when pulling the shots and getting them into the desired temperature range. Cremina is really not about exact temperature anyway so as long as you are not overheating you are good to go.
7-8 second half-flush should get your first and second shots into the good range. For 2nd shot you might lock-in cold portafilter and for 3rd shot you should for sure lock in cold portafilter once or twice and for each shot after that I would recommend cold portafilter lock-in 2 times.
If you have OE temp strip then keep first bar unlit at all times before pulling the shot or shot temperature will be too hot. Even if its unlit i.e. it just went out, it is too hot and you need to cool down some more.
When machine is left idle for 30 minutes you can just walk in and pull shot and it will be about right.
Also I found that OE temp strip should have lower range with 90°C/194°F bar being actually last bar. Ideal strip for Cremina should start at 65°C/149°F and go up to 100°C. That will give us very good range to judge the group head temperature. Since group head at 74°C/165°F gets us nice temperature range that would be really useful. If anyone knows where to get such strip please let me know.
I also posted study on my little blog as well... Hope this helps and let me know what you think or if you have questions and suggestions.