A guide to managing e61 Heat Exchnager machine brew temperatures
A link to a youtube video showing this process is below, I would suggest watching the video (may be best on a computer screen) & also reading.
To begin with I would like to say that managing HX (Heat Exchanger) brew temp is a minefield! Not only because of the technical & design difference between machines but also because there is a mass of information out there, some good, some not so good! Eric Svendson, creator of the "Erics Grouphead Thermometer" did a fantastic job in building the GH thermometer & has also posted a lot of useful info including two methods to manage brew temp named "Flush & Go" & "Flush & Wait". Before we continue with my method & the testing I think it would only be rite to give a quick overview of these two methods to help build the picture, once you understand what is going on here things become easier to take on board.
Flush & Go (F&G)
With the Flush & Go method we flush a sufficient amount of water out of the HX to cool the water inside the HX to below target brew temp. The GH (Grouphead) remains above target brew temp.
We then pull the shot immediately (or with a small delay). 'Too Cool' water exits the HX & is heated by the 'Too Hot' group. This results in some heat transfer & we have our brew water.
With the F&G method GH thermometer readouts will appear below actual brew temps during the shot. This is because the thermometer is an inch or so behind the puck & the 'Too Hot' group still has time to warm the water on its way past the thermometer to the puck.
An example of F&G could be as follows.
Remove PF (portafilter) & prep puck
Flush machine until temp starts dropping
Stop flush at 96Ā°C Lock PF in & GO
During the shot the GH thermometer will read below actual brew water temp at the puck (this will vary machine to machine but somewhere around 1-3Ā°C would be a ballpark figure)
The method above may give somewhere are a 93Ā°C temp at the puck
Flush & Wait (F&W)
With the Flush & Wait method we flush a sufficient amount of water to cool the mass of the GH below target brew temp. The puck would them be prepped & the shot initiated at after 'waiting' until the GH thermometer hits a pre determined point.
With this method the GH is cooler than target brew temp. The water in the HX has likely recovered to full temp (there or there about, this is really not critical as you will see moving forward). On pulling the shot 'Too Hot' water exits the HX & is cooled by the 'Too cool' group. This results in some heat transfer & we have our brew water.
With the F&W method GH thermometer readouts will appear above actual brew temps during the shot. This is because the thermometer is an inch or so behind the puck & the 'cool' group still has time to cool the water on its way past the thermometer to the puck.
An example of F&W could be as follows.
Flush machine until GH thermometer drops to 93Ā°C
Group will continue to drop
As soon as group begins to recover & temp starts rising remove PF & prep puck
Lock PF back & 'Wait'
Wait until GH thermometer hits 90.5Ā°C & pull shot.
During the shot the GH thermometer will read above actual brew water temp at the puck (this will vary machine to machine but somewhere around 1-3Ā°C would be a ballpark figure)
Both methods will get you where you wan to be. F&W is a little easier as we have nailed down a few of the variables. HX water is going to be about the same temp & GH is at whatever we decide. F&G is a little more tricky. Once the group starts dropping temp as we flush the thermometer is changing rapidly, a slight delay may mean a target flush to 96Ā°C would quite quickly become 96.5 or 95. Not the end of the world but we can do better.
Now that I have given a general overview to the two most commonly used methods out there I want to say that both of these methods work. Both get you where you want to be but my biggest concern is the amount of water that is required to implement them. I know a lot of people have plumbed in machines however I would say a lot of people live in a hard water area & don't want the expense of filtrations systems or simply cannot be bothered with it. Also consider people like me who have a tank only machine. Once fully warmed up my Rocket Appartamento will take between a 150-200ml cooling flush. Follow that with 50ml for the shot & maybe a 30ml screen flush & I get less than 10 shots to a tank & multiple drip tray 'dumps'.
My frustration with misinformation, lack of support from retailers/manufacturers, long cooling flushes, constantly having to dump the drip tray & recycling bins full of empty water bottles forced me to look into how to get around these issues myself.
This is a guide that I have built for myself. It compromises of charts, videos & general discussion around what exactly is going on with a e61 HX machine but most importantly it includes an easily repeatable process to hit whatever brew temp you like at whatever boiler pressure you like, shot after shot without flushing. Please consider this is MY routine that works for me & with my machine. In general e61 HX machine will perform in a similar manner as the engineering is similar, however there are many factors that will change between machines so please see this guide as an example rather than the rule.
What will this guide show me?
Two sets of six 30 second shots pulled in a b2b (back to back) manner at a 'reasonable' workflow. Both sets took between 30 & 40 mins to record.
One set at 1.1 bar max & one set at 1.5 bar max.
You will see GH temp & 'at Puck' temp during all extractions (in the video) & data will be logged into graphs for reference & review.
You will see a method to produce "hot" "medium" & "cool" shots at both 1.1 & 1.5 bar with no flushing (other than screen flushing post shot) or changes to Pstat/PID during the sequence of extractions.
You will see some attempts to produce a "flat profile" shot where I am trying to eliminate the "HX Hump" at the beginning of the shot that this method produces.
You will also see a comparison of 1.1 & 1.5 bar shots pulled in the same manner producing almost identical brew temp profiles! A chart is included with 1.1 & 1.5 together.
The method
Before we talk about the method there are a few basics to understand. Understanding the basics will hep make sense of the process.
An e61 Grouphead is pretty much a solid block of brass & usually weighs around 4kg.
The amount of water required to produce a double shot of espresso would usually be between 30 & 100g. This is taking into account 'puck absorption' & filling any pre-infusion chambers & is somewhat of a 'stab in the dark' but the point I am trying to convey is that it is much less than the weight of the group.
Considering the water during a 'normal' extraction flows slowly there is plenty of contact time between the water & the GH. The GH does an excellent job of tempering the water & bringing it up or down as required.
This suggests that controlling GH temperature plays a large roll in the actual temperature at the puck. It is my belief that controlling GH temp is WAY more important than HX water temp. This will be demonstrated here & the results are shocking.
The method I have been using is to cool the GH with a small battery operated fan. This does not affect HX water temp much although the return will be a little cooler but not massively relevant to the outcome.
Position a small fan so that it rests on the top cap above the mushroom & blows directly down over the group
Cool the group to just above a pre-determined temp (group will continue to drop)
Pull the PF & prep the shot
Lock the PF in & wait for the GH to rebound to a pre-determined temp
Pull the shot at the pre-determined temp
With this method GH thermometer readouts will track above actual water temp at the puck. This is the same as with the Flush & Wait method as the GH is cooler than target brew temp & HX water is hotter than target brew temp.
The best part with this method is that we can fan the group to different temperatures & change actual brew temps shot by shot if we like. Boiler pressure/ temp appears to not drastically affect actual brew temp at the puck with this method.
This method has proven EXTREMELY accurate & repeatable shot to shot. I originally set out to try & produce a "hot", "medium" & "cool" shot but my results show we can target within 1Ā°C fairly easily & even go between temps from shot to shot at any reasonable Pstat/PID setting.
Finally some numbers...
Hot Shot - Fan to 93Ā°C - Pull Shot on rebound at 92.5Ā°C - At puck temp 95Ā°C
Medium Shot - Fan to 91Ā°C - Pull Shot on rebound at 90.5Ā°C - At puck temp 93Ā°C
Hot Shot - Fan to 89Ā°C - Pull Shot on rebound at 88.5Ā°C - At puck temp 91Ā°C
I have inserted the data referenced for the six shots at 1.1 bar & 1.5 bar. These shots were pulled over roughly a 40 min period over the course of two nights (one night at 1.1 & the other at 1.5 bar). A screen flush was performed after all shots & the PF was knocked out, dunked in hot water & put back in the group between shots unless otherwise stated.
Here are a few more graphs to demonstrate some of the findings.
Firstly we have a comparison between shot 1 and 4 at 1.1 bar.
Next up we have a comparison between shot 2 & 6 at 1.1 bar.
As you can see shot 2 (the blue line) had a higher 'hump' at the beginning of the shot. I suspect this is probably to do with the whole machine being just a little hotter.
Finally we have a comparison between 1.1 & 1.5 bar pressure.
As you can see although the 1.5 bar profile does take an upward climb towards the end of the shot the profile over a 30 second shot is similar. This shows just how much impact GH temp comes into play here.
As a bonus here is my best attempt at producing a "flat profile".
Please note this shot is at 1.3 bar which is where I have decided to now leave my machine after all of this testing. This is also a 40 second shot.
Machine was warming up from a cold start, PF was pulled at 90Ā°C, puck was prepped, 5 second flush at 90.5Ā°C, PF locking in & go.
This is an easy shot to pull, no 'fanning' required. This is now my go to morning shot every day.
Conclusions
I think from looking at the data it is clear to see that we can manage HX brew temperatures fairly easily utilising a small fan. We can also fairly accurately brew at a variety of temperatures without adjusting boiler pressure. The data also shows that boiler pressure/ temp does not directly have a significant impact on brew temp at the puck. Boiler temp will obviously influence the temp of the GH as the thermosyphon will run either hotter or cooler, however GH idle temp with this method has little bearing on the final result as we can regulate the temp with a fan anyway.
What do I recommend?
Well first off let me say, I am no coffee expert. I am a Telecoms Engineer. My job is problem solving, dealing with some simple & some complex faults on a daily basis. If I think something can be done better then to me that is the problem & I want to solve it. What I have done here is simply prove that a solution to my problem works in practice. I don't think this method will please everyone & that is fine. I am reluctant to tell anyone what to do, but rather share what I have found & what worked for me. What I will do however is share how I now approach managing HX temps after all of this learning.
My machine comes on at 06:45am every morning on a WiFi timer. I am down by 07:00am & flick the GH thermometer on. I keep an eye on it as I sign on for work, make the wife a cup of tea, make the kids some drinks, watch them beat each other up etc... Once the GH hits 90Ā°C I pull the PF prep my shot, wait for it to hit 90.5Ā°C, quick screen flush, lock it in & go. This produces that nice flat 93Ā°C(ish) shot, easy!
If I want to make another shot any time soon I leave the machine on & fan down as the process above explains & carry on.
At the weekends the machine comes on as normal at 06:45am, same deal, I pull my first drink of the day as the machine is warming up. If we are going out I will turn the machine off, if we are staying in I will leave it on. The machine warms up fully & when I want another shot I will simply fan it down to the desired point & carry on as the process above explains.
For back to back shots I will generally move quickly & keep on top of the group temp (before it rises too high), sometimes hitting it with a bit of air. You get the idea. Keep the GH where it needs to be & you are good to go assuming you are not a super human barista who can knock shots out on the minute every minute I would assume HX water will be 'there or there about' where it needs to be. Please bear in mind all Rocket machines are fitted with a 3mm thermosyphon restrictor in the upper plumbing. This regulates (slows) the flow of the thermosyphon which in turn slows down the rebound of the group. If you have what Dan Ken of Home Barista would describe as a "Dragon" meaning a HX machine probably without a restrictor recovery time may be too fast to stay on top of. Fanning between shots pulled quickly will likely be still be required.
Considerations
The first consideration would be to discuss a point I just touched on. HX rebound time. This method relies on maintaining the temp of the GH & always initiating the shot at the same pre-determined GH readout. The other variable at play is HX water temp. I know I have said that HX water temp is almost irrelevant & I still believe this to be the case for the most part. However, large swings in HX water temps will show up in the results. How much? Hard to say but worth considering.
Flow..... Now flow is an interesting variable. Considering we are relying on cooling the "too hot" water by passing it through the group that is below target brew temperature the rate at which the water passes through the group make a difference. Too slow & the temp at the puck will cool below target brew temp. Too fast & the Hot HX water will not be attenuated enough. How much these results vary is another test in the making however I would like to acquire a Flow Control kit for my machine before perusing that avenue.
One thing worth mentioning is that errors in grind that result in a 'too slow' flow may be slightly buffered by the effect of a cooler brew temp at the puck. Meaning if you ground too fine & are likely going to be pushing into 'Over Extracted' tastes, these 'tastes' may be somewhat muted but the fact that the brew water ran a little cooler. Possibly making the e61 group even more forgiving?
Another consideration worth touching on is the commonly talked about 'advantage' of a HX machine vs a Dual Boiler (DB) & that is brew water 'freshness'. It goes without saying that generally HX machine have a quicker turn around of water being used for brewing when comparing to a DB machine. This is because the HX is significantly smaller than that of a conversional brew boiler on a DB machine. Not flushing a HX machine would obviously slow down the turn over of water (which is kind of why this whole project started for me) which may in turn show up in the taste. I couldn't tell, maybe you could, if you would like to 'freshen' things up a bit a 5 second screen flush prior to each shot will not negatively impact temp stability, it may in fact help produce a flatter shot if that is what your after, which conveniently leads me onto my next point.
'Flat' profiles vs 'Humped' profiles. Throughout this whole process here I have been mentioning the 'HX hump' & have also mentioned ways to avoid it. Please consider this is only for experimental purposes. There has been a lot of discussion around 'flat vs humped', 'inclining vs declining' temperature profiles & to be honest I think the 'jury is out' as to which is better. I'd say don't worry about it, play with both if you like, have some fun.
Further experimentation
Although I feel I have reached a conclusion & am now happy with how to manage my Rocket machine I simply cannot leave it there. This process needs automating. Unfortunately machine manufacturers generally want to focus on their higher end machines & simply slapping a PID on a HX & claiming it somehow magically produces normal brew temps is simply not the way forward.
Lelit are the only manufacturer I am aware of who have done anything that resembles a solution, the Mara X. They have however gone about it completely 'arse about face' if you ask me. I am not knocking them as I think innovation is important & it is great that they are trying, but with all the resources at their disposal to simply regulate brew temp by effectively ramping boiler pressure up & down (based on return thermosyphon water temp) is urrggghhhh, well it works, sort of but I just don't like it. Hard to really explain but I think they could have come up with a much simpler solution. I have one in the making that would actually work better, no boiler swings, no flushing (it is not a fan but I will not share until I have proven the concept).
I have automated the fan process myself with a small brushless DC fan linked up to a PID with a thermocouple mounted to the underside of the group. The PID can be programmed to turn the fan on/off at whatever temp you like. Maintaining GH temps this way results in a fluctuation of around 1Ā°C at the group. Brew water at the puck would likely fall within the same range & brew profiles will possibly run 'flatter' due to the constant cooler idle that this process results in (return leg on thermosyphon will be cooler for extended periods may drop the HX temp a little, certainly stopping it 'superheating'). The set up for this is a little ugly, I am still at the proof of concept stage, playing around with a few options but I have a much better idea that has popped into my head that I will share if I ever get around to it.