This post describes the temperature instability issues with single boiler home coffee machines, highlights the reasons and the ways of overcoming them. Installing PID controller seems the best way to improve temperature stability and introduce possibility of temperature adjustment to achieve better espresso flavours and consistency between shots. The remaining part of post describes my own experience of equipping Gaggia Classic coffee machine with PID controller. The post might be interesting for home coffee enthusiasts with DIY skills.
It is all about temperatureTemperature is one of important factors affecting the taste and the flavour of espresso. The recommended water temperature that comes out of the group is supposed to be in a range between 90 and 96 C. The best temperature for particular type of the beans depends on the roast - for example, for the North-Italian roast, the right temperature, as literature suggest, is between 95 and 95.5 C. Dropping it just one degree lower will make acidic flavours apparent. Having temperature higher than that will add bitterness to the taste.
While the temperature is a key factor, single boiler home coffee machines, not only normally do not have functionality of adjusting it, but also, more important, the temperature they provide is not stable enough. The temperature in coffee machine is required to be stable throughout the shot pulling. If temperature drops due to fresh water pumping in boiler while you are pouring the shot, which is a problem of home machines as they have a small boiler, such temperature lowering should be minimised.
The device responsible for temperature control is called thermostat. Its simple role is to maintain a required temperature by continuous cycle of turning on the heating element of the boiler, when the temperature is lower than required and turning it off, when the temperature gets above required value. The problem, however, is that, even entry level machines you potentially can make a decent espresso with (I consider Gaggia Classic and Rancilio Silvia machines as absolute minimum you need to have if you actually want to make Espresso at home), have a fixed-temperature bi-metal thermostat (that uses the same principle as an electric kettle switch). As you may expect, accuracy of such thermostat leaves much to be desired and it has a relatively slow response to temperature changes - this results in temperature fluctuation unacceptable for espresso. You may have a look at the temperature graph below taken from this forum post.
Temperature (in Fahrenheit) is shown on Y axis and time is on X axis. You can easily calculate the fluctuation between peaks - it is about 17 C (or 30 F). The default temperature for this thermostat is actually 107 C (225 F), which it tries to maintain.
There are some techniques you may come across in the internet that are believed helping home coffee machine owners to pull better shots. One of them is called "time surfing" the idea of which is to start pouring at the certain point of boiler heating cycle (determined by the heating indicator on the panel). The other suggests to use steam switch to reach higher temperature and then start pouring having counted a certain number of seconds to start at the right temperature (e.g. here). These techniques are obviously better than nothing, but still a "blind" way of achieving better extraction. The best way to achieve the higher temperature stability and add possibility to adjust the temperature is to use PID controller on domestic coffee machine.
What is PID controller?PID is small electronic device that controls temperature. It replaces the functionality of dumb bi-metal thermostat with a high precision smart control. It continuously monitors the temperature of the boiler using high-accuracy RTD temperature sensor and controlling heating element power circuit much more granular and precise way than bi-metal thermostat. Temperature fluctuation on the machine equipped with PID normally does not exceed 0.3 C. When shot is pulled, the temperature drop due to introduction of the fresh water is minimal, as PID controller continuously keeps required temperature and respond to changes immediately.
My DIY projectThere are several manuals on the web describing how to install PID controller to different types of espresso machine. Here I will share my experience of adding PID to my Gaggia Classic. I have chosen slightly more expensive way, but paid more attention to electrical safety and minimised intervention to existing circuits of my coffee-machine, making PID installation fully reversible if required.
First things firstDisclaimer: The modification described below involves tampering with 220V electrical circuits in equipment that has contact with water - this may result in electric shock, injures or death, as well as property damage. The modification experience is described for information only and could be used as reference by qualified person with good understanding of electrical circuits and safety. Installing PID controller on coffee machine will void its warranty. Anyone installing PID using this publication is doing that at own risk and assumes full responsibility for all undertaken actions. Author of this publication will not be liable for any damage or injures caused to you or your property as a result of using information from this publication.
MaterialsI shall start with the list of the items I had to buy.
The main three components of PID kit is PID controller itself, temperature sensor and Solid State Relay (SSR). The PID and temperature sensor I ordered directly on Auber Instruments website. These guys are actually developing them, not just replicating, like those who sell its cheap clones on eBay. The PID model is SYL-1512A, this is a universal controller in compact 1/32 DIN case. The manual is easy to understand - device is pre-set for being used with SSR, so all you need to do after installation is to configure it to use RTD sensor, set the desired display unit, set the temperature to 105-106 C and run automatic calibration. I have also ordered a matching case to make it look more professional. It is high quality aluminium powder-coated case comes with front panel and also has a hole for cable on the back side.
The temperature sensor I have ordered was PT100M4, this is high-accuracy platinum RTD sensor with M4 threading on the probe and 45cm Teflon-coated cable (the left one on the picture below). It is ready to be used instead of original bi-metal one (the right one on the picture), no modification is required.
Solid State Relay (SSR) I have ordered from eBay. It is 10A SRR which is far above the current used in coffee machine, making its usage safe. The role of SSR is to control boiler high voltage power circuit by low voltage signal triggered by PID. In addition to that, I have bought a number of other items like connectors, terminals to make modification look better and less invasive. You can access the full Bill of Materials here. In total I spent £75 including postage. The only thing I did not buy was the power cable which I already had. Notice, that Auber Instruments also offer Coffee Machine Kits. This is basically the full kit to add PID to your coffee machine including all required components. It might be a good option for someone who does not want to mess with soldering, terminals, cables, etc. for me it was more interesting to spend some time and complete my DIY project.
Circuit diagramBefore putting everything together, I would like to make clear what actually is going to be achieved. Here you can download an original circuit diagram for Gaggia Classic. I sketched its copy for demonstration purposes.
Recall what PID is doing: it continuously monitors the temperature of the boiler using high-accuracy RTD temperature sensor and controlling heating element power circuit much more granular and precise way than bi-metal thermostat. Thus, PID device should be playing the role of existing bi-metal coffee thermostat ST1. To achieve that, coffee thermostat ST1 needs to be replaced with RTD temperature sensor, which will be used by PID for temperature readings to control heating element power circuit of coffee machine (previously controlled by ST1) using SSR. In addition to that, PID controller needs to be powered up when coffee machine is turned on. The modified diagram is shown below:
Putting everything togetherFirst step was to prepare cables. I used a couple of cables to connect SSR to heating element circuit. It was 2 mm cable suitable for 10A current, similar to one used in the coffee machine already. I used spade terminal on one end to connect to coffee machine circuit and spade-tongue terminal on the other end to connect to SSR.
Another cable was used to connect SSR to PID. SSR is triggered by low voltage (8V), so cable used was relatively thin. Again, spade-tongue terminal on one end for SSR connection.
Both cables were connected to SSR terminals.
Cables to power up PID were needed as well. I used three cables, but in fact PID manufacturer claims that earth is not needed since the case and controls are made of plastic. The power consumption of PID is less than 2 W, so I used thinner cables here, about 1 mm in diameter. As these cables are connected to terminals that are being used by existing circuits, so piggy back terminals were used on these cables.
Cables preparation is completed. Now let's move to assembling steps.
If you are using my steps as a guide, make sure that coffee machine is not powered before doing anything to it, remove power cable from the connector on the back of machine to be completely sure.
Next thing I did is fixing SSR to the back wall of coffee machine. I used one M4 screw with a washer to do that.
As per diagram mentioned earlier, I detached cables from bi-metal coffee thermostat (at the bottom of the boiler) and connected them directly to cables attached to AC terminals on SSR (1 and 2 on the picture above).
Using small adjustable spanner, I unscrewed and removed bi-metal coffee thermostat (shown on the right on the image below).
I applied some thermal paste to RTD sensor, before screwing it up at the former bi-metal thermostat location. Notice that I rolled up the cable into small ring to make screwing easier.
Then I sorted power cables. Neutral and Earth went directly to power connector on the back.
Live one was connected to the power switch. I used steam switch terminal as it gets powered once machine is turned on and has more space around for piggy back terminal.
The cables inside machine were organised using plastic strap and fitted through the ventilation holes outside for PID connection. I made sure that no cable is touching the boiler, as this is important. There were 7 cables that went outside: 3 for 220V AC power, 2 from SSR and 2 from RTD sensor. Notice, I have moved SSR to the left - in previous position it made difficult to fit the top cover, but since this picture was made later, you may see SSR located at previous place on some other images below.
All cables needed to be fed through the small hole in the controller case and connected to PID according to diagram.
Power cables were connected to terminals 1 (Live), 2 (Neutral) and 3 (Earth).
SSR cable was connected to SSR output (terminals 9 and 10). RTD sensor was connected to sensor input (terminals 7 and 8, notice that 6 and 7 were also connected together using piece of wire).
Once everything was connected, I screwed the front panel and the case was assembled. I attached a piece of adhesive magnetic sheet to the case, so that it could easily be fixed to the side of coffee machine like on the picture below. Also I installed spiral cable wrap to that short outside section to make it look neater (not shown on the picture).
As I mentioned, PID configuration is fairly straightforward. For PID parameters I selected auto-tuning which set optimal PID parameters automatically.
The main advantage of PID controller is that temperature can be adjusted easily whenever needed. Notice that set temperature is above the actual water temperature in the boiler, it is roughly 10 degrees above. 105 C on the controller corresponds to 95 C brew temperature. If this is not the right one for your espresso, it can easliy be changed using up/down buttons on the PID.