Argos Physics: How the Argos Works

The Argos is a lever espresso machine that prioritizes shot to shot brew temperature stability. It does not prioritize pre infusion pressure. The dial on the Argos controls brew temp which is the temperature of the water as it leaves the grouphead and continues through the puck.

The grouphead is a heatsink

To achieve the desired brew temp, let’s explain a few concepts. The stainless steel construction of the Argos and most importantly the grouphead is very good at absorbing heat. If you have a metal cup and you pour boiling water in it. The metal cup will get hot very quickly and the water will also cool down some proportional amount more than if you poured boiling water into a cup that is not a good heatsink. This same process happens with the grouphead on the Argos. 

So you can imagine that if our brew temp is set to 92°C. We cannot just set the boiler temp to 92°C because the grouphead being colder will pull heat away from the water as it moves through. The boiler temp would only ever be the same as the brew temp if the grouphead temp was also the same as the brew temp, which in practice does not happen. 

Therefore we must compensate by setting the boiler temp some amount higher than the grouphead temp so that we get as close as possible to our target brew temp. We have spent many hours fine tuning the formula to calculate target boiler temp based on the grouphead temp and brew set temp. 

pid controlled boiler

Once we have the target boiler temp calculated, the PID controller takes over. Its job is to keep the boiler at that target temp. There are many great articles explaining the concepts of PID, but we’ll try to explain it shortly here. The boiler in the Argos is either on or off. The PID controls the boiler. Its job is to get to the target boiler temp as fast as possible and maintain it without overshooting.

Let's say our brew set temp is 92°C and grouphead temp is 25°C then our target boiler temp is 125°C to compensate for the grouphead pulling heat out of the water and our current boiler temp is 22°C (cool room temp water). The PID has 125°C as the goal, it sees we are at 22°C so it turns the boiler on and keeps it on. As the boiler temp gets closer and closer to the target boiler temp the PID will start cycling the boiler on and off, to gradually get as close as possible to the target temp as fast as possible without overshooting it. At this point you’ve reached the target boiler temp to allow you to pull your shot at the brew set temp. 

Why is my pre infusion pressure so low?

For all brew set temps and group head temps, the Argos’s primary goal is to maintain temperature stability. That comes at the cost of pre-infusion pressure. Pre infusion pressure comes from the boiler temp being above the temp of boiling water, creating steam and therefore pressure. The higher the boiler temp, the higher the pre infusion pressure. As the grouphead heats up, boiler temp has to go lower so that we maintain brew temp stability. Therefore our pre infusion pressure goes down. The graph below demonstrates this.

This is one of the most important aspects to understand about the Argos. If you're pulling shots back to back it means that your second shot may require you to manually pre infuse (Mini Fellini) with the lever vs relying on just boiler pressure. Your third and fourth shot will almost certainly require it. This is because the grouphead will continue to heat up as you run hot water through it.

pre-infusion mini fellini

To pre infuse with the lever in spring configuration means letting the lever up partially and allowing the spring to apply pressure until a few drops come through and then pulling the lever down again to refill the grouphead and then letting go completely to pull your shot. You might call this a Mini Fellini. The process is the same for direct except it would mean pulling the lever down applying pressure until a few drops come through and bringing the lever up to refill the grouphead and continue again with your shot like normal.

The first shot pulled on the Argos when the grouphead is cool will usually be the best/easiest due to these properties. Even with these differences in pre infusion pressure and this workflow, we’ve found shots to still taste delicious and we’ve received feedback from our customers that support this as well. 

With a Mini Fellini finesse, they press, a technique that's far from less,

Spring lever pulled with an artist's grace, espresso blooms in its warm embrace.

— Mike, Argos Owner

Now that we understand shots are easiest to pull when the grouphead is cool, it also means that pulling shots back to back in quick succession as soon as it’s at brew temp again is usually the best. As the grouphead will still absorb heat from the boiler (slowly) even without pulling shots, due to the top plate connecting the boiler to the grouphead. So the longer you wait between shots, the hotter the grouphead gets and the lower boiler pressure is.

The proof is in the plots

We use a Scace to tune our algorithms and make sure we’re reaching the target brew temp as close as possible. Here is a plot of four consecutive shots which includes the grouphead being filled (0 to 10 grams). The brew set temp is 89°C. The Y axis is the temperature. The x axis is the output in grams which basically represents 30 to 60 seconds. You can see that shot to shot, the Argos’s brew temp is very stable.

Here we zoom in past the grouphead being filled and we can get a closer look at the temp readings.

Here is a plot of grouphead temperature compared to boiler pressure. Based on the principles we discussed earlier this plot should make a lot of sense. As the machine warms up and right before shot one, we reach peak boiler pressure. This is when the grouphead’s temperature is the furthest from the brew temperature. As you pull shots and water flows through the grouphead, it heats up, requiring the boiler temp to lower and therefore boiler pressure to lower.

Classic Mode

You’ve learned about what makes the Argos unique and some trade off’s we’ve made in order to prioritize brew temp stability. After gathering feedback from customers of the Argos we’ve realized there is an opportunity to sacrifice some brew temp stability in order to have higher pre infusion pressure. We call that Classic Mode. Learn more about what Classic Mode does, how it works, and why you might use it here!

We hope this provides a deeper understanding of how the Argos works and our goals when creating this machine.

If you have any questions, feel free to email us at support@odysseyespresso.com!

— Ross & William