PID Control and Steaming Power - why we should care!
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Over many years of reviewing machines I have seen an increasing use of PID controllers on espresso machines. Unfortunately this has extended from the brew boiler to the steam boiler and is in my opinion a poor choice for the steam boiler. Firstly lets take a step back and look at what we want from a Steam boiler in the 2 main types of machines.
- We want steam power
- we don't want to overheat the brew water in the heat exchanger
Two goals that are almost mutually exclusive and it is a balancing act. You may think a cooling flush can always resolve it, but this is not always the case as very large cooling flushes are not a great thing. As a general rule the lowest you can have the boiler temperature and achieve good steaming the better it is.
- We want powerful steam
- we might be power limited (e.g. boiler priority)
- We usually have an upper limit to prevent overheating of electronics etc..
- We want the smallest boiler that will perform well
- On some machines if steam boiler preheats brew boiler water, you may be limited on steam boiler max temp (e.g. Brewtus), but not the Duetto due to it's preheat design.
Much easier to have powerful steam in the Dual boiler scenario.
However, in both types of machines when boiler pressure drops, we want the steam heating element FULL ON until boiler pressure is back to it's preset level. This is highly desirable, because it widens the band where we have the balancing act in HX machines, lowers the maximum boiler temperature required in DB machines and means a less powerful heating element is required.
So why is PID control of the steam boiler bad!
Well it's not bad if properly implemented, unfortunately this is not the case on most espresso machines. The lower the temperature the higher the power supplied to the steam boilers heating element, but as it rises the power is being progressively reduced; exactly what we don't want when steaming.
PID control is meant to return the temperature of a system back to the set point without massive overshoots. In effect the temperature drops a bit and it pulses the heating element a bit and back up it goes, if it drops a lot it switches it on continuously until it gets nearer the set point and then pulses it a bit and the gradually returns it to the set point. So it does NOT apply full power if the steam pressure drops and in fact only applies full power when the temperature drops to the bottom of any proportional band set, this could be as much as 6 degrees or more!
In practice and is a true life example…I was testing a machine with PID control of the steam boiler and when steaming the pressure was dropping to around 0.7 bar, but it would not rise higher despite a powerful steaming element. The reason was that the PID was limiting the power supplied (because of it's programming and need to prevent overshoots etc..), the 1400w heating element effectively becoming a 700W one. Many PID machine use a PID setup that has the same shared parameters for the brew boiler and steam boiler. Unfortunately the different boilers require totally different control systems.
What can we do.
- If you own a PID HX machine and you want it to be a stronger steamer, set the PID parameters quite aggressively. In effect use it as an on off controller, which is exactly what the pressurestat is doing. If it was previously set as a PID with conservative parameters, you should find a significant improvement in steaming. This may then allow you to reduce the temperature while still having good pressure and achieve smaller cooling flushes and better brew temperature stability.
- If you own a dual boiler machine with PID steam control and you want better steam performance, fit a pressurestat if you can (and use it to switch any SSR). If you are in the USA, then the electrical systems often pose a bigger challenge and this may be especially helpful
Now don't get me wrong, I like solid state relays and electronic temperature control, but not when it's implemented incorrectly.
Ultimately we need the manufacturers to realise this is an issue and fit coffee machines with proper dual channel PIDs that don't share settings between the boilers. The steam boiler channel should then be set as a simple on off electronic temperature control. They won't listen to me, but they might to you as the consumer! The next dual boiler machine I do the high level design for, will have this and many other issues addressed.