Quick Mill Carola PID Test & Analysis:
In this post, I will describe a test while I monitored the Carola's heater continuously for an hour. This test reveals:
1. How the Carola's PID behaves.
2. How quickly the machine's boiler warms up.
3. How quickly the machine recovers after pulling shots.Intro:
There's been some interesting discussion about the Carola's PID behavior and performance. For example, is the Carola's heater controller a true PID or something less (like an electronic thermostat)? In the end, I'm hoping for some of you to help shed some light on this. Test Method:
The easiest way to track the PID's behavior would have been to hook up a data logger to the heating circuit. What I had at hand was an iPhone.
So, instead I took a 72 minute video of the heater light and analyzed the brightness of the pixels in each frame. The video below video depicts this test method as Carola transitions from the "heating up" to "maintaining set temperature."Raw Data:
The raw data reveals how the heater pulses. For example, this graph shows in more detail the transition mentioned above.
The PID consistently works on a 2 second cycle, during which the heater will pulse for a certain amount of time. The length of this heater pulse ranges from 1.5 seconds (while heating up) down to 0.1 seconds. Definition: Duty cyle - the percentage of time that the heater is on.
Thus, the numbers indicate that the duty cycle ranges from 75% (1.5/2) down to 5% (0.1/2). Below we see the PID switch from a 40% duty cycle to a 5% duty cycle.Results:
Below I have graphed the duty cycle for the entire 72 minute test.
This graph has a lot of interesting information. Some notable observations as we move from left to right across the graph:
• The PID reaches a maximum duty cycle of 75% (which is 1.5 sec ON / 0.5 sec OFF, repeated).
• The boiler reaches temperature after about 10 minutes. The heater's ramp down only takes about 30 seconds, but there are a few intermediate steps as it ramps down (i.e. it doesn't just immediately switch between two modes.)
• To maintain the set temperature, the PID alternates between a 5% and 40% duty cycle roughly every minute. This is the main part that concerns me regarding whether it's truly behaving like a PID and/or keeping temperature very constant.
• As the overall machine continues to heat up (from 10-28 minutes,) the 40% duty cycle time durations gradually become shorter and more consistent. The overall machine is reaching steady-state and requires less energy to stay at temperature.
• After pulling one shot at 29 minutes, the machine does an extra long 40% duty cycle and appears to recover after about 2 minutes. The effect of this shot can be seen for about 10 minutes, as the 40% duty cycle time durations taper off.
• After pulling 2 successive shots at 53 minutes, recovery takes about 5 minutes, and the machine even kicked into a 70% duty cycle for a bit. The overall machine seems to get back to steady-state after 10 minutes.Summary:
In summary, the PID does seem to have some level of intelligence going on, and it's always actively doing something to try to hit a target. But it seems a bit crude, so I'm not sure if it's really doing what most PIDs do.1. Behavior
It only has about 3 types of behavior - a 75%, 40%, and 5% duty cycle, all of which operate on a repeating 2 second time period. The PID alternates between these three duty cycles for various amounts of time. 2. Warm Up
As described in my review above and in other reviews, the boiler water gets up to temperature after about 10 minutes. But the overall machine has not yet reached steady-state until about 20-30 minutes. So while you can pull a shot after 10 minutes, it may be advisable to do a flush to help the brew head get up to temperature.3. Shot Recovery
The boiler temperature takes 2-3 minutes per shot to recover, and after you are done pulling shots the overall machine seems to take about 10 minutes to get back to steady-state. Please Share Your Expertise or Experience!
So that's more than anybody ever wanted to know about the Carola's PID behavior! But obviously I find this kind of thing fun to tinker with.
What I'm really curious about is whether this type of behavior is typical of all espresso machine PIDs, or if it's just how the Carola's cheapish "PID" works (or if it's not really a true PID or more like a proportional controller with a few extra tricks up its sleeve.)
Please feel free to offer your opinions, expertise, or experience with the Carola or other machines! Thanks.