Gaggia Classic Boiler: step response, lag and temperature drop rate

Partly for my own notes, here are the results of some simple experiments with the Gaggia Classic boiler, to better understand its behaviour. Prior to these tests, the machine started out at ambient temperature of around 21°C. The temperature sensor is the TSIC 306 mounted in the same position as the original brew thermostat. There was no portafilter fitted. The tests were carried out on 240V AC mains in the UK, and this is the original Gaggia Classic with an Aluminium boiler.

This first graph shows the temperature (blue line) in °C versus time in seconds for a step input of heat at full power for 4s. The red line shows the timing of the heat pulse. Note the lag of about 4s before any measurable change in temperature after the element is switched on. In this example, the starting temperature was 21.7°C, the peak temperature was 29.8°C and the temperature rise was 8.1°C.

2016-05-11_gaggia_4s_step

This second graph shows a step input of heat at full power for 8s. Again, there is a lag of about 3s before the temperature changes. Here the starting temperature was 23.2°C, the peak temperature 39.1°C and the temperature rise was 15.9°C. The original data can be downloaded as a CSV file.

2016-05-11_gaggia_8s_step

Here’s a longer sequence, showing the first 35 seconds of the boiler heating up from “cold” after the heating elements are first switched on. The heating elements were on for the full duration of the graph below. Again, it takes 4 seconds before the first change in measured temperature. After 12 seconds, the temperature rise becomes almost linear, with a rate of increase of about 1.83°C/s (straight line fit to a linear subset of the data in Excel). The original data can be downloaded as a CSV file.

2016-05-11_gaggia_temperature_rise

For the final test, the machine started at about 26.9°C and was driven up to 93°C under PID control then allowed to naturally cool down, to see how fast the temperature drops. Again, the data can be downloaded as a CSV file.

2016-05-11_gaggia_temperature_drop

The initial temperature drop from 92.9°C to 90°C takes 29.25s, which is about 1°C every 10 seconds (0.099°C/s), within the small region between 90°C and 93°C where our temperature controller operates.

Measuring Ulka EP5 Pump Inductance

Having previously tried to estimate the pump inductance of the Ulka EP5 Pump, and without really being certain of the accuracy, I decided to try and actually measure it. Unfortunately, this is made difficult by the series diode built into the EP5 pump. Although I had a spare pump, I didn’t want to completely destroy it by opening it up, so I decided to try and reach the terminal by performing “minimally invasive surgery” on the pump…

To start with, shining a torch through the plastic pump body reveals the location of the metal tab terminal on the solenoid, and the series diode:2016-05-04_ulka_exam1

Having done this, a Sharpie pen was used to mark a point above the metal tab, which was approximately where the “U” is in the ULKA logo as shown by the small black dot below:

2016-05-04_ulka_exam2

Then a pin vice was used to carefully and slowly drill a 1mm hole until it reached the surface of the metal tab, while taking care not to drill too far:

2016-05-04_ulka_exam3

Having done this, we can now directly access both terminals of the solenoid and completely exclude the diode from our measurements. Using a low cost LCR component tester (fish8840 Taobao) it is possible to measure the inductance and series resistance as shown below. Successive readings showed 847.8mH, 856mH and 859.3mH with an average of about 854mH. The series resistance was about 165Ω as measured previously.2016-05-04_ulka_coil_LR

As both sides of the diode are now accessible, we can also check the diode forward voltage as shown below. This shows Vf=706mV and C=40pF:

2016-05-04_ulka_diode_VFCWhen finished, the hole will be filled with epoxy to seal it back up again and make it safe.

Conclusions

The inductance was measured as 854mH and the series resistance was 165Ω. The diode forward voltage is 706mV. These tests are carried out at a low voltage, and without any hydraulic load on the pump. It is possible that the solenoid plunger position may also affect the measurements. Note that the measured inductance is significantly different to the previous estimate.