I brief update. After I ordered and received stainless steel thermal probe from ACS, today I replaced the old brass one, please see the pictures below.
After replacing the faulty one with richly PTFE covered new probe I started the machine with both boilers on.
The steam boiler reached 127 deg C (1.5 bars) and the heating element was switched off by the PID control.
As I live in a soft water region and I also have got an in-line ion exchange resin I wonder what the black thing is that covers the probe. After being in 10% citric acid solution for 5 minutes the probe was cleaned (and ready to go?).
The irony is that over three yeasr I have switched on the steam boiler less than ten times for half an hour each time.
Does the colour prompt for descaling? I have always considered the limestone to be white-ish. I took a picture from our kettle, that is heavily used (since 2009) and never has it been descaled.
I will appreciate your input.
Cheers.