PID has been installed on a heat exchanger.
On every group were installed at the output of platinum sensors (located under the shower).
Within a one-shot temperature changes are +- 0.3 C max.

We had a little tricky finding the optimal parameters.
quote them:
souf = 0.7
P = 0,1
I = 60
d = 10
Ny = 0,1

This is the best performance. Good response temperature drop and rapid recovery to a given point.

Heating power in 2000
Boiler 12 liters.



During idle time:
TOP (Thermometer) - left the group.
in the middle (Thermometer) - the right group.
The lower (PID) - a boiler.



Stopwatches (counting from zero, auto-reset before the shot). Extremely handy thing.

Nice job!

Failure modes with an RTD
- open circuit = high reading
- short circuit = low reading

I suppose it's possible for a pressurestat to fail closed, which is why there is an over pressure relief valve, but still, if I were doing this myself, I'd have the RTD *and* the pressurestat, with the pressurestat set high as a safety.

Belt and suspenders.

I guess I'm conservative when it comes to safety design.

Where explosion is one possible scenario then most certainly two or more safety devices, or one safety and one control. Do any espresso machines have a "fusible plug" that blows out at a preset temperature and has to be replaced?
KDM

allon wrote:I suppose it's possible for a pressurestat to fail closed, which is why there is an over pressure relief valve, but still, if I were doing this myself, I'd have the RTD *and* the pressurestat, with the pressurestat set high as a safety.

My first thought about safety system - mechanical pressure switch should not be dismantled (up to a maximum value). But having studied the issue, I realized that it was unnecessary. PID will not work when a tattered sensor. I think.

Don't you have a thermal fuse on your heating element looped into the circuit? If not, you should likely get one. That's what I used on my Elektra when I PIDed it.

It seems there is, in the immediate vicinity of the heating element. Nonrestorable:)

What do you mean by non-restorable? You just wire it back in if it's out. And if you need to replace it—which I very strongly recommend if it's not new—they're usually pretty easy to find, at least here in the US.

Not recoverable - in the sense of one-off. If burned - must be replaced. Usually in modern coffee machine it is mounted somewhere near the heating elements.
Do you think there is cause for concern? I have not studied the chain for thermal protection, but for some reason believe that the protection is present. Coffee machine in 2011, the Italians could not make a joke so:)

You can actually get resettable hi-limit fuses, though honestly I think after they go once you should probably replace them anyhow.

I do think it's prudent to have a back-up circuit-breaking safety element on a machine. I doubt it's that much of a risk, but the consequences can be quite unpleasant and expensive, so why risk it?

The "fusible plug" I was speaking of is a device that melts at a preset temp and opens the boiler to the atmosphere. It is a safety of last resort, should the control and the overpressure valve fail.
Duke

http://en.wikipedia.org/wiki/Fusible_plug

PS: I surfed around and found these on line for under $3