Just contemplating a possibility here. On a large boiler, a PID determines the constant temperature of a large amount of water. This allows you to pull a number of shots in a row at a consistent temperature.

But if one were to add a PID to a machine with a very small boiler.... One double shot empties like half the boiler or more, a reality we can see if we put a thermometer on that boiler. At full flow, it knocks the boiler temperature down by 30 degrees or more as the room temperature water from the reservoir gets pumped in to push the hot water out. Of course the heating element kicks in immediately, but it takes time to bring the fresh water up to temperature. A PID would likely not put that heater on much quicker than is already occurring.

So all a PID really does for you in the small boiler setting is to determine the starting temperature! This has value of course, but if you have an accurate temperature gauge you can accomplish the exact same thing by surfing.

Am I missing anything in this analysis?

One the boiler gets to temperature, it will stay more more temperature stable with a PID. It may also take slightly longer to get to temperature because the PID will slow down the heat as it approaches the set temperature.

Another consideration is that the brew water is drawn from near the top of the boiler in the SBDU machines that I know of. Thus, a thermometer located on the outside of the boiler does not reflect the temperature profile of the brew water.

A PID reduces the need for temperature surfing by reducing the deadband to almost nil. A conventional button thermostat is still going to have a wide deadband that you have to work around. While no doubt it is possible to surf to a particular boiler condition and pull a shot, the advantage of a PID in a SBDU is twofold:
1. it just makes life a lot easier. Walk up and pull.
2. it makes temperature adjustment practical. Raising or dropping average brew temperature by a couple degrees is trivial with a PID, but would require developing a new flushing and surfing procedure for each and every brew temperature because the behaviour of the button thermostat cannot be changed.

baldheadracing wrote:3. Another consideration is that the brew water is drawn from near the top of the boiler in the SBDU machines that I know of. Thus, a thermometer located on the outside of the boiler does not reflect the temperature profile of the brew water.

A PID reduces the need for temperature surfing by reducing the deadband to almost nil. A conventional button thermostat is still going to have a wide deadband that you have to work around. While no doubt it is possible to surf to a particular boiler condition and pull a shot, the advantage of a PID in a SBDU is twofold:
1. it just makes life a lot easier. Walk up and pull.
2. it makes temperature adjustment practical. Raising or dropping average brew temperature by a couple degrees is trivial with a PID, but would require developing a new flushing and surfing procedure for each and every brew temperature because the behaviour of the button thermostat cannot be changed.

3. On my ViaVenezia, I've located my thermometer probe directly next to the brew thermostat. It's instant read, and though cheap is quite good. So it should be fairly close in its reading, and if the shot I just made is any indication it is indeed close. It read 200.2 when I started pre-infusing, went up to 204, then dropped to 189 over 24 seconds as I pulled the shot. In the final four seconds I opened up the flow control all the way, and it dropped to 169. It dropped a few degrees more after I was done, then started going up again.

2. You'd still have to purge a bit to drop the temperature, right? There is no cooling element in any espresso machine I know of...

With the PID on the Silvia, my end-of-shot brew water temperature is two/three degrees F cooler than the beginning-of-shot temperature (measured on a Scace).

And no, one doesn't have to purge to cool. Boilers shed heat, and so it takes two-three minutes for the PID temperature to fall to the new set value.

If one wants to brew right now, one does indeed have to purge if the boiler temperature is too high. And I don't think there's a 53mmScace... and even if there was, $600 is a bit steep for that function when I spent $15 on the machine itself

Why would the boiler temperature be too high? I set the temperature to match the coffee. If I want to change the temperature up or down for the next shot, the PID will reach the new set value well before I finish dosing, grinding, and puck prep. (As an aside, my PID's PT100 RTD sensor is in the boiler's thermowell casting so it is both a more accurate and a more precise estimate of in-boiler temperature than a surface-mount thermocouple.)

Regardless, deadband is deadband. You can do things to reduce deadband, but that's on you to do. If you want deadband handled automatically, then a PID is one answer. If you want to control the brew temperature profile manually, then you can, just like folks did on old machines that had no thermostat or pressurestat (or pump).

Some old machines used the operator as the PID... He constantly kept an eye on the gauges and moved the flame closer to or further from the lowest part of the boiler to control how much heat it was taking. Opening and closing the inlet to keep the boiler at the right level with his other eye on the sight-glass. If he had a third eye he could use the machine.
If you enjoy temperature surfing, then feel free to do it. Be happy that it can be done with a rocker switch on the little machines.

What it boils (pardon the pun) down to is this: I'm rarely pulling multiple shots in succession. In almost all instances, I'm pulling just one double shot and then powering the unit down. I derive many benefits from purging; it heats the portafilter**, warms the cup, and clears out old grinds that might still be there. So there's no incentive to lose the initial boiler overshoot. And after purging I simply wait for the $2 thermometer to let me know the boiler has reached my target temperature, and away we go.

An aftermarket PID, otoh... while it would nicely regulate the boiler temperature, I would still want to purge anyway. And my understanding is that a PID like the Auber, if it fails, fails to the ON position - which would pose a significant risk if left unattended. In short, in my circumstance the PID is a non-zero cost modification, with multiple non-zero risks, for essentially zero benefit.

Thing is I just picked up an exact duplicate of my machine, more complete (has the cleaning tool) and completely clean. Almost like it had never really been used. And someone asked me to make an offer on a PID designed for this machine, so I was tempted to experiment.

**I do not keep my basket in the portafilter when doing puck preparation.

PID's and SSR's can fail open or closed. If there is a fail closed, then there are one or two thermostats on the boiler that will shut down the heating element. (Newer machines have two; older machines have one.)

Regardless, a PID seems to not be worth it for you for one-shot-only. FWIW, I've removed the PID (and the thermometer) from the Europiccola that I use for making one-shot only.