PID mod for boiler/steam temperature w/ two RTDs... an idea

Equipment doesn't work? Troubleshooting? If you're handy, members can help.

#1: Post by xabean »

Ok - so I've had a Gaggia Classic Pro for ~2wks now, have dialed in my grind and can pull a shot in 25-30s. So far so good.

But I'm frustrated with A) the price of PID kits online (~$150 US), compared to kits on amazon (~$40 US) and B) how you ultimately lose steam functionality because the PID kit locks the temperature to just one set point, rather than two. C) how often my Gaggia Classic Pro cycles the heating element on and off. It can't hold temperature stable.

So here's my idea. My Classic Pro has two thermostats that cut off power to the heating elements at 145c, and 107c -- DM1288 steam thermostat, and DM1168 coffee thermostat. Both are M4 thread that screws into the boiler. One gets bypassed (the coffee thermostat) when you flip a switch.

There are thermocouples/RTD probes that have M4 thread connections. Most of the kits only have *one* RTD, and stick it on the coffee thermostat position. Why not put two RTD probes, one on the coffee thermostat position, and one on the steam position?

If the RTD this output curve for a 1000 ohm RTD here is correct, and is as linear as it appears then we can put two of the exact same RTD probes on the boiler, one connected at the coffee thermostat position, and then one at the steam position. The trick would be to make the PID controller think the temperature *at the steam position* is lower than it actually is in reality.. so we need to decrease the resistance. How do you decrease the resistance of a resistor? You jumper across it with another resistor. This gives two paths for current to flow across increasing current capacity and thus lowering resistance. With a little math; we can figure out how to intentionally offset the temperature read by the PID controller.

So lets say for the sake of simplicity the coffee temp is 100c, and the steam temp is 150c. Lets say the boiler currently read through the coffee RTD reads 100c, or 1400 ohms. So to trick a the PID into pushing the temperature up to 150c, the RTD needs to read ~200 ohms less. So when the RTD is at 1600 ohms, it needs to be 1400 ohms. With this calculator app, we can calculate the "missing resistor" parallel value paired with 1600 ohms -- 11,000 ohms: ... l-resistor

Lucky us, 11,000 ohm is a standard value for a 5% or 1% resistor! ... or-values/ and they're like $0.68 USD off digikey ... EB/1553654

So a $0.68 resistor, a $40 PID controller, $?? for two RTD pt100 temperature probes, and $11 for a 40A solid state relay? Somebody check my work to make sure I'm not crazy?

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#2: Post by Nunas »

You might find some ideas here There's another thread going where a chap is trying to use one of these on a commercial machine. It's going to take some doing. MCE - replace pressurestat with PID

But this kit is made for your machine and appears to have way more functionality than most PID kits I've seen.


#3: Post by robmack »

Holding steam temp with a fluctuation less than a degree Celsius is not required for the application (i.e. to produce steam at a reasonable pressure for frothing). If the water temperature varies during steam generation by 5 - 10 degrees Celsius, it's still going to produce steam, enough for the frothing job. However, it is much more important to maintain a constant temperature at the group head to within less than a degree Celsius (if possible). Water temperature variation during the brew will affect coffee flavour a great deal.

The lower thermostat position on the Gaggia Classic boiler is in a location closer to the group head and is at a point within the aluminium of the boiler that has constant contact with the body of water. Such cannot be said for the steam thermostat position at the top of the boiler. So, placing the RTD in the lowest position give a greater chance that it will follow and maintain water temperature closer to the desired set point than would be possible in the upper position. If you place a RTD in the upper position, it's going to read the temperature of the aluminium that has been superheated by the steam-air vapour above the waterline. I don't know for sure but I would think the thermal inertia of the metal would cause the RTD not to follow the temperature variations as closely as they might occur. What I'm thinking here (and I'm not an engineer) is that the steam vapour has less thermal inertia than the body of water, so it is able to heat up and cool off faster than the water in the boiler. I just think the upper RTD will give false readings and cause an upset in your design. I may be wrong, as I said.
- Robert

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#4: Post by baldheadracing »

Pretty much every PID has an "alarm circuit" that one uses for the steaming function in an SBDU machine ... at least, that's been the case with every PID that I've ever purchased. No need for two sensors when only one is needed.

Here's a Silvia diagram as an example: However, there are many ways to design a circuit ... this was just the first one that came up in a search.


#5: Post by JRising »

What BaldHeadRacing said...
If you don't like the alarm displaying, it's silenceable, or you can leave your steam thermostat on the boiler and never see the alarm.