Posted Wed Feb 13, 2013, 7:53pm Subject: Custom Thermocouple for Gaggia Classic
Everyone knows, before you can PID your espresso machine, you have to have a thermocouple to sense the water temperature. I started a thread on installing a custom PID into my Classic, but I needed a thermocouple.
Granted, thermocouples are a dime a dozen out on the market. You can get them as cheap as $5 to several hundred dollars. No matter what price range, none of the readily available thermocouples out there tickled my fancy. They are either too big, too small (very rare), not liquid-tight, too long, etc.
First and foremost, and it shouldn't have been the first priority, but I didn't want to completely disassemble my machine to drill out and re-tap my boiler. These boilers are aluminum and the job would take about 2 minutes, but the disassembly and reassembly time was the deal-breaker for me.
What I DID find was a RTD on Auber's website which was advertised as being more sensitive and better performance than thermocouples. It also comes in M4-0.7, which is what the Gaggia takes (looks like same with Silvia). So I bought one of these bad boys for about $25+s/h and it turns out it is basically a thermal resistor (resistance changes as temperature changes). This wouldn't work in place of a thermocouple, which develops a VERY small voltage as temperature changes. I could build the electronics to capture resistance but that would be too much work.
So here's what I did. My first attempt was to find a M4-0.7 hex-head screw in brass. I checked the usual suspects (McMaster) and as far as they are concerned, don't exist. However, they do carry a M5-0.8 which I could turn the threads down and re-tap to M4-0.7. That would work!
So, --the first revision-- of my thermocouple had issues, mostly physical. Basically, the head of the M5 screw was too short and could not secure the cable and shield. I also had to use high-temp, "food-friendly" RTV to seal the cable. Long story short, it was messy.
Second attempt, looks much like the one in the photo, except no shrink tubing. Version 2 also had issues, not physically but electrically. Since the probe is long, the thermocouple element was prone to touch the thermocouple body and short with the shield. This isn't an issue as long as the shield did not touch anything. That wasn't going to happen.
So, the third attempt, like they say, "third time's a charm".
Features: - Brass body for better thermal conductivity versus most thermocouples out there are stainless steel. I've purchased several types of thermocouples (stainless steel) the last few months and I'll tell you something. I would not trust those in my boiler. Not all stainless steels are created equal. Most of the stainless steel thermocouples out there (cheap ones) are low grade stainless steel, which will rust. I can post pictures if anyone cares to see. These are just dipped in water for testing so only little exposure will oxidize the steel. - M4-0.7 thread to match the Gaggia Classic - BIG plus! I don't have to remove anything to install it. Just dump the water in the boiler. Reach in and unscrew the thermostat, teflon tape the new thermocouple and reinstall in-place. Easy-peasy. - Long probe, so it'll reach into the boiler to get closer to water temperature than boiler-body temperature. - Of course, designed to be liquid-tight. - Internally, electrically isolated to eliminate shorting. - Enhanced thermal conductivity. - Proper strain relief and crimped for a secure fit.
I have not installed this in my espresso machine yet, since my PID is still under development. It's mostly software and menu system that I'm working on at the moment and work is keeping me really tied down. Busy at work is a good thing - keeps food on my table.
Posted Wed Feb 13, 2013, 8:05pm Subject: Re: Custom Thermocouple for Gaggia Classic
Although I haven't installed this in my Classic yet (still waiting for a more complete PID system), I have tested it in a pot of boiling water. Water will not go past 212F so that was my criteria.
Maybe not everyone knows that every thermocouple is different. Because the theory and how thermocouples are made, it is not possible for 2 thermocouples to be equal, not intentionally anyway. So, I've found this one to be about 2% optimistic, giving me 216F. So, in my PID firmware, I've included a calibration value. Of course, after calibration, it held steady at 212F +/- 0.6F.
Posted Wed Feb 13, 2013, 10:28pm Subject: Re: Custom Thermocouple for Gaggia Classic
Just to clarify, you are talking about PID, but not a PID controller? You are talking about using PID algorithms on an Arduino? If you are using a PID controller, then the RTD sensor function is selectable as are various thermocouple types. The PID controller can send enough current to register a resistance change in the RTD. IIRC, you are going to set up an Arduino.
I am not as clear on how you are using the thermocouple. It appears long enough to go into the boiler, but the boiler is not set up for that as the existing tstat site is a blind tap just to hold the tstat in place. In addition, if you are going to drill and tap into the boiler, will the brass on aluminum seal. The tstat has a brass threaded post and there have been threads where the stud was broken off of the tstat. It is pretty soft and does not tighten well, but does not have to as OEM blind tap. It is also placed with thermopaste in the threads and under the tstat, but not into the boiler. Is that food safe if you are entering the boiler?
A little more specifics on how you made the thermocouple bolt and connection. Did I understand correctly that you used the M5 and rethreaded? What is the part, where obtained and how much?
Posted Wed Feb 13, 2013, 10:54pm Subject: Re: Custom Thermocouple for Gaggia Classic
It is possible I don't know what I'm talking about since I have not removed my thermostat as of yet, since my PID is not ready to be installed at thus time. I suppose my assumption is that the sensor element SHOULD be as close to the water as possible to actually give you a true temperature reading. If not, I can remake the thermocouple body.
I am not speaking about any off-the-shelf or custom PID. This thread is primarily about the thermocouple. I do have a PID on my powder coat oven that can do all that but for my application, I have no real desire to make it more complicated than I need to, which there are no ready-made RTD amplifiers on the market that will output a digital (I2C, SPI, or serial, etc) so I decided to go with a thermocouple for my PID implementation.
The V3 thermocouple in the picture is a full custom thermocouple, designed and manufactured in my shop. The first one was made from a M5 bolt and still sitting on my desk. The V2 was destroyed while trying to retrieve the sensor element. V2 was also machined from 3/8" brass hex stock. So what you see in the picture cannot be purchased off-the-shelf.
Once I get the firmware closer to done, I'll talk more about my PID algorithm. And yes, I am using an Arduino Nano for this project. I started with an Uno but it would be too big (why bother? It would be the same size as a store-bought PID). To make things more compact, a Pro Mini would be the answer - if it wasn't that it doesn't have enough I/O for all the features I'm implementing. Yes, it will be a psuedo-multitasking Espresso controller. And no, it won't make the coffee for you. That's all I'll share about the controller for now.
Anyway, whether this thermocouple body will fit or not, I'll find out when I pull the thermostat. Redesigning and turning the new part will only take an hour or so. If what you're saying is true about the boiler not setup to accept the probe-point, then the new part will be even easier and cheaper to make.
Posted Wed Feb 13, 2013, 11:12pm Subject: Re: Custom Thermocouple for Gaggia Classic
Thanks for the clarification. The Tstat does not enter the boiler cavity. I have one in hand and used the RTD sensor in its place. You could probably drill through, but I would be concerned about the ability to seal. Also, the boiler stat is surrounded by the element. Decent picture here with fairly short stud.
If you are going into the boiler, then you may want to make the probe long enough to try to get thermocouple tip well into the boiler, to get water vs boiler wall temperature. I am not sure that it really matters exactly where the thermocouple is as the water temperature only really matters at the puck, the rest is just an offset of that. Boiler water temperature is still not exactly puck water temperature. From what I read, Gaggia placed the boiler stat in a very strategic/reactive place on purpose.
I think that many of us would like to find an expensive thermocoupe connector to use, so if you come up with an easy bolt and assembly it would be interesting to know. I would like to have access to a proper brass bolt and thread size. It can be drilled out and one of the inexpensive thermocouples inserted. How are you attaching bolt to thermocouple? Thermoconductive epoxy such as Arctic Silver, thermoconductive and not electrical conductive.
Posted Fri Mar 8, 2013, 12:13am Subject: Re: Custom Thermocouple for Gaggia Classic
I finally had a chance to run a new thermocouple body. It's basically the same as the originally posted body without the ball-extension. The set up is the same. I'll be inserting another thermocouple element into the cavity, seal the cavity and crimping the end for a nice secure fit.
Obviously, I haven't tested this one, since as you can see, it's just the body. I hope to have some time tomorrow to finish it up and do some testing.
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