Heatmeter with only Thermocouples and a 16x4 LCD?

Hi there!

I'm thinking of building a HeatMeter to monitor my homebuilt Watersmoker during the long PP sessions. I really like the HeatMeter concept, with the raspberry pi doing the web page stuff and the sensor / PID work offloaded to the AVR / Arduino.
However, I am an absolute fan of thermocouples. Therefore, I would like to build a HeatMeter with only Type K thermocouple probes. I thought of using a few of the Max 31855 I already have lying on my desk.

Now to the questions:
- Did someone already do that, saving me from having to do the modification work? ;-)
- What is the maximum number of probes the HeatMeter Software is designed for? I know, I will have to adjust the Arduino part to the 31855s, but I thought of sticking with unmodified Raspberry software.
- What about the LCD? I have a 16x4 lying here. Someone already adjusted the Arduino software for that?
- And finally: As I have to adjust the PCB layout anyway: I have a standard Arduino lying here and thought of using this, adding a separate PCB for sensors and display… I know, it won’t be the smallest build, but that way I might be able to work with a single sided PCB I can easily etch at home.

Thanks,

Daniel

The Heatermeter has a maximum of 4 probes.

As for using all K-thermocouple it can be done using a breakout board instead of the on board probe jacks, I make one(thermocouple breakout board). The thermocouples are analog and not digital, so im not sure if the max would work, Bryan would need to chime in to answer that question.

As for a 16x4 lcd, yes they are compatible with the software and has a setting for them. I happen to be building one for someone, atm.

As for combining the Arduino and Pi, I don't think it will work without some major code and HM rework

You're in for quite a change. The current Thermocouple sensors are analog so they just interface directly into the ADC. You're looking at SPI bus digital chips which mean you need to add the library and add a new probe type.

There are at most 4 probes (all can be TC or thermistor) designed at both the ATmega and Pi level. The "database" (RRD) holds only 4 probes, the service daemon only understands 4 probes, the website is designed at every level for 4 probes. You'd have a giant job making it generic to support more.

16x4 display is supported already via software selection

You could use an Arduino, the HeaterMeter board is really just an Arduino with stuff added on and the USB interface removed. The Pi just runs on the RX TX lines.

Thanks for your detailed answers.

I am fine with 4 probes, I won't fiddle with that. Now I also understand why the HeatMeter uses a simple preamplifier only for the thermocouple - you can easily exchange resistive and thermocouple probes and just change the calibration curve...
On the other hand, the 31855 would enable doing the AD-conversion on a separated board, away from all the other RF noise (servo, pulse width for the fan, etc.).

I will have to think about this a bit more. The fastest way would be to just take the HM Board as kit and add a breakout board for 4 TC probes, as John suggested... This guarantees that the project is finished within a reasonable amount of time. On the other hand, I really like the idea of having the AD conversion done right next the the TC connectors, separated from the rest of the project - with a breakout board similar to the one proposed in this thread (really neat, simple one-sided PCB, or this one for Arduino). But I tend to let the projects sit on my bench for quite some time, after having finished the hardware side - I am just not the fan of doing much software work.