Thermocouple for 500F, P1?
T Chap
Posts: 4,223
Hello all. What is your go to thermocouple for P1, up to 500F. I am working on a powdercoating system that has an IR lamp several inches from the part to bake. I want to mount a thermocouple at the same distance from the lamps to the object being heated and try to maintain 400F. The lamps show about 750F on a hand held temp sensor aimed at the lamps elements. I have tested holding the lamps near the metal for 5 minutes and can exceed 400F aiming the temp sensor at the metal after it has heated, this means the metal will get too hot if not regulated. The other thing is the lamps are automated and move about 48" back and forth so the thermocouples move as well. That means either about 8' leads from the thermocouple back to the main P1 control board OR if long leads cant work then a dedicated P1 located on the lamps so the thermocouple leads are 12-16".
Any suggestions on the simplest thing I can order off the shelf? Thanks
Comments
Regular cheap Type-K should do. There's nothing special about the probe, it's just the two particular wires joined together at the tip. Cutting and rejoining can be done repeatedly until you're out of length. That means you can screw it, glue it, weld it to the material of interest.
The trick to low noise is to ensure the converter electronics is electrically floating. Preferably fully isolated.
PS: A thermocouple generates its volts where there is a temperature gradient - so that happens down the wires at distance from the target surface. It actually happens with all metals but is only measurable when using opposing metal types for the two wires.
Thanks, I think I should have added what is the go to ADC for these or is there a specific method to interface a thermocouple to P1. I assume it is just a voltage that is getting higher with temp and requires a lot of gain before an ADC.
ensure the converter electronics is electrically floating
As in have a separate LDO for the converter board for cleanest DC?
Yep to all. There is specific chips for the job, they have all the fancy bits built in.
Assuming the thermocouple is only one of many connections to the controller, best way is to use an isolated DC-DC converter for thermocouple/ADC supply and serial optocouplers for data link back to the controller. An SPI interface to the ADC is common.
J or K type will work for those temps. paired with a 31855 module if you want an easy application.
Great, thanks for the tips. Here is a K version on digikey with 31855 and thermocouple included, high temps.
https://www.digikey.com/en/products/detail/dfrobot/DFR0558/9559246
I'll get this tomorrow and check it out.
I use the sparkfun SEN-13266, with the omega connector PCC-SMP-K-(K)
If you don't already have code for reading the MAX31855, I just cleaned-up and moved my P1 object to ObEx. I wrote this a long time ago for an HVAC project.
-- https://obex.parallax.com/obex/max31855/
I had the adapter and TC out recently because I've been doing a bunch of reflow soldering in a toaster over and wanted to record the "profile."
Fantastic. I should have it tomorrow. Will have it running soon I hope.
I'm updating the objects (I have P1 and P2), so get them tomorrow as ObEx is a PITA when it comes to editing one's files on the site.
I have a board made up with eight of those spackfun SEN-13266. All SPI lines are bussed together. I just hit the chip select when I want to read. I used my own version of the JonnyMac P1 spin object (Thanks JM). Note, with long wire runs you'll get alarms on the 31855. You can just not use the alarms. If you use the proper gauge of thermocouple extension wire it should not be a problem.
Thanks. I was thinking to mount the 31855 module near the heating elements.
I just downloaded the current obex version and realize it is SPI. The module I bought with 31855 is I2C, which means they have put some controller on the board to convert it to an i2c module. I'll get the sparkfun versions next week, and see if I can get their i2c code running in the mean time.
I think I will pull off their controller chip, and add hack in some wires. It seems like too much a chore to chase down all their C>i2c code and dissect it.
This is the adapter I have. I have tested it with my P1 and P2 objects.
-- https://www.sparkfun.com/products/13266
There are inexpensive adapter boards (SPI) on Amazon, though it doesn't have the bladed connector like the Sparkfun PCB
-- https://www.amazon.com/UMLIFE-MAX31855-MAX6675-Thermocouple-Temperature/dp/B0B19M4497
There's not much to it. I found the code for DFRobot board on GIT, but it's not great -- no luck finding a generic I2C spec. They seem to think the only microcontrollers available to their customers fit into the Schmaschmino family. Still, since the code is so simple, I'll give it a whack as I'm watching TV tonight.
Thanks Jon. It’s much easier for me to hack the board than read through endless C code to understand the i2c. Thanks for posting the object, I’ll update when I get the module.
I've made a controller for a relow oven with a P1 some years ago: https://forums.parallax.com/discussion/154830/thermocouple-measurement/p1
The kit finally arrived from Digikey. I hooked it up to my custom Arduino Uno board and ran the demo with a logic analyzer connected. As suspected, the controller on the interface holds the SCL line low while getting the reading (via SPI) from the MAX31855. The delay is on the order of 160us. I did some cleanups to my P1 I2C library to ensure clock stretching works. I also updated it to leave the data line low at the end of a write -- this seem to make the LA interpretation better.
Here's the output from the DFR thermocouple module -- it seems to read a bit high (this probably the TC, not the board).
I attached the LA captures from the Arduino and the P1. The Arduino I2C is 100kHz. The P1 I2C is in Spin so it's slower; about 20kHz to account for clock-stretch checks.