Thermocouple temperature readings

contactform · 2016-07-23 13:26:50

Is it possible to read the temperature off of a J type thermocouple (2 wire) using the Raspberry Pi with the Unipi add-on board. If so how/ with what function blocks. I am trying to get a graphical HMI to display the temperature data from a plastic extruder, and it already has J type termocouples in place, so I just want to pull the voltage off of those.

jaroslav_sobota · 2016-07-23 18:00:54

No it is not. Reading a thermocouple needs a specialized amplifier, which is not present on the UniPi. Here is a link to a topic on UniPi forum, which deals with thermocouples: http://unipi.technology/forum/viewtopic.php?t=124

You might be interested e.g. in the Artila RIO-2018 Modbus module, which you can use to read up to 3 thermocouples via Modbus TCP.

tjbostian · 2016-07-23 21:00:26

Then how does a one wire thermocouple (like in the thermostat example in the REXsample folder) work?

jaroslav_sobota · 2016-07-24 15:17:29

1-Wire temperature sensors are not thermocouples. The most common 1-Wire temperature sensor is DS18B20 and it is a digital device which measures the temperature and provides the data via the 1-Wire communication bus. 1-Wire is a trademark of Maxim Integrated.

On the contrary, a thermocouple (type J, K, etc.) is an analog device which requires circuitry to convert temperature reading into a digital form. One of the most common thermocouple-to-digital converters is Maxim MAX31855.

The UniPi contains a 1-Wire bus master, therefore it is possible to connect 1-Wire temperature sensors directly. But it does not contain any thermocouple-to-digital converter, therefore connecting thermocouples to UniPi is not possible.

Hope this helps! If you need additional information, just let me know.

tjbostian · 2016-07-24 20:06:47

Could you use the analog inputs on the Arduino to measure the voltage off the thermocouple? I have limited space in this project, and I'm trying to keep the number of breakout boards and daughter boards to a minimum. So by hooking up the Arduino like in this example https://www.rexcontrols.com/articles/re … ts-arduino could I read analog data from it?

jaroslav_sobota · 2016-07-25 14:32:34

You would need an amplifier again, e.g. like this one: http://www.ebay.com/itm/Quad-Channel-J- … 1438662460 Should work almost out of the box.

But it involves 2x A/D conversion, which is uneffective, so it's better to use e.g. this one: http://www.playingwithfusion.com/produc … hp?pdid=31 which you can connect directly to the RPi via SPI (when UniPi is not present) or to the Arduino via SPI and then via USB to RPi (when UniPi occupies the pins of the Raspberry Pi). In both cases a bit of hand-coding will be necessary.

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#1 2016-07-23 13:26:50