MAX31855

There is something I would like to check with you guys. The item of interested is the thermocouple MAX31855. From adafruit website it states that it will work with any K type, however after further reading it states that "The MAX6675 and MAX31855 thermocouple amplifiers are not compatible with grounded thermocouples. "

Does it means that the thermocouple amplifier MAX31855 will not work with my thermocouple that I got it from RS.component which is hot junction grounded? I have insert the stock number and link for your reference. (RS Stock No. 621-2158) (http://sg.rs-online.com/web/p/thermocouples/6212158/)

One more question, from what I have seen most of the wiring is done in SPI, could it be done in I2C?

Would really appreciate your assistance in it.

Thank you very much!

The MAX31855 thermocouple signal conditioner will not work with a grounded thermocouple. The MAX31855 always performs three functions in each conversion. These are: 1) temperature conversion of the internal cold-junction reference, 2) temperature conversion of the thermocouple input, and 3) fault detection of the thermocouple.

The fault detection is the issue. In order to detect a shorted, open, or grounded thermocouple, the MAX31855 uses an array of internal semiconductor switches to disconnect the input from the conversion circuitry and connect it to fault detection circuitry. The fault detection circuitry will ground the T- side of the thermocouple and use that to detect a grounded input. It also detects opens and shorts. As far as I know, you cannot turn off this fault detection, nor generally would you want to. Being able to detect a bad thermocouple or wiring errors is a nice thing to have.

Also, you want to have the MAX31855 thermally connected to the point where the thermocouple leads attach to the board. Everywhere there is a dissimilar metal junction will generate a slight offset of the input voltage. Having the cold junction (inside the MAX31855) at the same temperature as the connection point will eliminate these errors. Place it as physically close as possible, and if possible, use a large common copper pad not connected to either inputs or any signal as a thermal plane, or something similar. This is referred to as an “isothermal” junction.

The link you provided to the RS 621-2158 part shows a length of type-K thermocouple wire with one end fused/welded. That is ideal for what you want, just dont connect either lead to ground or any other potential. The thermocouple must be left “floating”. A grounded thermocouple has one lead connected to ground, usually inside a probe. This type would not be usable. There is also a third configuration, which is an isolated thermocouple inside a probe with a shield overall, and the shield is grounded. This type would be usable since the thermocouple leads themselves are not connected to ground, but rather the shield. However, in your case, using two wires (Chromel and Alumel, type K) with a welded junction is exactly what you want to use. The welded junction is the sensing point.

Also, remember that the thermocouple is “polarized”. The Alumel wire is the T- lead, and the Chromel wire is the T+ lead. Here is a link to a color-code chart that I have always found pretty good: https://www.omega.com/techref/colorcodes.html In the case of the wire you are using, the green lead is the T+ lead, and the white is T-.

Hope this helps.