Max31855 wrong readings

ESPHome
1

Hello

Im trying to make a fireplace sensor, have a wemos d1 mini connected to a max31855 k-type
The probe i am using is this

I followed this guide to connect and configure it

the wemos seams to be able to communicate with the max31855
but the readings are off, switching the cables off the probe have no effect.
anyone have an idea of what could be wrong?

[15:51:21][D][max31855:102]: Got thermocouple temperature: 576.50°C
[15:51:21][D][sensor:092]: ‘Fireplace Temperature’: Sending state 576.50000 °C with 1 decimals of accuracy

2

Out of curiosity… Is the reported temperature with it measuring room temperature or the fireplace?

I can make the assumption of room because that temperature is high for the thermocoupler you linked. You might have a faulty thermocoupler or it is connected backwards (but you tested that).

3

Both, it makes no difference if it’s just measuring room temperature or the fireplace, there is no change at all.
The thermocoupler is brand new, unfortunately i have no other k-type to test if the MAX is working or not

4

Have a look at this thread: Thermocouple temperature readings completely off when LED strip is turned on

I significantly improved my readings with ferrite chokes and a capacitor across the thermocouple input.

Another thought: If you do not need to know the exact temperature inside your fireplace but just want to know if it’s hot or not, you could also consider a MLX90614 infrared sensor. I point this towards my fireplace from about 30cm distance and then compare the object temperature with the ambient temperature.

5

I don’t have any ferrite chokes at hand but i did put a 0.1uF capacitor across the thermocouple input and the MAX power supply.
Now when i attache the thermocouple to my fireplace it registers temperature changes. but the readings are still off by a lot.
And when i detach from the fireplace the value drops.
This should suggest the thermocouple is working right?
Can i calibrate it somehow by adding an offset?

[08:39:28][D][max31855:102]: Got thermocouple temperature: 576.00°C
[08:39:28][D][sensor:092]: ‘Fireplace Temperature’: Sending state 576.00000 °C with 1 decimals of accuracy
[08:39:48][D][max31855:102]: Got thermocouple temperature: 576.25°C
[08:39:48][D][sensor:092]: ‘Fireplace Temperature’: Sending state 576.25000 °C with 1 decimals of accuracy
[08:40:08][D][max31855:102]: Got thermocouple temperature: 576.25°C
[08:40:08][D][sensor:092]: ‘Fireplace Temperature’: Sending state 576.25000 °C with 1 decimals of accuracy
[08:40:28][D][max31855:102]: Got thermocouple temperature: 576.25°C
[08:40:28][D][sensor:092]: ‘Fireplace Temperature’: Sending state 576.25000 °C with 1 decimals of accuracy
[08:40:48][D][max31855:102]: Got thermocouple temperature: 576.25°C
[08:40:48][D][sensor:092]: ‘Fireplace Temperature’: Sending state 576.25000 °C with 1 decimals of accuracy
[08:41:08][D][max31855:102]: Got thermocouple temperature: 577.50°C
[08:41:08][D][sensor:092]: ‘Fireplace Temperature’: Sending state 577.50000 °C with 1 decimals of accuracy
[08:41:28][D][max31855:102]: Got thermocouple temperature: 578.00°C
[08:41:28][D][sensor:092]: ‘Fireplace Temperature’: Sending state 578.00000 °C with 1 decimals of accuracy

6

Possibly. Do you have any other thermometer for comparison? I am using a thermocuple in a gas BBQ and the temperature reading can easily be off by 50ºC compared to an analogue thermometer built into the lid.

ESPHome comes with a variety of filters that you can apply to fine-tune your sensor.