internal temp output reading?

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savard
 
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internal temp output reading?

Post by savard »

Hello Adafruit Representative,

I am using the serial thermocouple code (below) offered by Adafruit with sensor MAX31855 and K-type thermocouple.

------------------------------------------------------------------------

This is an example for the Adafruit Thermocouple Sensor w/MAX31855K

Designed specifically to work with the Adafruit Thermocouple Sensor
----> https://www.adafruit.com/products/269

------------------------------------------------------------------------

As I run the code, the output is displayed as:

Internal Temp = 25.56
C = 25.25
Internal Temp = 25.56
C = 25.25
Internal Temp = 25.56
C = 25.25
Internal Temp = 25.56
C = 25.25
Internal Temp = 25.56
C = 25.00
Internal Temp = 25.56
C = 29.50

My question:
1) Why are two temperatures being displayed? Is there an internal temperature sensor located in the board? I am using Leonardo board (ID: 849) and Data Logging Shield (ID: 1141).

Please kindly respond at your convenience.

Warm regards, Savard

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adafruit_support_bill
 
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Re: internal temp output reading?

Post by adafruit_support_bill »

Yes. There is an internal temperature sensor in the thermocouple amplifier. The thermocouple signal is proportional to the temperature differential over the length of the thermocouple. In order to know the temperature at the tip, you need to know the temperature at the other end.

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flosk8
 
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Re: internal temp output reading?

Post by flosk8 »

I have currently a temperature of 7.00 for the thermocouple and an inner temperature of 22.00.

So how can I calculate the temperature at the thermocouple tip?

Thanx

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adafruit_support_bill
 
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Re: internal temp output reading?

Post by adafruit_support_bill »

If the MAX31855 says 7.00, then that is the tip temperature. The MAX31855 chip takes the internal temperature into account to generate that number.

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flosk8
 
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Re: internal temp output reading?

Post by flosk8 »

How can that be? I have a temperature of around 22 C. And the thermocouple tells me it's 7.

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adafruit_support_bill
 
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Re: internal temp output reading?

Post by adafruit_support_bill »

Are you using a K-type thermocouple?
If you hold the tip of the thermocouple between your fingers, does the temperature increase?

Post photos showing your soldering and connections to the board.

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flosk8
 
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Re: internal temp output reading?

Post by flosk8 »

Mhhhh it seems not to be the soldering.

I have a two channel MAX31855 with two thermocoules one is showing the right temperature of about 20 and the other one only 7. It has to be the thermocouple and not the MAX31855 . Because when I change the ports its still the one that shows only 7.

I wonder whats going on here because they are both K Type with green and white pins.

Any Idea?

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adafruit_support_bill
 
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Re: internal temp output reading?

Post by adafruit_support_bill »

It could just be a mismarked probe. The color code for K-type in this country is -red/+yellow. But we have seen some samples where the colors are reversed.

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flosk8
 
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Re: internal temp output reading?

Post by flosk8 »

Nope I definetly poled them right. Green=+ White=-

Because its increasing temperature when i hold it in my hand.

But still its about 13 degrees to less.

Can it be that a thermocouple is buggy? This one has a two meter cable, can that be the problem? The 2 port MAX31855 is for 3,3V.

I also have the problem that even if the temperature remains the same I get changes up to +/- one degree when I read every second. What could that be?

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adafruit_support_bill
 
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Re: internal temp output reading?

Post by adafruit_support_bill »

Probably either a mis-marked (i.e. not a K-type) or improperly constructed thermocouple.
This one has a two meter cable, can that be the problem?
If it is not a K-type extension cable, that can definitely be a problem.
The 2 port MAX31855 is for 3,3V.
That shouldn't make any difference. The thermocouple voltages are in the microvolt range.
I also have the problem that even if the temperature remains the same I get changes up to +/- one degree when I read every second. What could that be?
That could be just noise. Since the amplifiers need to be very sensitive to work with microvolt-level signals, they are also prone to picking up noise. It is common to take the average of several readings to minimize the noise.

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