Friday, January 29, 2016

Teardown of Fluke 52 II thermocouple thermometer

A small board responsible for cold-junction compensation (CJC) during thermocouple measurement is a curious part of this thermometer.  Have you ever designed cold-junction compensation for a thermocouple front end?  Let's see how Fluke did it.

Main board is visible.


Progressively removing the case. The cold-junction compensation board is not yet visible. It sits below the main board and behind the black plastic plate with slots for thermocouple connector blades.
The cold-junction compensation board sits below the main board.  The contact between the pads and thermocouple connector blades forms the cold junctions, which the instrument has to compensate for.  Since cold-junction compensation has its own daughter board, and it's somewhat  insulated from temperature gradients that may be generated by the main board.


Interestingly, the CJC board is made with metal (copper) substrate.  It promotes heat transfer and reduces temperature gradient between the cold junctions and the sensor which measures their temperature.  (The CJC temperature sensors are better visible on another photo below.)

Side view.

Thermocouple connector added to show where the contact between thermocouple materials and copper pads on the cold-junction compensation board takes place.

Cold-junction compensation sits on its own daughter board, and it's somewhat insulated from temperature gradients that may be generated by the main board. 

CJC board. Temperature sensors visible. Thermocouple plug added to the picture to show where blades make contact.
Main view of the cold-junction compensation board.
  • SOT-23-3 ICs between each pair of the thermocouple pads are the sensors which measure the cold-junction temperature.
  • An EEPROM (AT25010 ) suggests that the CJC board is calibrated separately from the main board and the coefficients are stored in this EEPROM.
  • LT1034 is a dual voltage reference

Each blade of a thermocouple plug has an individual spring clip that presses the blade to the contact pad on the board.

Related

Maxim App Note 4026  Implementing Cold-Junction Compensation in Thermocouple Applications
Discussion about cold-junction compensation in this Analog Dialogue article (2010).