Thermocouples

Thermocouples are used on aircraft to measure brake temperatures and engine EGT (Exhaust Gas Temperature). The heating of two dissimilar metals (Chromel and Alumel) creates a small DC millivolt signal. Higher voltages equate to higher temperatures.

We had an aircraft (767-300) with a long history of the right strut brakes indicating "0" (with the left side brakes reading 1 or higher) after landing. The 767 doesn't show actual temps, just a scale 1-9.

The absolute first thing to do with a brake temp split is to grab a laser temp gun and measure the brake rotors. If the measured temperatures match...... you have an indication problem. If the brake temps do not match...... you've got a control (application) problem.



With this aircraft, the right strut brakes read in the mid 200's Fahrenheit. The left side brakes were over 600F. EICAS indication was 2 for all brakes on the left and 0 for the right.


I found a cheap signal generator on Amazon for troubleshooting this problem.





I wasn't sure if the damn thing would even work, but surprisingly it did. I simulated a 20Mv signal and fed that into the #7 brake sensor plug. I used the positive lead on the Chromel pin (1) and negative on the Alumel pin (3).




We had a reading of 5 on EICAS. (A 10Mv signal gave us a 2.)




This simulation, along with with the laser gun readings ruled out indication as the fault.

I'm sure this simulation method will work with engine EGT indications also.

When replacing thermocouple wiring, how do you know what color (green or white) is what material? 
  
     
  
Chromel (green) is magnetic. It should be attracted to a magnet.

We found the opposite for the pins. Alumel pins seemed to have a greater pull to a magnet.
 

MiliVolt Simulator - Proper Connection