Strange CHT instrument from AS

Tomas J · #1 03-09-2014, 12:59 PM

I have just monted a combinated EGT and CHT instrument from AS...
http://www.aircraftspruce.com/catalo.../micro4egt.php

The schema is very strange for connecting the CHT, as it told to connect negative cable to a plus pin (you can see it on a picture in the link). I did connect to the pins as they fit from the cable, but that made the pointer go backwards.
Then I shifted the cabels at the point where they are spliced near to the probe. Now the pointer go the right way.
But then I start thinking about it... Perhaps now it's wrong connected at both instrument and prob, and if so, does that matter? Is there different metall also in this cables, as it is in the ones near the probe, and will that give me a wrong indication. I have not been flying yet, so I have not seen if it shows a realistic value. I just tested to heat up the ring.
Do anyone know about this...?

Tomas J · #2 03-11-2014, 06:02 AM

I've wrote to the vendor and got this answer...

"Hi Tomas,
It is confuse, but for our gauges the Red wire is always (-) negative and the Yellow, Purple, and White are (+) positive. The recommendation is that you need minimum 7ft lead wires to keep the calibration more accuracy.
See attached installation instructions and let me know if you have any questions.
Best regards,
Louis Magana
Production Manager
Hewitt Industries of LA
Tel: 714 891-9300"

So, the pins on the extension lead are shifted from factory. Need to shift them to get the pointer in right direction.

mburch · #3 03-11-2014, 08:32 AM

Hi Tomas,

You have discovered something that confuses a lot of people... the standard industry color code for Type K thermocouple wire uses yellow for the positive wire and red for the negative wire. I'd like to meet the man who thought this up and ask him what he was thinking!

mcb

bird · #4 03-11-2014, 11:45 AM

Wow, I read the install in the link, it does say red to positive on the cyl temp.
However, the letter from the actual vendor says red is negative. Red is negative on thermocouples such as type k, s, etc. if you connected red to the negative and are getting backwards reading, check to make sure the connections have not been swapped elsewhere. Thermocouple standards state that red is negative.

Bird