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Bill,
Several have already answered some of your questions. I will address your question is grounded or ungrounded better. The answer is it depends upon the instrument that it is being attached to. Ultimately the instrument is going to change the analog TC signal into a digital value (A/D converter.) To save money, instrument manufacturers run several TC to a single A/D through solid state relays (multiplexing.) To further save money, they often tie the negative side of the TC (red wire on a J : red does not mean + in TCs, it identifies the type of wire) into a common bus (eliminates half the relays.) If designed in this fashion, ungrounded junction (also called isolated junction) TCs must be used. If grounded junction TCs are used in this arrangement, ground loops will occur (if any conductive path exists.) Ground loops will induce error into the signals and lead to erroneous reading. Instruments designed for grounded TCs will generally work with both types, but I have seen exceptions where an isolated would not work in a grounded instrument.
The end of the story is: check with the manufacture of the instrument and save yourself a lot of time..
Joe
----- Original Message ----- From: "Bill Bradburry" <bbradburry@allvantage.com>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Monday, August 28, 2006 1:15 PM
Subject: [FlyRotary] Thermocouple grounding
This question may be for Tracy since I am using his EM2, but I assume
that the answer would be the same with other monitors
I am using the 2nd Gen oil pan which has an oil temperature switch in
it. I have removed the switch from the housing and plan to make a "J"
thermocouple and install it in the housing. If I pot the thermocouple
such that it does not touch the sides of the housing, is this considered
to be ungrounded? The opposite if it touches? If I crimp a ring
connector on the end of the thermocouple and clamp it to say, the intake
manifold, is it a grounded thermocouple?
Which type is best...grounded or ungrounded?
Also, I am not clear on the instructions. Looking at CHT1 for example,
the white wire goes to P1-9 and the red wire goes to P1-12. An astrick
says to ground the red lead pin at the connector if an ungrounded sensor
is used...does this mean connect the red lead to both P1-12 AND one of
the ground pins listed, or connect it just to one of the P1 ground pins
listed?
Sound like grounded sensors are less complicated. How would you make
certain that the sensor is grounded in the oil temp housing example I am
using above?
Thanks for the assistance.
Bill Bradburry
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