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Date: Mon, 28 Aug 2006 21:52:34 -0400
From: Joe Ewen <jewen@comporium.net>
Subject: Re: [FlyRotary] Thermocouple grounding
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
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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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