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My experience using Tracy's EM2 with home made J thermocouples is that
they seem to work OK if at least one of the TC's is grounded. I didn't
run the extra ground wire at the connector because I was expecting to
clamp the TC to something that was grounded. As it turns out some of
the points I wanted to measure were not grounded. The readings seemed
to be OK and stable if any of the other TC's were grounded. I tested
all of the TC's at one data point (boiling water) and the readings were
within a degree or two of the boiling point. The TC under test was not
grounded.
The J thermocouples were made by twisting the two wires together and
melting them in an ox/acetylene flame.
Bob W.
On Mon, 28 Aug 2006 21:52:34 -0400
Joe Ewen <jewen@comporium.net> wrote:
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
>
>
> --
> Homepage: http://www.flyrotary.com/
> Archive and UnSub: http://mail.lancaironline.net/lists/flyrotary/
> --
Homepage: http://www.flyrotary.com/
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--
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