|
|
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/
> Archive and UnSub: http://mail.lancaironline.net/lists/flyrotary/
--
http://www.bob-white.com
N93BD - Rotary Powered BD-4 (first engine start 1/7/06)
Custom Cables for your rotary installation -
http://www.roblinphoto.com/shop/
|
|