Received-SPF: none
 receiver=logan.com; client-ip=204.127.225.91; envelope-from=rlwhite@comcast.net
Date: Mon, 28 Aug 2006 20:39:37 -0600
From: Bob White <rlwhite@comcast.net>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Thermocouple grounding
Message-Id: <20060828203937.60df9324.rlwhite@comcast.net>
In-Reply-To: <list-1353627@logan.com>
References: <list-1353627@logan.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit


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/