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From: "Al Gietzen" <ALVentures@cox.net>
To: "'Rotary motors in aircraft'" <flyrotary@lancaironline.net>
Subject: RE: [FlyRotary] Thermocouple grounding
Date: Mon, 28 Aug 2006 12:33:54 -0700
Message-ID: <000001c6cad8$e645ab30$6400a8c0@BigAl>
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Bill;

I just found that RWS is closed until Wed.; so maybe I can help you with
this.  'Grounded' means the junction end of the TC is in contact with a
surface that is at ground reference for your system.  If the TC is =
potted in
insulator and not touching a grounded surface, then it is 'ungrounded'.  =
If
you clamp the junction to the intake manifold, then it is grounded (I =
assume
the engine has a ground strap).  I would clamp the junction directly to =
the
surface you want to measure; and put some thermal insulation between it =
and
the clamp - apiece of kool-mat, gasket material, etc.

If the TC is not grounded, then the pin that the red lead goes to should
also have a jumper to a spare ground pin.

I just went through this with J TCs I use to measure coolant temps on a
section of aluminum tube.  Since the section of tube is isolated by =
sections
of hose on both ends, I assumed it would be ungrounded, and I jumpered =
the
red lead ends.  Apparently the coolant provided enough of a conductor =
that
the readings were clearly off.  I tied a wire from the tube to ground, =
then
went in and cut out the ground jumpers, and now things appear correct.

Best,

Al


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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