(B2431) wrote in
:

From: Jim Yanik
Date: 1/17/2004 10:17 AM Central Standard Time
Message-id:

(B2431) wrote in
:

From: Jim Yanik
Date: 1/16/2004 6:53 PM Central Standard Time
Message-id:

"John R Weiss" wrote in
news:Sp%Nb.73504$nt4.98595@attbi_s51:

"Phil Miller" wrote...

Yes really, on every jet engine TIT is Total Inlet Temperature.

Yep.

I ask because I came across an interesting paragraph in the
T56-A-15 service manual yesterday. Went like this...

...measures the turbine inlet temperature by means of
thermocouples...Eighteen thermocouple assemblies are mounted in
the turbine inlet casing of each engine...One thermocouple of
each assembly is connected to the turbine inlet temperature
indicating system, and...one...is connected to the electronic
datum control system. The 18 indicating system thermocouples are
connected in parallel [!!] by the indicator turbine thermocouple
harness assembly,

I'm surprised you ever doubted the truth on this one!


So, let's revise.

That's going a bit too far... Nobody can revise the Tarver
Chronicles! :-)


bimetallic thermocouples generate millivolt signals,and paralleling
them would not work.Standard practice is to series-connect them,and
compare to a reference junction.Do these assemblies include signal
processing to convert the mV signal to a digital form,which could
then be sent on a parallel bus?

--
Jim Yanik
jyanik-at-kua.net

On every aircraft I ever worked on thermocouples were in parallel if
there were two or more.

Dan, U.S. Air Force, retired


Well,I guess they aren't bimetallic thermocouples,then.
Anyone have any speculation on what sort of signal will work with a
parallel connection?

--
Jim Yanik
jyanik-at-kua.net


All thermocouples are bi-metallic.

OK, try this on for size. Jet engines use chromal-alumal (type K) and
recips use iron-constantan (type J) thermocouples.

The different bi-metal combos generate different volts/degree.

The wires are also
made of the same type of material. If you mount several thermocouples
in parallel then all plus wires match and all minus wires match. As
far as the cold junction end is conserned the hot end has one
thermocouple.

If you mount them in series you get plus -- minus -- plus-- minus
etc. This will introduce one heck of an error if it works at all.

Just connecting them to a copper wire bus makes another bi-metal
junction,too,although it's not in the area being measured,and thus more
stable.


If you ever get a chance to look at a jet engine look at the EGT ring
of thermocouples. You will see they are in parallel.

Dan, U. S. Air Force, retired

You wire thermocouples in series and the voltage outputs add
together(sum).Parallel them,and one voltage bucks against the other.
Paralleling thermocouples is like parallelling two batteries of different
voltages.One works against the other.

I worked on thermocouple calibration while in the USAF,as a PMEL
technician(Precision Measurement Electronics Laboratory).


--
Jim Yanik
jyanik-at-kua.net