making thermocouple temp probes?

[JazzMan]

I've got some 1/8" OD 316 stainless tubing on order from
McMaster-Carr, already have some thermocouple wire rated
for 2K°F, and will be buying some fittings for mounting
the probes in the exhaust manifold of my engine. My thought
was to TIG one end shut on a 2" length of the tube and use
a stainless compression fitting to lock the probe in place.
I have several questions. One is, do I need to pot the
thermmocouple wire inside the tube? If so, with what? Would
it matter if the potting agent was conductive? Do I need to
somehow weld the thermocouple junction to the end of the
tube being used as the probe body? Or would simple contact
be suffient?

For reference, the engine's been modified and I've been
editing the fuel maps in the ECM. Knowing the EGT at
individual cylinders allows me to address flow balance
issues in the intake manifold, for instance.

JazzMan
--
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**********************************************************
"Rats and roaches live by competition under the laws of
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live under the laws of justice and mercy." - Wendell Berry
**********************************************************

 

[Chris]

I've got some 1/8" OD 316 stainless tubing on order from
McMaster-Carr, already have some thermocouple wire rated
for 2K°F, and will be buying some fittings for mounting
the probes in the exhaust manifold of my engine. My thought
was to TIG one end shut on a 2" length of the tube and use
a stainless compression fitting to lock the probe in place.
I have several questions. One is, do I need to pot the
thermmocouple wire inside the tube? If so, with what? Would
it matter if the potting agent was conductive? Do I need to
somehow weld the thermocouple junction to the end of the
tube being used as the probe body? Or would simple contact
be suffient?

For reference, the engine's been modified and I've been
editing the fuel maps in the ECM. Knowing the EGT at
individual cylinders allows me to address flow balance
issues in the intake manifold, for instance.

JazzMan
--
**********************************************************
Please reply to jsavage"at"airmail.net.
Curse those darned bulk e-mailers!
**********************************************************
"Rats and roaches live by competition under the laws of
supply and demand. It is the privilege of human beings to
live under the laws of justice and mercy." - Wendell Berry
**********************************************************

Like your motto, Jazz.

Exposed thermocouple beads can be grounded at the point they're
measuring (sometimes there's no other way), as long as your instrument
measuring the thermocouple is floating (like a battery-powered
thermocouple meter). As long as you've got a floating meter, a
grounded bead and a SS compression fitting is a great idea. You might
want to put a sliver of mica between the compression fitting and the
T/C bead to ensure you're not shorting out the T/C by grounding it at
more than one point. Other than that, you're in good shape.

Sounds like you've got a good plan.

Good luck
Chris

 

[JazzMan]

Like your motto, Jazz.

Thanks! I stole it from someone else a while back. I have no
idea who the wendel guy is, but the saying just seemed to embody
an ideal I thought was worth knowing.

Exposed thermocouple beads can be grounded at the point they're
measuring (sometimes there's no other way), as long as your instrument
measuring the thermocouple is floating (like a battery-powered
thermocouple meter). As long as you've got a floating meter, a
grounded bead and a SS compression fitting is a great idea. You might
want to put a sliver of mica between the compression fitting and the
T/C bead to ensure you're not shorting out the T/C by grounding it at
more than one point. Other than that, you're in good shape.

Sounds like you've got a good plan.

I appreciate the advice. The t-couple wire pair is insulated in
a single woven material, and each individual wire is also
insulated. I'll have to remove the main insulation to get
the two wires to fit inside the tube. The potting agent would
be used to seal the two wires going into the tube to keep
water and dirt out. The patting agent I looked at in the catalog
has aluminum in it, so it would presumably be conductive. I
don't know if the agent would soak through the woven insulation
around the individual wires.

JazzMan
--
**********************************************************
Please reply to jsavage"at"airmail.net.
Curse those darned bulk e-mailers!
**********************************************************
"Rats and roaches live by competition under the laws of
supply and demand. It is the privilege of human beings to
live under the laws of justice and mercy." - Wendell Berry
**********************************************************

 

[Larry Brasfield]

message ....

I appreciate the advice. The t-couple wire pair is insulated in
a single woven material, and each individual wire is also
insulated. I'll have to remove the main insulation to get
the two wires to fit inside the tube. The potting agent would
be used to seal the two wires going into the tube to keep
water and dirt out. The patting agent I looked at in the catalog
has aluminum in it, so it would presumably be conductive. I
don't know if the agent would soak through the woven insulation
around the individual wires.

I would not worry about the potting agent, unless
it was sold as a moldable conductor. The aluminum
is probably an oxide, or particles that are oxidized
on the surface, and not particularly conductive. The
thermocouple acts as a low voltage, low impedance
source, so a little stray current would not change
the sensed voltage enough to matter anyway.

 

[Dick]

Don't know if this info is of any help but I made up a lower
temperature probe, based on a Pt100 sensor & thin walled 3/16" brass
tubing, by brazing in a very short length of brass rod to block off one
end of the tube.
Slid the sensor into the tube as far as it would go and back
filled the rest of the tube with a very fine, dry sand, then sealed open
end of tube with high temperature epoxy adhesive.
The sensor leads & joints are protected by some lengths of
narrow high temp sleeving and the sand was simply garden sand, washed
thoroughly, dried in an oven, sieved through a fine grade kitchen sieve
& finally ground very fine in the pestle & mortar usually used for herbs
& spices (best to do this when herself is out of the house !). To get
the sand past the sensor leads & into the tube fix a small paper cone
around the open end of the tube, fill it with the fine sand and then
gently tap the tube .........
I've been using this arrangement for some time as the sensor for
a PID temperature controller & it works very well, strong mechanically &
with a fast thermal response time.

 

[Bill]

I've got some 1/8" OD 316 stainless tubing on order from
McMaster-Carr, already have some thermocouple wire rated
for 2K°F, and will be buying some fittings for mounting
the probes in the exhaust manifold of my engine. My thought
was to TIG one end shut on a 2" length of the tube and use
a stainless compression fitting to lock the probe in place.
I have several questions. One is, do I need to pot the
thermmocouple wire inside the tube? If so, with what? Would
it matter if the potting agent was conductive? Do I need to
somehow weld the thermocouple junction to the end of the
tube being used as the probe body? Or would simple contact
be suffient?

For reference, the engine's been modified and I've been
editing the fuel maps in the ECM. Knowing the EGT at
individual cylinders allows me to address flow balance
issues in the intake manifold, for instance.

JazzMan
--

No, you do not need to pot the wire to the tube but it would be a good
idea to keep out water, grease, etc.

If the potting agent is conductive and if it causes a low impedence
between the 2 wires then you could get a secondary junction there. I
doubt that will be a problem with what you're doing.

You do not need to weld the TC junction to the tube cap although that
would probably give you a faster and more accurate measurement assuming
electrical noise is not an issue. A grounded junction would have less
susceptability to thermal gradient measurement errors in this
application.

Make sure you are getting sufficient immersion depth into the exhaust
stream otherwise you could get an error in your measurement due to
thermal shunting. You might try locating your sensor into the stream
and then move it slightly to get more or less immersion. If slight
changes in immersion depth change the indicated temperature by too much
it could mean you have insufficient immersion into the exhaust stream.

If you are getting an indication that you have insufficient immersion
you could try exposed junctions where you do not cap weld the tube but
rather let the wire junction be exposed to the exhaust stream and just
use the tube for mechanical support. You would need to seal the tube
at the cold end to prevent exhaust leaks or use a ceramic potting at
the hot end.

Bill Schuh
Watlow

 

[Jack Hayes]

I've got some 1/8" OD 316 stainless tubing on order from
McMaster-Carr, already have some thermocouple wire rated
for 2K°F, and will be buying some fittings for mounting
the probes in the exhaust manifold of my engine. My thought
was to TIG one end shut on a 2" length of the tube and use
a stainless compression fitting to lock the probe in place.
I have several questions. One is, do I need to pot the
thermmocouple wire inside the tube? If so, with what? Would
it matter if the potting agent was conductive? Do I need to
somehow weld the thermocouple junction to the end of the
tube being used as the probe body? Or would simple contact
be suffient?

For reference, the engine's been modified and I've been
editing the fuel maps in the ECM. Knowing the EGT at
individual cylinders allows me to address flow balance
issues in the intake manifold, for instance.

JazzMan
--
**********************************************************
Please reply to jsavage"at"airmail.net.
Curse those darned bulk e-mailers!
**********************************************************
"Rats and roaches live by competition under the laws of
supply and demand. It is the privilege of human beings to
live under the laws of justice and mercy." - Wendell Berry
**********************************************************

The exact TC you need are available commercially, normally they are sold
with a plug connector.

Jack

 

[JazzMan]

The exact TC you need are available commercially, normally they are sold
with a plug connector.

Jack

Got a link? I've looked through the McMaster-Carr catalog
and typically their sensors are far too long (clearance issues
in the engine bay around the manifolds) and too large in diameter.
I want to run as small a diameter as possible so as not to
affect exhaust flow. My perfect sensor would be 1" long and
less than 1/8" diameter, with a 1/16" NPT base so that I
could drill and tap the edges of the manifold flanges at
the heads and install the sensor directly into the exhaust
stream. I've never seen anything like that except in photos
of complex test equipment setups.

JazzMan
--
**********************************************************
Please reply to jsavage"at"airmail.net.
Curse those darned bulk e-mailers!
**********************************************************
"Rats and roaches live by competition under the laws of
supply and demand. It is the privilege of human beings to
live under the laws of justice and mercy." - Wendell Berry
**********************************************************

 

[Bill]

Got a link? I've looked through the McMaster-Carr catalog
and typically their sensors are far too long (clearance issues
in the engine bay around the manifolds) and too large in diameter.
I want to run as small a diameter as possible so as not to
affect exhaust flow. My perfect sensor would be 1" long and
less than 1/8" diameter, with a 1/16" NPT base so that I
could drill and tap the edges of the manifold flanges at
the heads and install the sensor directly into the exhaust
stream. I've never seen anything like that except in photos
of complex test equipment setups.

JazzMan
--

I hate to send you to a competitor but for a one off order try
Omega.com ($25).

I'd suggest you buy a 0.063 or 0.125" diameter mineral insulated probe
whatever length you need with some lead wire. These probes can be bent
to fit into the engine spaces as needed.

(http://www.omega.com/ppt/pptsc.asp?ref=JMTSS&Nav=tema07)

Buy the NPT fitting you want in a plug form and drill it out to accept
the probe then braze the fitting onto your probe sheath at the desired
immersion depth. I'd use a plain steel fitting instead of a SS fitting
since if you don't design for the thermal expansion of your metals you
could run into some problems. Use a lot of antiseize on the threads.

Bill Schuh
Watlow