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Hi
Joe;
Good point. IIRC, Bulent is using the EM2
which requires grounded TC's. But for other applications this could be an
issue.
Cheers...
Todd.
Todd,
Very nice job on this procedure.
I just wanted to note that anyone using the type
of TC (grounded junction) described in this procedure, should check that
their instrument(s) are suitable for use with grounded junction TCs.
Some instrumentation, multiplexes multiple TCs to a single AD (analog to
digital converter) and if only one leg of the TC channel is internally
switched to the AD, ground loops can occur which may adversely affect the
display value. With non isolated multiplexed instruments, isolated
junction TCs are needed.
Joe
----- Original Message -----
Sent: Monday, January 17, 2005 2:21
AM
Subject: [FlyRotary] EGT Thermocouple
construction procedure
Today I typed up the procedure
for making your own TC probes. It took me longer to type it than it takes to
make 2 of them. I hope this is clear enough to follow. I know that a few
pics taken during construction would be worth much more, however I didn't
have a camera with me at work when I made these.
EGT Thermocouple construction
procedure
Materials Required;
1.
.25"x0.49" 316SS tubing
2.
Solid "K" type thermocouple wire (positive &
negative)
3.
1/8" ceramic TC wire spacers. Some of these don't seem
to be made uniformly, (at least not the ones I had on hand) and you may need
to sort through a bunch in order to find some that will fit snugly inside a
¼" SS tube. They can be found at
http://www.omega.com/pptst/ORX_INSULATORS.html
4.
"K" type extension wire (preferably multi
strand)
5.
Silver Solder
6.
316SS 1/16" TIG welding rod
7.
Tightly wound spring with ¼" ID, approx 3" - 6"
long
8.
Tightly wound spring with ¼" OD approx 3" - 6"
long
9.
Parker or Sweglok tube fitting ¼"tube X 1/8"
NPT
10. Electrical
insulator cloth.
Procedure;
- Using a stainless tube cutter (extra hard
wheel), cut tube to desired length for TC probe. Deburr one end only,
leaving the other end with burr only allowing a small hole in end of
tube.
- Cut + & - solid TC wires several inches
longer than probe length.
- Twist the ends of these wires several times
to create the TC junction
- Slide several ceramic spacers onto the TC
wire.
- Insert this TC into the SS tube in the
deburred end, until the twisted TC junction extends approx 1/8" through
the small burred hole on the other end of the tube.
- Lightly clamp in vice with tip facing up.
- TIG weld closed the top of the tube,
including in the weld, the twisted pair of TC wires. This requires a
delicate touch and very little filler rod. Best to practice on some scrap
tube ends first. There should be no filler material extending past the OD
of the tube. Lightly grind or file off any excess if necessary. Clean up
using wire wheel if desired. This is your grounded tip
thermocouple.
- Slide on several more inches of ceramic
spacers on to the TC wires
- Strip approx 1" of the outer sheath off of
the extension wire and ½" of the insulation off the ends of the extension
wires.
- Silver solder these extension wires to the
TC wires. Ensure the polarity of wires is correct. Red is always negative
in TC's. The negative wire in "K" type is also magnetic while the positive
wire is non-magnetic.
- Wrap these junctions with an electrical
fibrecloth insulator, ensuring to keep wrap tight (1/4" OD). There are
probably other acceptable ways to insulate this junction including putty
or ceramic, but this worked well for me.
- Slide the tightly wound spring with ¼" ID
over the extension wire, over the solder splice to the TC wires (ensure
insulator wrap slides securely inside spring), over ceramic spacers and
finally onto the end of the ¼" tube. You will need to "screw" this spring
onto the tube end, which it will easily do, using the spring coils as
"threads". This will securely hold spring onto the probe and provide a
high temperature, flexible sheath over the TC wire splice, to provide
mechanical protection.
- Slide smaller spring with the ¼" ID over the
extension wire. This should fit quite tightly to the extension wire, so a
"screwing" action may be required to slide it along the wire. When you
reach the larger spring, the 2 springs can be "screwed" together with the
OD of the coils of the smaller spring mating perfectly with the ID of the
coils of the larger spring. Screw these together for 1" - 2" or just short
of the splice location.
- The tube fitting has a shoulder in it,
preventing the tubing from passing though. This is there as normally this
fitting is just used to couple tube to pipe. This shoulder needs to be
removed by drilling out with a ¼" drill bit. Once removed this fitting
will slide easily over the end of the TC probe.
- Drill & tap the manifold in the desired
location with a 1/8" NPT hole. The surface of the manifold can be
extremely hard, preventing even the best drill bits from starting a hole.
Use an angle grinder to just take a little off the surface... use this as
an opportunity to make a flat surface to insert the tube fitting in until
the outer shoulder contacts evenly.
- Once the tube fitting has been fully seated,
check the inside of the manifold to see if any part of the threaded
fitting protrudes into the exhaust path. Grind off with a die grinder if
necessary.
- With ferrule and tube nut loosely screwed
onto the tube fitting, insert TC probe until the tip is just protruding
into the exhaust gas path - or to desired depth.
- Tighten tube nut approx, 1.5 - 2 turns. This
will seat the ferrule permanently onto the tube. If you tighten too much
you will squeeze the tube to point where you will begin to hear the
ceramic wire spacers begin to crack under the pressure.
- Loosening the tube nut allows the TC to be
removed.
- That's it you're
done.
Todd Bartrim
RV9Endurance
13B Turbo Rotary
C-FSTB
"The world will always have a place for those that bring hard work and
determination to the things they do."
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