----- Original Message -----

From: Todd Bartrim

To: Rotary motors in aircraft

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;

1. 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.
2. Cut + & - solid TC wires several inches longer than probe length.
3. Twist the ends of these wires several times to create the TC junction
4. Slide several ceramic spacers onto the TC wire.
5. 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.
6. Lightly clamp in vice with tip facing up.
7. 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.
8. Slide on several more inches of ceramic spacers on to the TC wires
9. Strip approx 1" of the outer sheath off of the extension wire and ½" of the insulation off the ends of the extension wires.
10. 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.
11. 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.
12. 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.
13. 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.
14. 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.
15. 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.
16. 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.
17. 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.
18. 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.
19. Loosening the tube nut allows the TC to be removed.
20. That's it you're done.

Todd Bartrim

 

RV9Endurance

13B Turbo Rotary

C-FSTB

http://www3.telus.net/haywire/RV-9/C-FSTB.htm

 

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