This document contains the installation procedure employed under NASA contract NAS3-27038. They represent the state of the art at the time of this project

Wnuk, S. P., Jr.

English

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FINAL REPORT 
ON 
PROCEDURE FOR INSTALLATION OF PdCr GAGES BY FLAME SPRAYING 
TO 
Introduction 
NASA LEWIS ,RESEARCH CENTER 
CLEVELAND, OHIO 
UNDER NASA CONTRACT NO NAS3-27038 
By: S.P. Wnuk, Jr. 
Strain Gage Development Lab 
Hitec Products, Inc. 
Ayer, MA 01432 
This document contains the installation procedure emp loyed under NASA contract 
NAS3-27038. They represent the state of the art at the time of this project. 
References 
1. Lei , Jih-Fen, "Deve lopment and Characterization of PdCr Temperature 
Compensated Wire Resistance Strain Gage," Fourth International Conference 
of Machine Perception Technology, Cleveland , OH, September 1989 
2. Strain Gage Development Lab, Hitec Products, Inc. , "Flame Spray Attachment 
Procedures" 
3. Strain Gage Development Lab , Hitec Products , Inc. "Strain Gage Installation 
Accessories Catalog" 
4. K. Krake , "Proposed GWP-29 Thermocouple Installation," NASA Dryden , 
6/15/93 
<P 1993 Hitec Product s, Inc , Ayer, MA 01432 
Facilites: 
The facility requirements are covered in Reference 2 "Flame spray Attachment 
Proceedure" and should be adhered to. The facility should contain the following 
equipment as described in Reference 2: 
1 . Flame spray booth 
2. Gasses 
3. Air compressor 
4. Air conditioner/ dehumidifier 
5. Bench with microscope 
6. Grit blast equipment 
Flame Spray Equipment: 
The following items of equipment are specified for the installation of PdCr gages. 
See Reference 3 for description of part numbers (PIN), 
1. Hitec Products CM-1000 Powder Flame Spray System, PIN #200120 (for 
spraying Nickel Chromium Aluminum pre-coats). 
2. Norton Co. Rokide Flame Spray System, PIN #200201 
3. Vortex Air Cooler, PIN #200140 
4. Air filter, desiccant type, 20CFM minimum capacity 
Materials: 
1. Rod, Rokide, Hitec HTZ, 1/4" x 18" 
2. Powder, Nickel Aluminide, PIN #200149 
3. Tape, Flame spray mask, Telfon Fiberglass, PIN #200154 
4. Tape, FLame spray mask, Aluminum, PIN #200154A 
5. Methyl Ethyl Ketone (M.E.K.) solvent 
Tools: 
1. Machinists square, Starrett EDP 50118 14D 
2. Scribe 
3. Carbide tip scribe in aluminum holder 
1 
4. Tweezers, Dumont 3 C(2) 
5. Industrial razors , package, single edge 
6. Dresser stick, aluminum oxide, white, 1/4 x 1/4 x 4", 60 grit 
7. Pencil, red 
8. Pencil, silver 
9. Scale, machinist's, 6" w ith decimal graduations 
10. Microscope light 
11. Eye protection, clear with side shields 
12. Eye prQtection, #5 shade with side shields 
13. Ear protection 
14. Igniter 
15. Rubber mallet 
16. Welder's gloves, leather 
1 7. Shop coat - cotton 
18. Glass plate, 2" x 6" double thickness pane (4) 
19. Resistance meter, Beckman 850 or equivalent 
20. Artist's brush 
2 
Procedure 
The steps described in the text below are augmented with photos and sketches at 
the end of the text. 
1. Solvent clean gage bonding area using appropriate procedures for the 
mat erial being used. This presupposes that the parts are free from 
contaminants. If the parts are oxidized due to exposure to elevated 
temperature, the oxides must be removed by abrasion with 600 to 1800 
grit silicon carbide paper, or by light grit blasting with an S.S . White 
airbrasive unit (or equivalent). 
2 . Layout gage and thermocouple locations . 
Using machinist layout tools accurately layout gage and thermocouple 
center lines and outline the gage and thermocouple bond surface. For a 
120 ohm gage made with 10 mil diameter pins, the gage area required is 
0.4" long x 0.280" wide. The thermocouple bond area is .075" w ide x 
0.100" long. If desired, the Rokide pre-coat may be extended to insulate 
under lead joints. An additional 0.1" of pre-coat extending beyond the 
end of the gage is sufficient to protect cable-gage lead splices . This 
extended pre-coat makes the gage layout dimensions 0.500" long x 
0.280" wide. 
A ball point pin with a carbide ball may be used for layout. The 
carbide ball will rollover surface imperfections and, after the ink is 
removed with solvent, the light line made by the ball is visible. This line 
introduces a small compressive residual stress and is usually not 
objectionable. Do not use ballpoint pins with stainless steel or plastic 
balls , because these will wear out almost instantly. A machinist's scribe 
may be used if not objected to by the project management. Also, red 
colored pencil's have been used for layout. Colored pencil layouts are not 
as accurate as a scribe but not as objectionable. Do not use ordinary lead 
pencils because these contain carbon which should not come in contact 
with many high temperature alloys. 
3 . Mask thermocouple locations for grit blasting. 
Using 1" wide silicone tape (PIN 200154 SA) mask the part for grit 
blasting. Place the tape 0.010" to 0.015" outside of the scribe line 
around the thermocouple area to be grit blasted. The grit blast intensity 
diminishes to zero at the tape edge and therefore the additional 10-15 mil 
perimeter is required to fully expose the bond area to the grit blast. Grit 
blast the exposed area using a grit and blast pressure as recommended 
for the test material. A #30 to #46 Aluminum Oxide grit using 60 to 120 
PSI air pressure is usually required for good Rokide adhesion. Examine 
3 
the surface under the microscope to insure adequate roughness before 
removing masking tape. Clean gage and thermocouple area with MEK 
after tape removal. 
4. Nickel Aluminide Pre-coat 
Mask the gage bond area perimeter using Teflon Fiberglass flame 
spray masking tape (PIN 200154) . This tape is applied exactly on the 
scribe line perimeter. Do not touch the gage bond area with the tape 
adhesive because silicon contamination from the tape may cause poor 
bonding of coating . Press tape firmly in contact with the surface, 
especially where tape layers over lap. Tape must be well bonded to the 
surface to prevent lifting of tape caused by heat from the flame spraying 
process. 
Using the Hitec Products Min i-Gun for metal powder (PI N 200101) 
apply nickel aluminide metal pre-coat (PIN 200149) using standard 
operating procedures described in the operation manual. Apply 2-3 m il 
thick coating and examine for completeness of coverage using 
microscope . Coverage should be 95-99% complete . Move a light source 
at varying angles while examining the coating. A slight reflection from 
the original surface should be observable. Excessive coating thickness 
can cause warpage of the part on thin specimens . An ideal coating is 
one layer thick. Once the flame spray particles begin bonding to each 
other, they will cause distortion because of a differential in thermal 
expansion coefficients between the structure and the coating. A properly 
applied coating provides a surface texture equ ivalent to a coarse grit 
blast. This coating elim inates the need for grit blasting as a surface 
preparation method . The flame sprayed metal coating is easier to apply, 
and prevents part distortion commonly encountered with grit blasting . 
Also, nickel aluminide can be applied to materials that are too hard t o grit 
blast. 
On thin parts, less than 1/8" th ick, application of Vortex cooling air 
speeds up the coating process . The parts should not become too hot to 
touch during any time of the coating process. Good application 
technique will always keep the part cool. Application of the coating to 
hot parts will cause some undesirab le distortion. The cooling air must be 
desiccant f iltered to remove all oil or an oil - less air compressor shou ld be 
used. 
Remove masking tape and gently run an industrial razo r over the 
coating to remove any loose particles and blend the edges . An excessive 
buildup always occurs at the tape edge and must be removed . A clean 
wire brush (with very small bristles) may also be used to gently remove 
any loose particles. Be sure the wire brush is never used for other 
applications where it may be tainted with oil or other contaminants . 
4 
- --- ---
5. Apply Ceramic Pre-coat 
Remask using Teflon fiberglass tape. Place the tape exactly at the 
edge of the nickel aluminide pre-coat. Place the part in the flame spray 
booth in a suitable vise, and apply 30psi of vortex cooling air. Using 
Hitec HTZ rod, apply a 2-3 mil pre-coat over the nickel aluminide. The 
pre-coat will appear grey. If the coating appears white, it is too thick. 
Examine the grey coating under the microscope to be sure it is 
continuous. 
6. Apply Gage 
Remove gage slide from package and examine gage under 
microscope. Remove tape dot from the gage leads. Place slide on clean 
surface with leads pointing to right and cut near side tape carrier tab at 
the junction with cutouts. This is accomplished using a single edge 
industrial razor placed at the tab - cutout junction and press down, 
cutting through the tape carrier. With the razor still in position, grasp the 
solid tape tab with tweezer and remove. Using the tweezer, grasp the 
uncut tape carrier tab and carefully lift upward, removing the gage from 
the slide. 
While viewing the gage location under the microscope set at 7x, hold 
the gage in position just above surface with the tweezer. When satisfied 
with gage position press down on the gage lightly using finger of free 
hand. Do not remove and reposition as this destroys the tape adhesive. 
It is most important to have good tape adhesion. The gage should be 
located so there is an equal margin on the three free sides of the gage. 
Using a blunt tweezer, press on tape bars near leads to be sure all tape 
adhesive is in intimate contact with pre-coat. Do not press hard on tape 
above wire strand. i.e. Do not conform wire strands to the contour of 
the Rokide surface. Heat treated PdCr wire is somewhat brittle and can 
be broken with excessive pressure from tweezers. Examine grid wires 
adjacent to leads to be sure these wires are not above the surface. 
Cut a 1/2" long x .030" wide tape bar of aluminum flame spray 
masking tape (PIN #200154A) and place over leads as shown . Be sure 
tape does not go over or touch the one mil PdCr wire. Press the tape bar 
in place over leads using tweezers to bend aluminum to comply with 
shape of gage leads. This tape, once bent, will not spring back, as does 
the fiberglass tape, and has much more holding power than the fiberglass 
tape adhesive. Place a piece of 0.040" wide double thickness fiberglass 
shading tape over the 0.030" aluminum tape bar with equal overhang on 
each side. Place appropriately sized shading tape over each gage tape 
bar. Cut two 1/8" lengths of 0.030" wide double thickness fiberglass 
tape and place these over the platinum crossover wire at top of gage, as 
shown in Figure 9. These tape bars hold the platinum wire in place 
5 
during flame spraying. Measure gage resistance at ends of leads and 
record. Measure to nearest 0.1 Q. 
Next, the gage is boxed in by placing a single layer of fiberglass tape 
on all four sides and located about 1/64 to 1/32" away from the grid. 
The first tape layer should go over leads with tape starting half way 
across the lead tape bar. The second tape layer goes over top end of 
gage. The next two tape pieces are placed along the sides of the gage. 
Press all tape firmly in place using tweezers or fingers. Check to be sure 
grid is bonded by pressing on tape bars while observing for wire 
movement during pressing . If no movement occurs, gage is ready for 
tack coat. 
7. Tack Coat 
Place specimen in vise or suitable holder and position vortex cooling 
air nozzle so the cooling air is along tape bars and at about 20 degree 
angle from surface. Turn cooling air to 30psi. Ignite gun. Turn gun on. 
Spray at about 10" distance. Hold gun at idle in a position which imparts 
no heat into the specimen. Hold gun perpendicular to gage surface and 
apply Rokide with rapid passes followed by several seconds of dwell with 
gun in idle position. Spray technique is best developed on other gages, 
such as Pt-W types. The technique should apply no more than 400°F to 
gage grid maximum, (see Reference 2) and grid should return to room 
temperature before making the next pass. The gage should have no 
measurable change in resistance when correctly applied. 
The tack coat uses less than 8" of rod and takes 25 to 30 passes. 
The gage should be lightly pressed with the thumb before the 1 st pass 
and before each subsequent pass to be sure tape is in intimate contact 
with pre-coat. If tape adhesive lets go and several spray passes are 
made, the tape will loose all stickiness and the gage must be removed, or 
the tape bar replaced. 
The gage should be examined continually to view progress and 
spraying should stop when enough material is deposited to barely hold 
the gage in place. The leads will not be covered! They will just be partly 
covered. If the leads are covered, too much material has been applied. 
The Rokide is semi-transparent and in thin layers, the leads and grid are 
quite visible. It is better to periodically stop spraying and examine under 
a microscope, and add material if required, than to apply too much to 
begin with. The microscope for this examination should be turned up to 
6 
l 
I 
25x or 30x for a good look. What looks like inadequate coverage at 7x 
may at times really be adequate when viewed at 30x. 
8. Tape Removal 
Remove all perimeter tape down to the grid tape bars. While viewing 
• at 15x, using sharp tweezers, remove shading tape bars. Grasp tape 
carrier tab with tweezers and pullover itself and at 45 0 to the gage axis. 
Pull slowly while observing under scope. Be sure all adhesive stays with 
the tape. Do not pull upward on grid. Upward pulling can pull grid out of 
Rokide or stretch grid wire. Pull carefully. Pull slowly. The stretched 
adhesive will slowly let go of the gage. If a piece of adhesive breaks off, 
it must be removed. After tape removal, examine gage carefully and 
remove any debris using an artists brush. Then dip the brush in M.E.K. 
and apply to former tape area. If any tape residue remains, it will swell 
from the M.E.K. and become visible for removal. The artists brush or 
sharp tweezers are used to remove any tape residue. Check gage 
resistance. 
9. ReMask and Final Coat 
Apply a single layer fiberglass tape perimeter about 1/64 to 1/32 
beyond previous perimeter tape. Examine under 15x after all taping. 
Place gage in vise and turn on Vortex air. Spray final coat using same 
technique as before. When complete, there may be 2-6" of rod 
remaining. Do not apply too thick. Remember, the Rokide is semi-
transparent. Lead shape will be visible. If coating over leads turns all 
white the coating is too thick. Check gage resistance with gage at room 
temperature. 
10. Remove all tape. 
Dress any sharp corners using aluminum oxide stone. 
11 . Install thermalcouple per instructions by K. Krake. 
7 
Figure 1. 
Figure 2. 
Figure 3. 
Constant stress bar ready for gage installation. 
Nickel chrome aluminum pre-coat applied to gage bond areas. 
Thin insulating coat of alumina, 4% Zirconia is applied to gage bond 
area. 
Figure 4. Photo of free filament PdCr/Pt dual element strain gage with 
temporary teflon fiberglass carrier. The leads are .003" Dia. PdCr 
Gage is temporarily stored on glass slide covered with release paper. 
Figure 5. Sketch of a free filament PdCr/Pt dual element high temperature strain 
gage . Note the platinum compensating element is positioned on both 
sides of strain sensing PdCr grid to thermal balance temperature 
gradients across grid. The .003" Dia. leads are designed to minimize 
stresses on the weld junction and to prevent rotation of the lead 
during bonding. 
Figure 6. Gage is removed from slide with tweezers and positioned on test bar. 
Figure 7. Photo of gage in position on test bar. 
Figure 8. Photo of .030" wide tape bars placed over platinum crossover strand. 
Figure 9. Sketch of gage showing placement of tape over platinum crossover 
wire and the placement of aluminum teflon tape over the .003" 
diameter leads. Note the tape does not touch the .001" grid. 
Figure 10. Photo showing tack coat applied and tape bars removed. 
Figure 11. Gage perimeter masked in preparation for final coating. 
Figure 12. Completed gage installation on constant stress test bar. 
40IlNSTPDCR.GGE 
8 
• 
• 
FIGURE 1. Constant stress bar ready for gage installation. 
FIGURE 2. Nickel chrome aluminum precoat applied to gage bond area. 
9 
:;::'F~~~:,q ~< :. 1;.hi!l·: inS:U::t:a.ti:ng .. cqat --q£ "a.:+umi:Ii.a, .. 4%zir.conia is app'lied 
:::·::>P:;XX.:;X)~<:·<~-::~:.~:~:·::~~g¢\:~?~~:;:. ~re.~~~ ..... ' . .", ':.: . 
. ' 
"'FJ;GURE If. ' Photo of free ' filament PdC.r/Pt dual element strain gage 
.wf..th-··t'eJ!lporary . tefl,on fib"erglass carrier. The leads 
aX.e·.·.003" :diame-.ter ·~· .·PdC;:. gage is store.d on glass slide 
< cove.redw:tt h T~l~asepaper. 
". 
10 
• 
r .. 
[r nnnnn In 
r::r::r:: FF ~~~ FF ~~~ FF ~t=~ EF 
~ I=f= f:". ~t:: ~ 1-1-~ ~~ I- 1-1-F= f--I--1-1-- r== I--1-1- ~ 
.. : 
FF=F ~~ ~ FFF ~~ ~ ~~~ 1=1= 1= 
'-I-- 1-1- '-
.. 
o· 
o · 
" 
::::::::1= U~UUUU U~ ~I= 1= ;::=1= ~~ 1= ;::::::~ ~ ~ ~;::I= 1 1= I=-t:.:=1= l- I--
0 
FIGURE 5. Sketch of a free filament PdCr/Pt dual element high 
temperature strain gage. Note the platinum compensating 
element is positioned on both sides of strain sensing 
PdCr grid to thermal balance temperature gradients across 
grid. The .003" Dia. leads are designed to minimize 
stresses on the weld junction and to prevent rotation of 
the lead during bonding. 
11 
FIGURE 6. Gage is removed from slide with tweezers and positioned 
on test bar. 
FIGURE 7 . Photo of gage in position on test bar. 
12 
FIGURE 8. Photo of..030" wide tape bars placed over platinum 
crossover strand. 
."- "-- ----.,.~ .. ~"--
FIGURE 9. Sketch of gage showing placement of tape over platinum 
crossover wire and the placement of aluminum teflon 
tape over the .003" Dia. leads. Note the tape does not 
touch the .001" Dia. grid. 
13 
FIGURE 10. Photo showing tack coat applied and tape bars removed. 
FIGURE 11. Gage perimeter masked i n preparation for final coating. 
14 
• 
FIGURE 12. Completed gage installation on constant stress test beam. 
15 



Procedure for Installation of PdCr Gages by Flame Spraying

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