Gold ink coating of thermocouple sheaths

A method is provided for applying a gold ink coating to a thermocouple sheath which includes the steps of electropolishing and oxidizing the surface of the thermocouple sheath, then dipping the sheath into liquid gold ink, and finally heat curing the coating. The gold coating applied in this manner is highly reflective and does not degrade when used for an extended period of time in an environment having a temperature over 1000.degree. F. Depending on the application, a portion of the gold coating covering the tip of the thermocouple sheath is removed by abrasion.

FIELD OF THE INVENTION 
The invention relates to a method for applying a gold ink coating to the 
surface of a thermocouple sheath. 
BACKGROUND OF THE INVENTION 
Stainless steel sheathed thermocouples are commonly used to measure the 
temperature of a material in environment of high thermal radiation. It is 
known that measurement errors inherent in such thermocouple devices can be 
reduced by coating the sheath of the thermocouple with gold ink which has 
a very low thermal emissivity. To this end, various methods have been 
developed for coating a thermocouple sheath with gold ink. 
Briefly summarizing the steps of an exemplary prior art coating method, the 
thermocouple sheath is hand-painted with gold ink, then heated and cured 
at over a 1000.degree. F. for one hour, and finally hand polished. The 
steps of hand-painting, curing and hand-polishing are typically repeated 
at least once. 
This prior art method suffers a number of disadvantages and has been found 
to be generally unsatisfactory. For example, hand-painting and 
hand-polishing of the thermocouple sheath is time-consuming and costly. 
More importantly however, hand-polishing is inadequate to effectively 
prepare the surface of the thermocouple sheath for subsequent application 
of gold ink. As a result, a gold ink coating thus applied is not 
sufficiently smooth and shiny, and, since reflectivity is important for 
minimizing thermocouple measurement errors, the performance of the 
associated thermocouple is thereby limited. Further, as a result of 
ineffective hand-polishing, the gold ink coating deteriorates quickly when 
exposed to temperatures above 1000.degree. F. and becomes blackened and 
ineffective after only a few hours of use. 
SUMMARY OF THE INVENTION 
In accordance with the invention, a method is provided for gold coating a 
metal surface which overcomes the disadvantages of the prior art discussed 
above. The method of the invention comprises the steps of: i) 
electropolishing the surface to remove an outer layer of the surface, ii) 
oxidizing the surface by placing the surface in an air environment at over 
1000.degree. F., iii) dipping the surface into liquid gold ink to thereby 
provide a gold coating on the surface, and iv) curing the gold coating by 
placing the surface in an oven environment at over 1000.degree. F. 
A thermocouple sheath coated according to the method of the invention is 
more reflective than a sheath coated by a prior art method. Further a 
thermocouple sheath coating according to this method can withstand 
extended use within an environment having a temperature of 1000.degree. F. 
or more. 
Other features and advantages of the invention will be set forth in, or be 
apparent from, the detailed description of the preferred embodiments which 
follows.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the drawings, a preferred embodiment of the invention will now 
be described. As discussed above, the invention involves a method for 
applying a gold coating to a surface, such as the surface of a 
thermocouple, so that the surface can withstand prolonged temperatures of 
1200.degree. F. without blackening or otherwise degrading. Although the 
method can be applied to various other metal surfaces, the discussions 
below refer only to a thermocouple sheath surface, as illustrated. 
FIG. 1 shows a thermocouple 10 which includes parallel metal lead wires 12 
and 14 integrally welded to the inside surface of a stainless steel sheath 
16. Sheath 16 includes a hemispherical end in the vicinity of the junction 
with lead wires 12 and 14. Sheath 16 is generally cylindrical along the 
remaining length of wires 12 and 14. A ceramic insulator 18 is provided 
between lead wires 12 and 14 and sheath 16 as shown. Thermocouple 10 is of 
conventional design and is only shown to facilitate the description of the 
method of the invention provided below. 
A gold coating 20 is applied, by the method of the invention, to the 
exterior surface of sheath 16 both over the hemispherical and cylindrical 
portions thereof. However, the portion of gold coating 20 covering the 
hemispherical surface of sheath 16 is removed, yielding the structure as 
shown. 
Briefly summarizing the method, as shown in FIG. 2, for applying gold 
coating 20 to thermocouple sheath 16, the method comprises the following 
steps: first, an electropolishing step 30 is performed to clean and polish 
the sheath 16; second, an oxidizing step 32 is performed to create a thin 
adherent film (not shown in FIG. 1) on sheath 16; thirdly, a dipping step 
34 is performed wherein sheath 16 is dipped in gold ink to provide gold 
coating 20; and fourthly, a curing step 36 is performed to cure gold 
coating 20. Dipping step 34 and curing step 36 are repeated a number of 
times to provide a gold coating 20 with suitable thickness. Finally, the 
porton of gold coating 20 covering the hemispherical portion of 
thermocouple sheath 16 is removed to expose a portion of the sheath. 
Now considering the method steps shown in FIG. 2 in greater detail, 
electropolishing step 30 is performed in a conventional manner wherein 
sheath 16 is immersed in an electrolyte bath which removes a thin layer 
from the surface of steel sheath 16. Preferably, electropolishing proceeds 
until a layer of steel is removed from sheath 16 to a depth of about 
0.0005 inches to 0.001 inches. Care should be taken to avoid excessive 
metal loss at the interface between the electrolyte bath and the air. 
Although any conventional electropolishing process will suffice, the 
following exemplary, non-limiting process has been found to be 
satisfactory. The electrolyte used for electropolishing consists of a 
mixture of two parts by volume of phosphoric acid H.sub.3 PO.sub.4 (85.2%) 
and one part by volume of sulfuric acid H.sub.2 SO.sub.4 (95-98%) mixed 
together in a one gallon glass beaker. The temperature of this mixture is 
maintained between 68.degree. F. and 72.degree. F. A stainless steel 
electrode is mounted in the electrolyte bath with the surface area of the 
electrode being about 20 square inches. The electrode is connected to the 
cathode a 12 volt DC battery charger rated at 20 amps while sheath 16 is 
connected to the anode of the 12 volt battery charger. 
Table I provides the results of sample electropolishing steps wherein 1/8 
inch diameter 304 stainless steel rods were immersed one inch into the 
above-described electrolyte. The duration of electropolishing and the 
resulting amount of metal removal is shown in Table I. 
TABLE I 
______________________________________ 
Time in Seconds 
Approximate Inches per Surface 
______________________________________ 
15 .0005 
30 .0010 
60 .0015 
120 .0030 
______________________________________ 
After electropolishing, sheath 16 is completely cleaned with de-ionized 
water to remove electrolytes. Further, it is important that sheath 16 is 
completely protected from contamination by e.g. finger prints, dust, etc., 
prior to subsequent steps. 
During oxidation step 32, sheath 16 is placed in a furnace having a 
temperature of 1150.degree. F. for a period of 1 to 2 minutes. As a result 
of oxidation, sheath 16 acquires a thin and tightly adherent straw colored 
film (not shown in FIG. 1). The straw colored film provides a base for the 
subsequent application of gold coating 20. After oxidation, sheath 16 is 
cooled to room temperature. Again care is taken to maintain complete 
cleanliness of sheath 16. 
During dipping step 34, sheath 16 is dipped by conventional means into a 
vat of liquid gold ink. A suitable gold ink is "HANOVIA LIQUID GOLD INK 
No. 7621" supplied by Engelhard Industries Division, 1 West Central 
Avenue, East Newark, N.J., 07029. The gold ink has the appearance of thick 
cranberry juice and, after dipping, leaves a cranberry colored coating on 
sheath 16. 
To assure a uniform coating over the surface, the sheath 16 is dipped into 
the gold ink with the axis of sheath 16, indicated by line A--A of FIG. 1, 
maintained vertically. Lowering and lifting of sheath 16 into the gold ink 
bath should be performed slowly, i.e. at a rate of approximately 0.25 
inches per second, such that surface tension at the surface of the gold 
ink helps to provide a uniform coating of gold ink. Also, the liquid gold 
ink should be thoroughly mixed prior to dipping and the viscosity of the 
liquid gold ink should be maintained constant during the entire dipping 
step. To attain suitable viscosity, a conventional thinner can be added to 
the liquid gold ink. 
During curing step 36, sheath 16 is initially air dried for 10 minutes in a 
ventilated oven at about 250.degree. F. Again the axis of the thermocouple 
is maintained in a vertical postion. During drying, liquid gold ink 
accumulates at the tip of sheath 16. Any excessive accumulation of liquid 
gold ink at the tip is removed in a conventional manner, such as by 
"blotting" with absorbent material. Typically, excessive gold ink must be 
removed from the tip of sheath 16 several times during drying. While 
blotting, particular care must be taken to avoid contact with any of the 
other surfaces of sheath 16, i.e. with the cylindrical portion of sheath 
16. Vapors emitted from sheath 16 during this step are flammable and 
should not be inhaled. 
To complete curing, sheath 16 is placed in an oven having an air 
temperature of 1150.degree. F..+-.25.degree. F. for approximately 3 
minutes. As before, sheath 16 is maintained with the axis A--A in a 
vertical position. Adequate ventilation in the oven is necessary to 
disperse vapors emitted from coating 20 during curing. If the vapors are 
not adequately removed, coating 20 will have a dark colored surface after 
curing rather than a bright, shiny surface. Sheath 16 is then carefully 
inspected and cleaned with alcohol. 
The above-mentioned steps of dipping sheath 16 into gold ink, air drying 
the resulting gold coating 20 and then curing gold coating 20 can be 
repeated to provide additional gold coatings. Each dipping of sheath 16 
into "HANOVIA LIQUID GOLD INK NO. 7621" leaves a gold coating 
4.times.10.sup.-6 to 5.times.10.sup.-6 inches thick. Preferably, a total 
of two such coatings is provided. 
For most thermocouple applications sheath 16 is placed in direct contact 
with the surface of the material whose temperature is to be measured. For 
a thermocouple constructed for such an application, it is necessary to 
remove the gold coating covering a portion of the hemispheric end of 
sheath 16. This is accomplished by conventional abrasion techniques such 
as by abrading with 600 grit polishing paper or crocus cloth. Since, in 
use, sheath 16 may be positioned at angle other than perpendicular to a 
material surface, at least 1/4 of the surface area of the hemispherical 
end of sheath 16 is preferably removed. However, any portion of gold 
coating 20 deposited in depressions and crevices of sheath 16 need not be 
removed. After this abrasion step, the tip of sheath 16 is cleaned with 
alcohol. 
For applications using a contact thermocouple where it is desirable that 
the gold finish directly contact the metal surface to be monitored, none 
of gold coating 20 need be removed. 
In summary, the invention provides a method for applying a gold coating to 
a thermocouple sheath such that the thermocouple sheath is more reflective 
than a sheath coated by the conventional prior art method and such that 
the thermocouple sheath can withstand extended use within an environment 
having a temperature of 1000.degree. F. or more. 
Although the invention has been described with respect to exemplary 
embodiments thereof, it will be understood by those skilled in the art 
that variations and modifications can be effected in these exemplary 
embodiments without departing from the scope and spirit of the invention.