The Temperature-Sensing Element 



This sensing element is a 51R2 thermistor contained in a brass- 

 walled cylinder that is threaded into the end-cap. This brass cylinder 

 ha3 a 13/32-inch O.D., and is 1-1/2 inches long. It is bored in two 

 stages, one to take the insulated thermistor and the other to take the 

 threaded bakelite plug. The bakelite feed-through plug is drilled to 

 take the two thermistor wires. The cavity containing the thermistor 

 is filled with mineral oil, the bakelite plug is run in, then the feed- 

 throughs are sealed off. 



The Feed - Throughs 



These are for the signal switch and the water switch, and are 

 located on the above-mentioned end-cap. Both feed-throughs are 

 identical in construction; therefore, only one will be described here. 

 A 3/4-inc h-diameter hole, 1-1/8 inches deep, is bored into the end -cap, 

 which houses all of the parts in the feed-through assembly. Starting 

 at the bottom of the hole, a lucite disk (0. 740-inch diameter, 3/8-inch 

 thick) containing a clearance hole through the center for a l/8-inch- 

 dianeter brass rod 1-1/2 inches long, and a groove mid-way up the side 

 of the perimeter for a PRP-902-11 O-ring, is placed in the position 

 first. This forms a seal between the assembly and the wall of the hole 

 in the end-cap. After this seal, a brass disk (0. 625-inch diameter, 

 l/4-inch thick) with a l/8-inch shaft anchored to the center of the 

 bottom, and a PRP-902-7 O-ring in the peripheral area of the bottom 

 side of the disk, is placed on the lucite disk. The l/8-inch brass 

 rod is insulated, where it passes through the body of the end-cap, 

 with a length of insulating sleeve. The insulated lead is connected 

 to the top of this disk. The lucite disk and the brass disk, with 

 their associated O-ring s, form the water-tight seal for the electrical 

 feed-through. Above the brass disk is another lucite disk (0.72 5-inch 

 diameter, l/4-inch thick) with a l/l6-inch hole through it for the 

 insulated lead and attaching plate. This disk serves as an insulator 

 between the brass disk and the threaded hex-head plug (3/4 inch, 14 

 thread, by 3/8 inch) that completes the assembly. This plug has a 

 0.102-inch -diameter hole through it for the insulated lead, and a 

 bored brass sleeve (3/l6-inch O.D., 3/4 inch long) silver- soldered 

 on top, concentric with the hole, to serve as a mounting post for a 

 piece of surgical rubber tubing (l/4-inch O.D., 3 inches long). The 

 rubber tubing, or boot, serves as a re.swrvoir for the silicone high- 

 vacuum grease and as a stiff ener for the wire to distribute Hexing,. 

 The feed-throughs and rubber tubing are packed with vacuum grease, 

 carefully excluding all air bubble?, then the tubing is served off 

 tightly at both ends. These feed-throughs are designed to withstand 

 a pressure of 1,500 p.s.i.j however, to increase the safety factor 

 would require enlarging the feed-through assemblies to accomodate the 

 additional 0-rings, which would then cause numerous design complications. 



12 



