The above drawing depicts the salt pill in cross section. The salt pill consists of 1600 gold (99.999% pure) wires strung between 2 circular stainless steel perforated grid plates. The wires are arranged into bundles of 200 wires each. 4 of the bundles are silver soldered into gold plated copper rods called front end assembly rods, or FEA rods. The other 4 bundles are each separately silver soldered into a gold plated copper plug called the heat switch base. The rods and heat switch base are brazed into stainless steel sleeves, which are then welded into the stainless steel endcap. The rods are shown extending to the left of the salt pill. The heat switch base is shown at the middle of the cap at the left end of the salt pill, and is attached to the inner wire bundles. The spacing of the grid plate holes is .055 inches. As the wires are strung between the grid plates, they must alternate between a wire from the heat switch base and a wire from an FEA rod. This is to keep the heat transfer to and from the salt crystal uniform. After the wires are strung, the assembly is annealed for 2 days at 350 celsius. This is to bring the conductivity of the wires up after the working action of stringing them between the grids. After annealing, the wires are attached to the right hand grid plate using stycast epoxy, and the excess wire is trimmed. The final welding is then performed on the end cap to attach and seal the rods and heat switch plate. The assembly is then inserted into a stainless steel can, and the seam is welded. Finally, the other end cap is welded onto the can, making it hermetically sealed, except for the presence of the 2 fill tubes, 1 shown at the extreme right of the drawing. The can is then given 2 stages of plating. First, the can is plated with copper strips running the length of the can from the heat switch plate at the top, to near the center of the bottom end cap. This is to allow the stainless steel to change temperature faster when the salt pill is changing temperature. The stainless steel is a poor thermal conductor in comparison to the salt, copper and gold which comprise the rest of the salt pill. Then the can is given a layer of gold plating which covers the entire outside of the can and protects the copper strips from oxidation, and accidental contact with the caustic solutuion used to grow the salt. At this time, a salt crystal is built up inside the can. The can is oriented with the FEA rods down. The rods are attached to a thermoelectric cooling device using copper braids. A solution of ferric ammonium iron dodecahydrate and 7% sulfuric acid in de-ionized water is injected into the can. The depleted solution is withdrawn after 6 hours, and the can is rinsed with de-ionized water to prevent crystals from forming high up on the wires, and new solution is then injected into the can. This cycle is repeated every 6 hours until the crystal has grown up to the upper grid plate. The washings are performed to prevent the crystal from forming fluid filled voids within the body of the crystal. These voids could occur if the crystals on the wires were allowed to grow and close off the top before the lower crystals could form, and this would prevent the depleted solution from being removed. Once the crystal is grown, the fill tubes are cut off and end caps are welded on. The salt pill is then totally hermetically sealed. The hermetic seal is essential because the enclosed salt will dehydrate over time if exposed to outside air. This would render the salt pill inoperative. In addition, the salt pill must be kept below 39 C. This is because the salt will begin to melt at that temperature, also rendering the salt pill inoperative.
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A photo of about 500 out of 1600 wires to be threaded between the grids.
A photo of the wires terminated at wirewrap pins to allow annealing before stycast is applied
A photo of the 1600 wires installed in the ADR grid plates, before 350C anneal
A photo of the ADR wires showing the 4 FEA rods, and the center cup which comprise the termination of the 8 bundles of 200 wires
A technician applies Stycast epoxy to the ends of the wires with a syringe prior to final trim.
A photo of the wires, grid plates, rods, and endcap awaiting insertion into the housing (can).
Preparing the mask for plating of the copper strips.
A technician prepares the plated copper strips for an additional plating of gold.
The ADR salt pill housing in the plating bath getting the outer layer of gold applied.
A photo of the ADR salt pill after assembly of the can, just prior to salt growth.
A closeup of the top end of the salt pill showing the copper plated strips (plated over with gold) radiating from the heat switch plate
A closeup of the bottom end of the salt pill showing the detail of the copper striping
A photo of the ADR salt pill in the lab where the salt crystal was grown. The hotplate/stirrer in the foreground, the housing is attached to the thermoelectric cooling in the background
More photos coming soon
Last updated 4-8--98