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=Connections= ==Tubing== The VTT body is machined from a block of extruded polystyrene foam (XPS). XPS has excellent physical properties for this application, but unfortunately, it is easily deformed and damaged by rough handling. Dings and dents on the exterior will not affect function, but deformations where the coldfinger and tubes enter the body can cause leaks and poor performance. Take measures to protect these areas from accidental impact and prevent the movement of connected parts. Three passages, or "ports", are machined into one face of the VTC to accept tubing. Looking at that face, the left port is for compressed air, the center port is the LN drain, and the right port is for the LN supply. All tubing is 1/4-inch in diameter. The LN supply tubing should be made of FEP. The other tubes can be of any convenient plastic; polyurethane and LLDPE are excellent options. The ports are machined slightly undersize to create a snug fit on newly inserted tubing. Repeatedly inserting and removing the tubing will enlarge the port and should be avoided as much as possible. A clear, plastic "tube hitch", with holes matching the port positions, is provided with the VTC to secure the tubing in place. Also provided is a short, bent drain tube. (A longer tube may be used, if preferred, to direct the drained LN to a specific location.) Use the following procedure to install the tubes: # Before installing the tubes, slightly chamfer the outer edge of the tube end so it will not gouge the interior of the passage during insertion. (A hand held pencil sharpener serves well for this.) # Make sure the tube hitch is oriented to match the port holes. Pass the tube ends to be inserted through the appropriate holes in tube hitch, so they protrude into the hitch. # Install a 1/4-inch circlip on each tube, on the interior side of the hitch, taking care to not scratch the tubes, or over-stretch the circlip. The circlip has two functions: it is a depth stop, and it prevents the tube from being pulled through the hitch and out of the port. #: NOTE: The LN drain tube must be inserted into its port a fairly precise depth, as close to 11 mm (0.43 inches) as possible. Install its circlip 11 mm from the end. The air and LN supply tubes may be inserted anywhere from 11 to 20 mm. # Insert the tubes into their respective ports, gently working them into place. It is easier to insert them each little by little than to finish one before moving to another. Avoid introducing any side forces on the tubes in the ports. Stop when the circlips reach the VTC body. # Slide the hitch along the tubes up to the circlips, wrap the hitch around the VTC body and snap the hitch ends together. Insert a thin, flat, hard support (such as a metal ruler) under the hitch snap to keep it from denting the VTC body. ==Wiring== Five pairs of wires are bundled into a cable and routed out of the VTC through a hole near the top of the assembly. Note that the hole is oversize and minor lateral movement of the wires is possible. This clearance enables the passage of cold nitrogen gas over the heating element wire, to cool it during operation. Do not plug or obstruct the hole. Four of the wire pairs are Type T thermocouple (temperature sensor) connections. The fifth pair is for the heating element and is permanently connected to a violet and gray "ribbon" cable (2-conductor, 24 AWG). Provide controlled heater power into these two wires. The two wires are equivalent; voltage polarity does not matter. The top thermocouple is sheathed in stainless steel. Its two wires have a brown FEP outer jacket. This sensor monitors the temperature at the very bottom of the coldfinger. It is the primary temperature sensor for the VTC, and feedback from this thermocouple should be used to regulate the heater power. The other three thermocouple wire pairs have a blue PVC ribbon cable construction, with a red stripe to indicate the reference lead (-). Each thermocouple monitors the temperature at a specific location. In order from top to bottom, they monitor (1) the top of the coldfinger, (2) the liquid nitrogen (LN) reservoir level, and (3) the heater element temperature. The top of the coldfinger (1) is usually the warmest part of the coldfinger trap. The thermocouple there may be used to establish the range of temperatures in the coldfinger. The LN reservoir level thermocouple (2) rapidly approaches -196 Β°C as the LN level reaches the top of the reservoir. This thermocouple can be used to signal a request for or control the flow of LN into the VTC. The heater element thermocouple is located just outside of the VTC body, in contact with the element wire insulation. Normally, this is the hottest point of the VTC. A safety mechanism should constantly monitor this temperature and automatically interrupt heating element power if it reaches 75 Β°C.
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