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==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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