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=General Description= [[File:VTC assembly.png|alt=A rectangular box VTC attaches to a vacuum line coldfinger. It contains a small reservoir for LN, a resistance-wire heater, and a thermal conduit. The conduit transfers energy between the heater and the LN at a controlled rate so the bottom tip of the coldfinger can be maintained at a precise temperature by using feedback from a thermocouple. Additional thermocouples attached monitor the LN level, the element wire temperature, and a secondary temperature at the top of coldfinger. |thumb|A 3D rendition of a Variable Temperature Coldfinger (VTC).]] A VTC attaches to a vacuum line coldfinger, to control its temperature to a precise setpoint anywhere in the range from -196 to +50 Β°C. It contains a small reservoir for LN, a resistance-wire heater, and a thermal conduit. The conduit transfers energy between the heater and the LN at a controlled rate so the bottom tip of the coldfinger can be maintained at a precise temperature by modulating the heating element power using feedback from a thermocouple. Additional thermocouples monitor the LN level, the element wire temperature, and a secondary temperature at the top of coldfinger. In addition to electrical power for the internal heating element, the VTC also requires supplies of liquid nitrogen ("LN") and clean, dry compressed air. In Aeon-designed systems, all of these resources are controlled by a combination of software and several additional physical devices. However, the VTC can be safely and accurately controlled by other means, as long as the required resources are provided and properly managed, and the appropriate limitations are always observed. ==Electrical Power== The VTC heating element is simply constructed from Teflon-insulated 36 AWG Nichrome 60 resistance wire, and has a total room-temperature resistance of approximately 100 ohms. Thus, virtually any convenient electrical power source that delivers between 5 and 120 volts RMS or DC may be used. The upper voltage limit ensures that the peak current never exceeds 1.7 amps. '''Warning!''' Delivered power must always be limited by a control device to no more than 12 W when cooled by liquid nitrogen and no more than 6 W otherwise. Exceeding this limit will destroy the heating element and may cause a fire. Additionally, the maximum temperature for any part of the VTC, including the heating element wire itself, is 75 Β°C. Temperatures exceeding this will damage the foam insulation. Power to the heating element may be modulated by voltage control, current limit, pulse duty cycle, or pulse frequency. When powered by 120 VAC, a duty cycle of 0 to 3% (pulse frequency 27 Hz) is appropriate for operating in ambient conditions. When cooled by liquid nitrogen, the maximum recommended duty cycle is 8%. ==Liquid Nitrogen== To operate at temperatures below ambient, the VTC requires a supply of liquid nitrogen (LN). LN is delivered into the VTC using 1/4-inch diameter FEP tubing. Other types of tubing do not work well. Atmospheric moisture condensation troubles and LN losses can be minimized by thermally insulating the FEP tubing between the supply and the VTC. Custom lengths of recycled-denim-based tubing insulation specifically designed for this purpose may be purchased from Aeon. The LN supply pressure should not significantly exceed atmospheric pressure. In particular, it should not come directly from a pressurized vessel, such as a typical "low pressure" liquid nitrogen cylinder, which usually delivers liquid at 22 psi. The VTC is designed to accept a trickle or "drip feed" of LN by gravity alone. Inside the VTC foam body is a small reservoir chamber that holds about 11.7 mL of LN. In normal operation, this reservoir fills to the top and then continuously overflows out of a drain tube connected at the top of the reservoir. The overflow should be continuous but minuscule. The LN supply control system should adjust the trickle flow rate into the VTC to achieve a drain flow rate of about one drop per second, or about 3 mL / min, with the VTC operating at the highest normal-operation regulated temperature for the implementation. (Note that the total LN consumption will be higher than 3 mL / min, because the LN that boils off as a gas doesn't appear at the drain port.) ==Compressed Air== The VTC uses low-pressure clean, dry, room-temperature compressed air or nitrogen to rapidly empty its LN reservoir and warm the VTC to ambient temperature. The gas should be delivered at 5-20 psi using 1/4-inch plastic tubing (polyurethane and LLDPE are good options). Adjust the supply pressure to the minimum required to instantly eject the LN from the reservoir out of the drain tube.
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