VTC

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General Description

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 °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 °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.

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:

1. 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.)
2. 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.
3. 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.

4. 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.
5. 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 °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 °C.