The CC is a cubical chamber located within the EE. Below is a photograph of the interior of the CC. On the right is the cell, in this case our standard calibration cell. On the left is the liquid-air heat exchanger and fan. Between the heat exchanger and the cell is the permanent calibration heater. In the foreground at the bottom is a liquid trap that collects any liquid ejected from the cell and protects the pressure measurement sensor located outside the enclosures.
Each wall has an interior 6.35 mm aluminum panel, covered by two layers of 19 mm rigid polystyrene foam insulation. An additional 6.35 mm aluminum panel is attached to the outer surface of this insulation and a final layer of 19 mm rigid polystyrene foam insulation covers the entire assembly. Rigid PVC angle material holds the panels at the edges and provides a slight separation. This arrangement makes the panels thermally independent. The inner dimensions of the chamber are 25 cm by 25 cm by 25 cm. The outer dimensions of the chamber assembly are approximately 35 cm by 35 cm by 35 cm. The front of the CC is a gasketed and insulated hinged door with mitered edges that fit snugly with the adjacent sides of the chamber. The CC is attached to the bottom grille of the EE by four 14 cm long by 19 mm diameter phenolic stand-offs.
The walls of the CC form the Active Insulation (AI). Each outer aluminum panel has 16 series-connected 1 Ω resistors attached to its surface that act as heaters. An imbedded temperature sensor is located at the center of each outer panel. A corresponding sensor is located at the center of the adjacent inner panel. The heating resistors connect to the outputs of six independent feedback-loop controllers. The set point for each feedback loop is the temperature of the inner aluminum panel. With this arrangement, the output of each controller forces the temperature of each outer panel to match that of the corresponding inner panel. The term “Active Insulation” (AI) refers to this temperature-tracking technique. The technique produces near-adiabatic conditions. In other words, no heat transfer to or from the CC interior takes place through the chamber walls.
There are several components inside the CC. A liquid-air heat exchanger and circulating fan occupy the left side of the chamber. A regulated DC voltage operates the heat exchanger fan. A tachometer sensor within the fan permits monitoring of the fan speed by the system software. The fan speed varies slightly with chamber temperature as the air viscosity changes. At constant temperature, the nominal fan speed is 1600 RPM, with a one-sigma standard deviation of about 0.2 RPM. We avoid a variable power contribution to the calorimetry measurements by deliberately leaving the fan speed unregulated. Fan power dissipation remains close to 0.86 watts during experimental runs. Between the heat exchanger and the open area where cells are located is the permanent calibration heater. Typically, a stand constructed of phenolic is attached to the bottom panel to hold the cell in the approximate center of the open space of the chamber and permit free circulation of air around it.
There are ports in the chamber walls for electrical connections, optical instruments and gas and liquid handling. Two ports are located on the bottom of the chamber to admit wiring for the heat exchanger fan, calibration resistors, cell power, AI panel heaters, and temperature sensors for the cell and AI panels.
Three small optical ports are located on the right side of the CC. One port is primarily used for a borescope to allow viewing of the cell. A matched port is located in the side of the EE allowing borescope access from the laboratory outside the enclosure. A small video camera optionally attaches to the borescope. The other two ports were designed for a laser or other device to illuminate a portion of the cell. A small phenolic tube in these ports, closed at the ends with microscope cover glasses, reduces heat transfer and provides a clear optical path.
In order to prevent dangerous pressures inside closed cells, MOAC has an automatic cell venting system. A 3 mm I.D. PVC tube connects the cell, through a liquid trap in the CC, to a capacitance manometer and solenoid relief valve outside of the EE. The manometer output is monitored and recorded by the main computer. Pressures outside of the range 700 Torr to 800 Torr trigger the opening of the solenoid valve. This valve is normally open which ensures that in the case of power failure, the valve stays open. A syringe or other volumetric device connected to the pressure line permits the determination of the total headspace volume as well as providing an indication that the cell is sealed.
Some experiments require periodic additions of fluid to the cell. A 1.6 mm I.D. TFE refill tube runs from the interior of the CC to the outside of the EE, where it can be connected to a syringe. The interior end of the refill tube connects to the cell when required, allowing it to be refilled during operation without opening the calorimeter. A retainer prevents displacement of the syringe plunger by the pressure in the cell.
 Caddock Electronics, Inc.; Type MP915; 1 ohm, 1%, MF, 15W, TO-126 (www.caddock.com)
 Lytron Corporation; Model 6105 G1SB Copper Tube Heat Exchanger; (www.lytron.com)