5.5.7.4  Gas Mixing Station

The gas mixing system works by metering three gas supplies into a common mixing tank. The mixture is then bubbled through alcohol in a tank within a small refrigerator which has been modified for safe operation in a flammable gas system. Because the gas flow, not the pressure, is regulated by the metering system, pressure switches have been installed to monitor the mixer outlet pressure and provide feedback to the flow-control system. The mixing, bubbling, and pressure control systems are all built into the same relay rack. They are collectively referred to as the Gas Mixing Station. A flow diagram is shown in Figure

Mass Flow Control System

The flow rate of each component gas is controlled by a mass flow controller which delivers a constant mass of gas per unit time. (Tylan General model FC-280AV) The mass flow is independent of pressure, although a minimum differential pressure across the controller is required for proper operation. The valves are factory-calibrated for Nitrogen (N₂). The system controlling them is field-programmed to compensate for different gasses. Flow channel 1, currently assigned for CO₂, has a mass flow controller calibrated to deliver a maximum of 100 sccm (standard cubic centimeters per minute) N₂ (74 sccm CO₂). Flow channels 2 and 3 have controllers with a full scale range of 1000 sccm N₂. With the calibration factors taken into account the maximum flows are 500 sccm Ethane (channel 2) and 1450 sccm Argon (channel 3). Manual valves (MV-201 & 221, etc.) are provided which allow one to bypass the mass flow controllers and use a needle-valve / rotameter set (MV-211, -212, -213) if desired. The needle valve must be closed during normal operation using the mass flow controllers.

  

Figure: Block Diagram of Mass Flow Control System.

The mass flow valves are controlled by a Dynamass Flow Control System (Vacuum General, Inc. model DM-2401). This unit is outfitted with four flow-control channels (two model FM-8 two-channel modules) and could be upgraded to eight flow channels if desired. Refer Figure for a diagram of this system. Currently HAWGS has only three of the four channels instrumented. The FM-8 receives a flow measurement from its associated flow controller, adjusts it by the calibration factor for the gas being used, and displays the result on the front panel. If the measured flow differs from the desired flow as set in the FM-8 by an operator, a correction signal is sent to adjust the valve in the flow controller. The DM-2401/FM-8 system allows the user to define up to four mixture/flow settings. Refer to the Dynamass System manual and to section 3.3.3 Setting a Flow Rate for more detail on operating the flow controllers.

The measured flows of the three component gasses are combined in a small blending tank in the back of the mixing station. The resulting mixture is delivered to the alcohol bubbler through a line which is teed to an overpressure relief valve (RV-271) set for 25 psig. This prevents overpressuring of the blending tank, the bubbler, or the delivery lines.

Alcohol Bubbler

  

Figure: Vapor Pressures of Isopropyl and Ethyl Alcohols calculated using the CRC Handbook parameterization.

Because the interesting alcohols for use in wire chambers have a feeble vapor pressure at room temperature, it is not convenient to purchase bottled gas with alcohol already added. A practical means of adding alcohol vapor to a gas is to pass the gas through a reservoir of the liquid alcohol which is maintained at a specified temperature. At a given temperature, the vapor pressure of the alcohol may be known, and this vapor pressure represents directly the partial pressure of the vapor in the gas mixture. The vapor pressures of organic compounds may be calculated from information in the CRC Handbook of Chemistry and Physics, where it has been parameterized as

Log10 P = (-0.2185 A/K) + B

where P is the pressure in Torrs, K is the temperature in Kelvin, and A and B are parameters provided in the Handbook for a number of compounds. For isopropanol within the temperature range -26.1^oC to +232.0^oC, the parameters given are A=10063.5 and B=8.996156. For Ethyl Alcohol the parameters are A=9673.9, B=8.827392.

At 0^oC, for example, this formula gives the vapor pressure of isopropanol as 0.0115 Atm. (1 Atm. º 760 Torr). If the gauge pressure of the bubbler gas + vapor is 1 atmosphere (2 Atm absolute pressure), as intended for Hall A, then the fraction of alcohol vapor, by partial pressure, is about 0.57%. Figure  shows the vapor pressures of these alcohols as a function of temperature.

Note that the bubbler temperature defines the vapor pressure and thus the Òdew pointÓ for the vapor in the gas. If the gas comes in contact with any surface which is colder than the dew point (the temperature of the bubbler) the alcohol vapor will condense on that surface. This is why it is important that all components of the gas system be maintained at a temperature above that of the alcohol bubbler. Because gas in the Hall A chambers is at about 1 atmosphere absolute pressure while that in the bubbler is at twice this pressure, the dew point for the gas in the chambers is lower than the bubbler temperature. The bubbler system consists of a refrigerator, a bubbler tank, a cold reservoir, a warm reservoir, and a fill tank. A float valve automatically maintains the liquid levels in the bubbler tank and the cold reservoir. Alcohol enters the bubbler tank only from the cold reservoir so that its temperature has already been established. The warm reservoir, sitting above the refrigerator, is equipped with a sight glass and serves as the main on-line alcohol storage vessel. When the level of liquid in this tank becomes low it must be manually refilled from commercially supplied bottles using the fill tank. The refrigerator used to maintain the alcohol bubbler temperature has been modified specifically to make it safe for containing flammable gasses and liquids.

Filling the alcohol reservoir is not trivial. Please refer to and carefully follow the procedure detailed in section 3.3.2 Adding Alcohol.

Delivery Pressure Control

Gas will be metered to each detector element through a needle valve. To achieve a constant flow through a needle valve a constant differential pressure must be maintained across it, so it is necessary to provide a fairly constant supply pressure out of the mixing station. This comes only at a price, as the mass flow controllers deliver a fixed flow rate regardless of pressure (within practical limits). If the detectors in Hall-A consume less gas than the mixer supplies, the pressure in the supply lines will increase. Similarly, if less gas is mixed than is consumed the pressure will decrease. To provide a usefully constant pressure of about 15 psig in the supply line, a pair of pressure switches has been installed in the mixing station outlet. The first of these, the Primary Pressure Control Switch, is set to open at 16 psig and close again at 14 psig or below. When the pressure is low this switch is closed and the Flow Control System is commanded to use the flow rates set into its PROGRAM C ("high flow"). When the Primary Pressure Control Switch opens, PROGRAM-D ("zero flow") is selected. By setting PROGRAM-C to provide just a little more gas than required by the detectors, the supply pressure can be maintained at between 14 and 16 psig. with a cycle time of several minutes. This pressure variation will result in a flow rate variation of no more than about 15%, which should be of no consequence for the detectors.

A second pressure switch, the Overpressure Alarm Switch, is calibrated to open at 18 psig and re-close at 14 psig or below. If the delivery line pressure manages to exceed the 18 psig threshold it indicates a system failure of some sort and the gas interlock system is tripped by this switch. Manual operator intervention is then required to re-establish gas flow.

Pressure control of the inert gas supply, used to purge the detectors, is provided by a conventional single-stage regulator (PR-301) mounted inside the delivery rack. This regulator receives 45 psig inert gas (the same gas delivered to mixer flow channel 3) and provides 15 psig gas to the INERT supply line to Hall-A.