Control System for a four component Gas Mixing Unit Suvendu Nath Bose, Satyajit Saha, Sudeb Bhattacharya, Saha Institute of Nuclear Physics, Kolkata Description The 8 L mixer volume has ten cascaded mixing volumes inter-connected to enhance mixing while passing through the elongated path of gas flow. The pressure and the temperature inside the four-input Mixer Volume [MV] are continuously monitored and displayed electronically. Four input gas lines of argon, tetrafluoroethane or R134A, isobutane and SF 6 connect the system to the four gas cylinders through Quick-Connect [QC] and in each channel, the gas passes through a Three Way Valve [TWV], an Electromagnetic Valve [EV], a Mass Flow Controllers [MFC] – with optional by-pass lines via Control Valve [CV] for manual mode of operation, and a Non-Return Valve [NRV] to the four-input MV. The auto-lock feature of the QC isolates the system from ambient when the input line is disconnected and the TWV in the input helps in purging the input pipe, required while replacing the gas cylinder in any channel. The NRV restricts reverse flow, if any, of gases in the pipe line from the MV. One TWV in the output line of the MV connects the MV either to the detector [RPC] through a CV and QC, or to a vacuum pump required for evacuation of the system. One Safety Bubbler connected parallel to the output QC releases the mixed-gas to exhaust line in cases of occasional over pressure, if any, in the RPC. The return line from the RPC is connected to the exhaust via a Bubbler placed in the Electronic Controller [EC] unit. Control Circuit Principle and Schematic diagrams The Electronic Controller [EC] operates the EV, displays the percentage setting and the final “volume flow rate” for every channel, as well as the pressure monitor [PM] and temperature monitor [TM] readings. It also controls the percentage setting in each channel, and the final flow rate of the mixed gas. In Static Mixing Mode, the control switch for EV in each channel is operated to select the channel, and the displays for PM and TM are used for mixing the gases in desired proportion. The MFCs are bypassed through CVs for sequential filling of the component gases. The final flow rate is controlled by the CV at the out put line, and other displays are not used. In Dynamic Mixing Mode, the control signals for each MFC is generated electronically following: Final gas volume Flow Rate = K (P[Argon] + P[R134A] + P[Isobutane] + P[SF 6 ]), where K is 10% to 100% of the maximum flow rate. ∑ P[X] = 100% and P[X] the is the percentage of the constituent gas [X]. The MFCs for argon, R134A, Isobutane and SF 6 are selected to feature a FS flow rate to be 2.5 L/Hour, 2.5 L/Hour, 0.5 L/Hour and 0.1 L/Hour. respectively so that the following percentage-controls are possible: Argon and R134A from about 40% to 50 % Isobutane from about 1% to 10% and SF 6 from about 0.1% to 2% The control scheme is as shown, where potentiometer P0 sets K, which controls the final flow rate, and potentiometers P1, P2, P3 and P4 set the opening of the MFC used for argon, R134A, isobutane and SF 6 respectively. The reference voltage is derived from a temperature compensated zener-diode and resistors R1-4 set the minimum percentage setting in each channel. Each potentiometer P[1-4] drives Voltage to Current converter [V to I] for converting the generated control voltage to the current required to drive the 4-20mA control current loop of each MFC. The control voltages thus generated are summed up and used to display the percentage flow with the help of the analog divider [A/B] as shown. The output current from the MFC indicating the actual volume flow rate, through the 4-20 mA current loop, is again converted to voltage by current to voltage converters and are displayed as L/Hr. Testing & Results Each Digital Volt Meter used in display and converter [V to I and I to V] have been tested and calibrated using Fluke Calibrator 5500A. The error percentage in converters is less than 0.5%. Test data with clean air in each MFC on variation of flow rate and the final ratio setting indicate good agreement with the MFC specifications for different gases and flow rates within uncertainties of less than 5%. Measured average concentrations by RGA set up: Argon: 37.1 ± 1.4 % Isobutane: 7.2 ± 0.5 % R134A: 55.7 ± 1.9 % for a preset ratio of Argon : Isobutane : R134A :: 34 : 7: 59 Plumbing Scheme DVMs for Status display Control scheme for MFC Component placement details Resistive Plate Chambers [RPC], planned to be used in the ICAL detector of the INO project, need precise mixing of four different gases, as well as precise control of the flow rate of the mixed gas. The designed system has the capability to pre-mix [Static Mixing Mode] the gasses stored in a pressurized vessel for subsequent use, and the unit can also dynamically control the ratio of the four constituent gases and deliver it at a controlled flow rate [Dynamic Mixing Mode] through the mixing volume to the RPC.