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Microprocessor driven digital flow controllers allow one to program, record, and analyze flow rates of various gases with a computer via an RS485 interface (optional RS232 is available). Controllers can be programmed for various control functions including flow setpoint, totalizer, stop totalizer, read totalizer, totalizer from preset flow, stop at preset total, auto zero, and more.
Principles of Operation
Metered gases are divided into two laminar flow paths, one through the primary flow conduit, and the other through a capillary sensor tube. Both flow conduits are designed to ensure laminar flows and, therefore, the ratio of their flow rates is constant. Two precision temperature sensing windings on the sensor tube are heated, and when flow takes place, gas carries heat from the upstream to the downstream windings. The resultant temperature differential is proportional to the change in resistance of the sensor windings. A Wheatstone bridge design is used to monitor the temperature dependent resistance gradient on the sensor windings which is linearly proportional to the instantaneous rate of flow. The output of the Wheatstone bridge is converted to digital format with a 12 Bit ADC (analog to digital converter). An on-board microprocessor and nonvolatile memory store all calibration factors and directly control a proportional electromagnetic valve. The digital closed loop control system continuously compares the mass flow output with the selected flow rate. Deviations from the setpoint are corrected by compensating valve adjustments, with PID algorithm thus maintaining the desired flow parameters with a high degree of accuracy. Output signals of 0 to 5 Vdc or 4 to 20 mA are generated indicating mass molecular based flow rates of the metered gas.
The digital interface operates via RS485 (optional RS232) and provides access to applicable internal data including FLOW SETPOINT, ACTUAL FLOW, ZERO ADJUSTMENTS, and LINEARIZATION TABLE ADJUSTMENTS. The analog interface provides 0 to 5 Vdc, 0 to 10 Vdc and 4 to 20 mA inputs and outputs.
The FMA6500 automatically nulls the sensor zero offset whenever the flow setpoint is below 2% of full scale. To accommodate this feature the control valve must fully close under that condition. Provisions are made to either disable, force or store the current auto zero via digital commands.
The firmware for the FMA6500 provides functions to register total gas quantity. The total mass of gas is calculated by integrating the actual gas flow rate with respect to time. Digital interface commands are provided to: SET the totalizer to ZERO; START /STOP totalizing the flow; READ the totalizer; START the totalizer at a preset flow STOP the flow at a preset total.
The FMA6500 is capable of storing primary calibration data for up to 10 gases. This feature allows the same FMA6500ST to be calibrated for multiple gases while maintaining the rated accuracy on each.
Conversion factors for up to 256 gases are stored in the FMA6500. Conversion factors may be applied to any of the ten gas calibrations via digital interface commands.
High and Low gas flow ALARM limits are programmed using the digital interface. Alarm conditions are reported via the digital interface or can activate the contact closure outputs.
OMEGA software supports programmable flow modes, allowing execution of custom programming of up to ten steps. Various flow configurations include ramping, linearized increasing and decreasing modes.
The AUTOTUNE function allows the FMA6500 to automatically optimize control response for the gas under actual process conditions. During the AUTOTUNE process, the instrument adjusts PID gains for optimum step response and determine key control valve characteristics (only available on units with less than 80 L/min maximum flow).
Two sets of dry contact relay outputs are provided to actuate user supplied equipment. These are programmable via the digital interface such that the relays can be made to switch when a specified event occurs (e.g. when a low or high flow alarm limit is exceeded or when the totalizer reaches a specified value).
Means are provided to force the control valve fully open (purge) or fully closed via either the analog or digital interfaces.
Whenever power is first applied, the FMA6500 runs a series of SELF DIAGNOSTIC TESTS to ensure that it is in optimum working condition.
The flow setpoint, measured gas flow and associated totalizer data is scaled directly in engineering units via digital interface commands. The following units of measure are supported: % of FS, mL/min, mL/hr, scfm, scfh, sL/min, sL/hr, lbs/hr, lbs/min, and one user defined unit of measure.
1 x 10-9 smL/sec of Helium maximumto the outside environment.
Balanced Power Supply
The FMA6500 operates on ±15 Vdc. The current requirements for the positive and negative power supplies are balanced such that the current in the power supply common connection is minimized. Maximum power consumption is 13.5 watts at ±15 Vdc.
FMA6500 Series Transducers should not be used for monitoring Oxygen gas unless specifically cleaned and prepared for such application. See ordering instructions below.
Accuracy (including linearity): 15 to 25°C (59 to 77°F) and 0.7 to 4 bar (10 to 60 psia): ±1% of FS, 0 to 50°C (32 to 122°F) and 0.3 to 10 bar (5 to 150 psia): ±2% of FS, ±1% of FS at a specific temperature and pressure with special calibration
Repeatability: ±0.15% FS
Turndown Ratio: 50:1
Response Time: 0.6 to 1.0 s to within ±2% of setpoint over 20% to 100% FS
Temperature Coefficient: 0.05% of full scale/°C or better
Pressure Coefficient: 0.01% FS/psi (0.07 bar) or better
Leak Integrity: 1 x 10-9 smL/sec Helium maximum to the outside environment
Optimum Gas Pressure: 1.73 bar (25 psig)
Max Gas Pressure: 34.5 bar (500 psig)
Max Diff. Pressure: 3.4 bar (50 psig) for up to 10 LPM, 2.8 bar (40 psig) for 15 LPM and greater
Gas and Ambient Temp: 5 to 50°C (41 to 122°F)
Output Signals: Linear 0 to 5 Vdc (2000 Ω min load impedance); 0 to 10 Vdc (4000 Ω min impedance); 4 to 20 mA optional (0 to 500 Ω loop resistance)
Communication Interface: RS485, standard; RS232, optional
Transducer Input Power: ±15 Vdc, 450 mA maximum
Wetted Parts: 316 stainless steel, 416 stainless steel, FKM O-rings; Neoprene or Perfluoroelastomer O-rings optional
Connections: Standard 1/4" compression fittings up to 30 LPM models; for 60 LPM models and greater: 3/8" compression fittings
Circuit Protection: Circuit boards have built-in polarity reversal protection; resettable fuses provide power input protection
Calibration Options: Standard 10-point NIST calibration optional up to 9 additional 10-point calibrations may be ordered for an additional charge