In this installment of Accurate Calibration Gas Using MFC Based Dynamic Gas Blenders, Tom discusses the proper development and maintenance of a Dynamic Gas Blender.
Developing and Manufacturing an Accurate Dynamic Gas Blender
Starting with a highly accurate MFC is only one part of developing an accurate gas blender. Another is how well a company develops the software and controls that will continue the accuracy chain.
When using MFCs controlled by an analog voltage, a minimum of 12-bit DAC and ADC should be designed into the instrument. Equally important is the accuracy of the precision reference voltage applied to these devices. With 12-bit resolution, increments of .0012 VDC are applied to the MFC to achieve precision control. Both the equipment and the MFCs should be CE certified. This ensures that the instrument will not be susceptible to electrical interference. The system should be built using 316 SS electropolished tubing, fittings, and MFCs. Alternate tubing and seal materials may be compatible with different gases. Proper selection can save considerable costs.
When developing software that will control a gas blender, improving the MFC specifications should be the primary focus. Some blender manufacturers, who use thermal mass flow controllers, have been known to install an MFC as it was received from the manufacturer without conducting any further calibration. Depending on the accuracy of an MFC manufacturer’s specification without verifying them with your own tests can result in a blender with errors greater than specified. A good gas blender manufacturer always uses a calibration standards lab to confirm an MFC’s factory specifications.
Double Check the Flow Standard
Every MFC installed in the a gas blender should be calibrated using an accurate flow measuring instrument that has a specification of .2% of reading or better. To achieve the highest, most accurate results with a blender, a minimum of eleven calibration readings should be gathered on the flow standard. The calibration data should then be entered into the gas calibrator software and used as a reference table. A flow-correcting algorithm; e.g., linear interpolation should then be applied by the blender software to improve the accuracy and linearity of the MFCs. The gas blender also should have an option that permits purging of the entire gas path using an inert gas.
The same principle should be applied to K-factor correction. Once an MFC manufacturer has been selected, that manufacturer’s library of K-factors should be used by the blender control software to make corrections as alternate gases are used.
Figure 4 illustrates one blender’s MAINTAIN PORTS mode. This mode permits the operator to enter the gas types being fed into the equipment’s input ports and MFCs. The software has a built-in standard that permits it to automatically compare the process gas to the original calibration gas for each MFC installed (up to 9 MFCs). Each bottle of gas can have up to nine individual component gases in a balance gas (typically nitrogen). The system automatically calculates and applies the K-factor for all the gases relative to the calibration gas of the MFC. This figure also shows three component gases in a nitrogen balance.
Figure 4. Model 2000 application of K-factor correction