BASIC PRODUCT INFORMATION & TROUBLESHOOTING FOR AERA® MFCS AND MFMS
The following information provides general product information for Aera MFMs and MFCs and a basic diagnostic troubleshooting guide to help you determine the source of a problem with one of these products.
Product Description
Calibration Reference and Flow Unit
MFM and MFC Handling Precautions
Theory of Operation
Conversion Factors for Non-Calibrated Gases
Basic Troubleshooting
Product Description
Advanced Energy Aera mass flow meters (MFMs) precisely measure the mass flow of various gases using the principle of heat transfer along a capillary tube. By measuring this heat transfer, the MFM develops a linear output signal of 0 VDC to 5 VDC over the selected flow range.
The MFC contains an MFM and an internal gas control valve. This valve adjusts the gas flow to match the set point signal (from a potentiometer or other source).
For both MFCs and MFMs, AE provides the necessary accessories, including power supplies, digital panel meters, potentiometers, line cords, and gas filters, allowing you to easily construct a complete gas system.
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Calibration Reference and Flow Unit
For calibration reference, Aera MFMs and MFCs use a standard temperature and pressure (STP) of one atmosphere at 0 °C (32 °F). While this reference reflects the generally accepted industry standard, other manufacturers may use different STP references. Such products will have different actual gas mass flows than Aera MFMs and MFCs. You should take account of these differences if mixing devices calibrated to different STP conditions.
Gas flow for Aera MFMs and MFCs is described in the following units:
Other calibration units such as percent, grams per minute and cubic meters per hour are available on request.
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MFM and MFC Handling Precautions
Use the following precautions when handling, installing, and using Aera MFMs and MFCs:
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The standard calibration position for Aera MFMs and MFCs is with the fittings horizontal and the electrical connector up. However, Aera MFMs and MFCs can be mounted in any position, though you may encounter small zero shifts with nonstandard positions. You can compensate for these shifts by adjusting the product's zero point.
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To keep vibration from loosening MFM and MFC electrical connectors, firmly attach the connectors with screws.
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Where high levels of electrical noise exist on the AC supply line, install a noise filter on the input side of the power supply.
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Thoroughly purge the MFM or MFC before using reactive gases (such as SiH4, SiH2Cl2, and HCl) or when replacing the gas cylinder.
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Piping the purge gas over long distances can cause contamination and subsequent corrosion in the gas system if leaks in the piping allow atmospheric gases to combine or react with the process gases.
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To prevent contamination of the MFM or MFC, use a gas filter designed for semiconductor manufacturing equipment just prior to the MFM or MFC.
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For best results, use MFCs in the upper part of their flow range.
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Aera MFMs and MFCs are typically supplied with Viton O-rings. These O-rings are not suitable for use with NH3, SF6, H2S, and SO2, etc. For proper sealing materials when using these gases, contact AE Global Customer Support.
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Theory of Operation
When gas flows through the MFM or MFC, it is split into two streams. A larger stream is routed through the bypass assembly, and a smaller stream, proportional to the larger stream, is routed through the sensor. The MFMs and MFCs are designed so that the ratio between these two streams remains constant over the measurable flow range of the instrument.
The sensor consists of a small stainless steel capillary tube with two thermal resistors wound around the outside of the tube. Current passes through the resistors, which thereby act as heaters, warming the sensor tube. When gas is flowing, heat is transferred from the upstream resistor to the downstream resistor, and the degree of imbalance is proportional to the mass flow rate of the gas (and independent of factors such as temperature and pressure). The resistors form part of a bridge circuit, which senses the temperature imbalance. The detected level of imbalance is then amplified to produce an output signal.
In an MFC, a comparator circuit senses the output signal of the MFM and compares it to the set point. The comparator circuit generates an output signal from this comparison, which in turn drives the control valve to close or open until the set point and the MFM output signal are equal.
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Conversion Factors of Non-Calibrated Gases
All Aera MFMs and MFCs are calibrated for a specific gas and flow range, which are indicated on the unit nameplate. You can use the units with other gases than those indicated on the unit as long as the gas is compatible with the O-ring materials.
Because the sensitivity of the MFM differs depending on the gas, you must apply a correction factor to the output reading if you choose to use a gas other than the one for which the unit was calibrated. For information on specific conversion factors, contact AE Global Customer Support.
NOTE: The accuracy of an MFC used with a conversion factor will be less than for an MFC calibrated specifically for that gas. When the conversion factors or densities of calibrated gas and the gas used differ significantly, you may encounter problems in setting the control valve.
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Basic Troubleshooting
The following checklist is designed to help you identify whether the problem is an internal problem with the MFC or a problem external to the MFC. Use the checklist to check for external causes. If the checklist does not resolve the problem, you can then refer to the MFC manual, which may contain information for resolving internal MFC problems or you can contact technical.support@aei.com for additional help or return/repair information:
- Are the isolation valves open when you want gas to flow?
- If NO, the isolation valves are not open, check the following:
- Is there adequate valve operating pressure (from 75 psig to 90 psig)? (Most pneumatic valves require an operating pressure in this range.)
Is the correct controlling tube connected to the correct isolation valves?
- If YES, the isolation valves are not open, proceed to the following questions.
- Is there adequate MFC inlet pressure (from 10 psig to 30 psig)? High-flow MFCs require at least 30 psig.
- Are the +/- 15 VDC voltages present at the MFC connector? The MFC will not operate without these voltages connected.
- Is the set point signal present at the MFC connector (from 0.1 VDC to 5 VDC)? Aera MFCs require a minimum set point voltage of 0.1 VDC.
- Is the set point signal at the correct voltage?
- If an adjacent MFC is functioning properly, swap cables from the functioning MFC to the one you are troubleshooting.
- If the problem is resolved, one or more of the cables are not functioning properly.
- If the problem continues, there is an internal problem with the MFC.
- For additional, unit-specific troubleshooting information, see the user manual that came with the device, or contact technical support (technical.support@aei.com).
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