To calculate the gas conversion factor for a specific gas, you divide the gas flow rate of that gas by the gas flow rate of nitrogen that produces the same output voltage from a mass flow controller (MFC).
Here's a breakdown of the process:
Understanding Gas Conversion Factors
Gas conversion factors are crucial when using Mass Flow Controllers (MFCs) because MFCs are typically calibrated for a specific gas, often nitrogen (N2). When you use a different gas, you need to account for the difference in its properties (like viscosity, heat capacity, and density) compared to nitrogen. The conversion factor allows you to relate the MFC reading (which is based on nitrogen) to the actual flow rate of the gas you're using.
The Calculation:
The core principle is to find the flow rate of nitrogen that produces the identical response in your MFC as a particular flow rate of your target gas.
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Step 1: Determine the MFC Output Voltage. When flowing your desired gas at a specified flow rate. Record this output voltage.
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Step 2: Find the Equivalent Nitrogen Flow Rate. Adjust the nitrogen flow rate through the MFC until the MFC output voltage matches the voltage recorded in Step 1. Record this nitrogen flow rate.
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Step 3: Calculate the Conversion Factor.
Gas Conversion Factor = (Flow Rate of Target Gas) / (Flow Rate of Nitrogen)
Example:
Let's say you want to use Argon (Ar) and the MFC is calibrated for Nitrogen (N2).
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You set the Argon flow to 10 standard cubic centimeters per minute (SCCM). The MFC provides a voltage output of 2.5 volts.
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You then switch to Nitrogen and adjust the flow until the MFC also reads 2.5 volts. This occurs when the Nitrogen flow is at 8 SCCM.
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The gas conversion factor for Argon, relative to Nitrogen, would be:
Gas Conversion Factor = (10 SCCM Argon) / (8 SCCM Nitrogen) = 1.25
This means that to achieve a 10 SCCM flow of Argon, you would need to set the MFC to read as if it were flowing 8 SCCM of Nitrogen.
Why is this important?
- Accurate Flow Control: Using the correct conversion factor ensures you are delivering the precise amount of gas required for your process or experiment.
- Process Repeatability: Consistent and accurate gas flow is essential for reproducible results.
- MFC Protection: Operating outside of the MFC's intended parameters can damage it.
Important Considerations:
- Calibration Curves: Some MFC manufacturers provide calibration curves or charts for different gases, which can simplify this process. These curves directly relate the MFC's setpoint to the actual gas flow rate.
- Gas Properties: The accuracy of this method relies on the stability of gas properties under the operating conditions.
- MFC Type: The specific method and considerations can vary slightly depending on the type of MFC you are using (e.g., thermal MFC, pressure-based MFC). Consult the manufacturer's documentation for detailed instructions.
- Operating Conditions: Ensure the temperature and pressure are stable during the measurements of both the gas and the nitrogen flow rates. Variations in these conditions can affect the gas density and therefore the accuracy of the calculated conversion factor.
In short, calculating the gas conversion factor is achieved by comparing the flow rates of the target gas and nitrogen that result in the same MFC output. This ensures accurate and controlled gas delivery in various applications.
