There are many reasons why this can occur, such as a change in mounting orientation, ambient temperature, or process pressure. While mass flow controllers (MFCs) from Brooks Instrument are known for stability and minimal long-term drift, the flow signal or process variable (PV) from an MFC may drift over time.
To ensure the best possible accuracy, it is recommended to zero the MFC at initial installation and periodically thereafter. The best way to know when your MFC needs to be re-zeroed is to see if there is flow signal (positive or negative) at a zero flow condition. Keep in mind, MFC valves are precision control valves, not positive shut off valves, so a small amount of valve leak-by may exist.
To zero your device, it is recommended that you follow these steps:
- Allow for a warm-up time of at least 45 minutes to get to normal operating temperatures
- It is preferable to zero at process conditions (meaning inlet pressure should be the same as the normal device operation)
- Close a downstream shutoff valve and open the MFC valve using a setpoint or valve override (VOR) open command. This will fill the lines up to the shutoff valve and pressure will equalize across the MFC.
- Allow 30 seconds minimum for stabilization
- Monitor the output (PV) signal
- For digital devices, press the zero button until the LEDs indicate the device is re-zeroing. The LED indicator turns green when the zero process is complete. PV signal should be at zero. Repeat if necessary.
- Alternatively, digital devices can be re-zeroed using service tools (BSS or BEST) from Brooks Instrument.
- For analog devices, monitor the output signal and adjust the zero potentiometer until zero is achieved.
- Remove VOR open command and/or set point
- Open the downstream shutoff. Monitor the PV for any flow indication. If you are seeing flow, this could be the valve leak-by as described above.
Brooks Instrument MFCs are zeroed at the factory. An initial zeroing may be necessary upon installation. By setting up the device, as outlined in the first five steps above, you can determine whether an initial zeroing is needed, or if it is good to go.
If you need assistance please contact our Technical Services team or your local Brooks Instrument sales representative for assistance.