A setpoint of 500 SLPM but only 400 SLPM delivered, a nonzero reading after closure, or a reading that oscillates does not immediately prove that an MFC or MFM is damaged. Gas supply, pressure differential, leaks, configuration and electrical conditions can all affect the result. Check external conditions first, then the instrument, moving from simple checks to more involved tests.
1. Describe the symptom precisely
- Stable reading, but delivered flow does not agree: check gas identity, conversion factors and calibration.
- Oscillating or jumping reading: check source pressure, pressure differential, leakage and electrical interference.
- Nonzero reading after an MFC is closed or commanded to zero: distinguish zero drift from valve leakage.
Record the command, reading and inlet/outlet pressures before making changes. A meter has no control valve or flow-control setpoint; its checks concern the reading and the external gas path.
2. Check the gas source and pressure differential
Check cylinder pressure, other branches consuming gas and regulator outlet stability. Then measure both inlet and outlet pressures.
Adequate inlet pressure does not necessarily mean adequate pressure differential. Downstream backpressure, undersized tubing or too many restrictive fittings can reduce available differential pressure, preventing the controller from reaching its setpoint and slowing stabilization. Compare both pressures and their difference with the ordered operating conditions.
3. Examine the regulator outlet pressure
An unstable regulator can make the controller continually correct its valve position. Possible causes include a regulator too large for the actual flow, aging spring or diaphragm components, operation beyond rated flow, or unsuitable piping and vent conditions.
Measure pressure near the MFC inlet while keeping the flow command constant. A pressure trend or oscillation indicates that the upstream supply needs investigation. Choose a suitable pressure measurement point; the article's suggestion of a straight section more than 1 m long is an example, not a universal installation requirement. Regulator sizing or an appropriately designed buffer volume may help, but must suit the system's pressure rating and response requirements.
4. Check leakage and restrictions
A leaking fitting can change delivered flow; a blocked upstream filter can reduce available pressure and slow response. Leak-test new or disturbed connections before service and replace filter elements according to the supplier's condition or maintenance criteria. See the leak-testing guide.
5. Confirm warm-up and a genuine zero-flow condition
Thermal instruments require stable thermal conditions. Follow the model's warm-up instructions; the original article uses at least 15 minutes as a general example.
Zero only under confirmed no-flow conditions with stable temperature. Closing an MFC control valve alone is not proof that flow has stopped. Adjusting zero while gas is still flowing can conceal a gas-path problem and invalidate subsequent readings.
6. Check the gas and conversion method
A stable but consistently high or low reading may result from using a different gas without conversion. A configured digital model with a supported gas library can report converted flow after the correct gas is selected. Confirm library coverage for the actual model; the number of gases is not assumed to be identical for every version.
With a 0–5 V or 4–20 mA analog model, convert readings—and controller commands—using the agreed coefficient. See gas-factor conversion. Variable mixtures may need a different calibration strategy.
7. Check the valve, signal and interference
- Repeated drift or residual flow after zeroing can indicate contamination or damage at a controller valve seat; contact the manufacturer.
- Irregular readings call for supply and terminal checks. The current product supply is 9–36 VDC, commonly 24 VDC. Route signal cables away from power wiring and variable-frequency drives; use shielding and grounding according to the equipment instructions. Confirm RS485 addresses and communication settings.
- A compatible FL-100 or host software can help compare command and feedback, separating a signal-chain issue from a gas-path issue. For an MFM, compare its reading against an appropriate reference rather than looking for a controller setpoint.
Symptom-to-check reference
| Symptom | Check first | Initial action |
|---|---|---|
| Flow cannot reach the command | Insufficient differential pressure or supply; blocked filter | Measure inlet/outlet pressure and inspect the filter |
| Reading consistently high or low | Wrong gas factor or calibration mismatch | Confirm gas/factor and compare with a suitable reference |
| Reading jumps or oscillates | Regulator pressure, small leaks, interference | Log inlet pressure; leak-test; inspect shielding/grounding |
| Nonzero reading after controller closure | Zero drift or valve leakage | Establish genuine no flow; check zero; then test the valve |
| Slow response or large overshoot | Restriction or excessive gas-path volume | Check the filter and piping volume |
When to contact the manufacturer
Repeated zero drift, ineffective approved purging, suspected sensor contamination, or disagreement beyond the specified accuracy requires professional assessment. Do not open the instrument yourself. Record the gas, calibration range, pressures and observed readings for the engineer.
Upstream filtration, control of moisture/oil, and calibration at an interval appropriate to the application reduce recurring problems. Annual calibration may be a useful starting point, but the process and the model's instructions determine the interval.
For selection, installation or technical enquiries, contact Flowmethod: Mr. Dou, +86 180 3818 3381; doufeijin@hotmail.com. See Contact for the full contact list.