There are no two identical leaves in the world, but there can be two flow meters with consistent readings. If you find that the readings of two flow meters are different, please do not directly determine that there is a problem with the flow meter.
Alicat flow meters and flow controllers are based on the principle of laminar pressure difference for measurement, with built-in temperature, absolute pressure, differential pressure sensors, and viscosity data of various gases. After switching to the corresponding gas mode, they can measure various parameters such as temperature, pressure, (working condition) volume flow rate, (standard condition) mass flow rate, and cumulative flow rate of different gases.
The typical response time of Alicat flow meter sensors can be as fast as 10ms, with a measurement range of 0.5% -100% F.S. Some models can have lower measurement limits as low as 0.01% F.S. The accuracy is generally ± (0.8% Rdg+0.2% F.S.), and some models can achieve ± (0.5% Rdg or 0.1% F.S.).
If the flowmeter or controller has not been calibrated for a long time, you may be concerned about its accuracy. We suggest sending the flow meter or controller back to the after-sales service center for annual calibration to maintain high accuracy. Of course, you can also choose other methods to check if its accuracy exceeds the standard.
Most customers will return the flow meter to us for calibration, while some customers will send the flow meter to the metrology institute for accuracy evaluation. Some customers will also connect multiple flow meters in series for reading comparison.
Regardless of the method used, the basic method is to connect the tested flow meter in series with another reference flow meter or flow controller that is close in range, has higher accuracy, and is trustworthy. Then, air or nitrogen is introduced to control different flow sizes within the range and observe whether the readings of the two flow meters are consistent. If the readings are consistent, it indicates that the accuracy of the tested flow meter is high.
During the comparison process, if you find that there is a difference in the readings between the tested flow meter and the reference flow meter, there is no need to be disappointed, as there may also be other reasons causing the difference in flow readings between the two. You can check against the factors listed below.
Part 1/Selection of Reference Flowmeters
(1) Is the reference flow meter accurate and of the same level or higher accuracy as the tested flow meter? It is recommended to use a flow meter with higher accuracy as a reference standard.
(2) Has the reference flow meter been calibrated recently? It is not recommended to use long-term uncalibrated flow meters as reference standards.
(3) Is the reference flow meter close to the range of the tested flow meter? The reference flow measurement range should be close to or slightly larger than the tested flow meter.
(4) What is the measurement principle of a reference flow meter and is the measurement result a mass flow rate? It is recommended to choose a mass flow meter as the reference flow meter, or a flow meter with a reading that can be converted to standard mass flow rate. If the measurement result is the operating volume flow rate, it will not be comparable due to the system pressure loss and the pressure loss of the flowmeter itself.
PART 2/Pipeline connection issue
(1) Are there any air leaks in the pipelines, connectors, and threaded interfaces? If the system leaks, it will result in inconsistent readings between the two flow meters.
(2) Is the pipeline between the two flow meters too long? The length of the connecting pipeline between the tested flow meter and the reference flow meter should be minimized as much as possible.
PART 3/Parameter Setting Issues
(1) Are the STP or NTP standard temperature and pressure settings of the two flow meters consistent? Both must be set to the same standard temperature and pressure.
(2) Is the flow meter reset before the test when there is no airflow passing through? If the flow reading is not zero when there is no airflow, it is recommended to use the reset button TARE to reset before conducting a bump test.
PART 4/Medium Gas Problem
(1) Is the gas type selection consistent between the two flow meters? Is the gas mode consistent with the actual gas tested? It is necessary to ensure that the gas mode selected by both is consistent with the actual gas tested.
(2) Is the temperature and pressure of the test gas within the normal operating range of the two flow meters, and is it close to the calibration temperature and pressure of the two flow meters? It is generally recommended to use gas at room temperature and pressure for testing. If the temperature is too low or too high, and the pressure is too high, it may cause additional flow errors.
(3) Is the test gas dry and pure? If the gas contains a large amount of water vapor, particulate matter, oil fumes, and impurities, it may cause additional measurement errors.
PART 5/Other Questions
(1) Is the data record the standard mass flow rate? Mistaking the volumetric flow rate of the working condition as the mass flow rate can also lead to a reading deviation between the two. The units of mass flow are generally SCCM, SLPM, NCCM, NLPM, etc. The unit of volumetric flow under operating conditions is generally CCM, LPM, etc.
(2) Is the working power supply voltage of the flow meter the rated voltage? Low, high, and unstable voltage can all lead to reading deviation.
(3) If the flow measurement signal uses analog signals, the interference of the system on the measurement signal should be ruled out, as signal interference may cause deviation in the reading results.
(4) During the testing process, it is possible to observe whether the pressure, temperature, or other sensor signals of the flow meter display normally. If there are obvious abnormalities, it may lead to inaccurate flow readings.
If you have carefully checked all the above factors and the flow readings of the two flow meters are still inconsistent, please send your flow meter or controller back to us for calibration.