Comparison of Gas Mass Flow Meter Accuracy
In our interactions with customers, we often receive feedback that when flow meters from different brands are tested in series, discrepancies are found between their readings. Does this mean that one of the meters does not meet accuracy standards? The answer is not that simple. As precision measurement instruments, discrepancies in flow meter readings are often caused by a combination of factors. This article will analyze common causes to help you scientifically interpret the results of comparison tests.
Common Causes of Discrepancies in Flow Meter Readings
01: Cumulative Error from the Flow Meters Themselves
When two flow meters, both with an accuracy of 1% F.S., are connected in series for comparison, the difference in readings may approach 2% F.S. in the worst-case scenario, leading one to mistakenly believe that one of them is out of specification.
02: Inconsistent Gas Type
If air is used for testing but the gas type measured by the two flow meters is not the same, the readings will differ.
03: Inconsistent Definition of Standard Conditions
When comparing the standard condition flow rate (mass flow rate) of two flow meters, it is essential that the definition of standard conditions is consistent. Standard condition pressure is typically 101.325 kPaA, while standard condition temperatures include 0°C, 15°C, 20°C, and 25°C. Before testing, ensure that the standard condition temperatures are identical (note that this refers to standard condition temperatures, not actual temperatures). Flow meters can be switched between these four standard condition temperatures.
04: Excessive Range Discrepancy
Comparing the accuracy of a flowmeter with a 100 SLPM range to one with a 1 SLPM range is meaningless, as flowmeters often have poorer accuracy in the low-range segment. A flowmeter with a 100 SLPM range (assuming an accuracy of 1% F.S.) may have an error as high as 1 SLPM when measuring a flow rate of 1 SLPM.
05: Differences in Temperature and Pressure
If one flowmeter operates under high pressure while the other operates at atmospheric pressure, pressure-induced drift will occur; similarly, different temperatures will introduce temperature-induced drift.
06: Pipeline Connection Issues
Pipeline leaks or loose fittings can cause the actual flow rates through the two flowmeters to differ. When the flowmeter has a small measurement range but the intermediate pipeline is long and large-diameter, the volumetric effect becomes significant, leading to inconsistent readings between the two flowmeters. If the outlet of one flow controller is directly connected to another flow meter at close range, it can also affect the accuracy of the other flow meter.
07: Misunderstandings Arising from Different Measurement Principles
For example, laminar differential pressure flow meters and thermal flow meters differ significantly in response speed (measurement sensitivity) and gas switching principles, which can easily lead testers to use incorrect testing methods. For example, when the gas supply pressure fluctuates, the laminar differential pressure principle responds very quickly, causing the measured values to appear to fluctuate as well, whereas the measured values from a thermal flowmeter will appear relatively stable. Additionally, a common misconception is that if two nitrogen flowmeters based on different principles are both fed with air for comparison and the results are converted using their respective conversion methods, the resulting values are likely to be inconsistent.
08: Differences in Traceability
Flow meters rely on traceability to establish measurement standards, and traceability varies by manufacturer. Domestic flow meters are typically traceable to the National Institute of Metrology, while imported brands may be traceable to other institutions. Even if both flow meters fall within their nominal accuracy ranges, their readings may still differ to some extent.
In addition to the reasons mentioned above, factors such as zero drift, sensor accuracy, and stability can all affect measurement values.
How should flow comparisons be conducted correctly?
The two flow meters under test must primarily meet the following criteria:
- Similar measurement ranges;
- Consistent standard conditions (temperature);
- Consistent gas type, matching the actual gas being fed through the system;
- Similar test conditions (i.e., temperature and pressure); measurements are best taken at ambient temperature and pressure;
- Pipe diameters should be appropriate, and the piping should be unobstructed. If the flowmeter under test has a small measurement range, the intermediate piping should be as short as possible to minimize the impact of volume effects; if the measurement range is large, the two flowmeters should be spaced apart to prevent the upstream flowmeter from affecting the flow pattern of the downstream flowmeter. Ensure that the piping is free of leaks and blockages, and that the pipe diameters are appropriate.
In summary, if the readings from two flow meters do not match, one should not hastily conclude that one of them is inaccurate. It is necessary to review the testing process and compare results with equipment from an authoritative institution. A more convenient approach is to use a P-series handheld flow meter to obtain a certificate from an authoritative institution, designating it as a flow standard device. This allows for flow comparisons of your flow meters and flow controllers across various scenarios.