Faster Flow Calibration for Gas Analyzer Audits

Posted by: Danielle Adams

Alicat’s “Whisper” series portable flow meters reduce the time it takes to conduct flow calibrations and audits of gas analyzers and gas dilution calibrators used in ambient air monitoring and other applications.

Auditing Gas Analyzers and Gas Dilution Calibrators

Gas analyzers and a pair of gas dilution calibrators in a typical ambient air monitoring shed. Alicat Whisper flow calibrator pictured at bottom left.

Gas analyzers and gas dilution calibrators in a typical ambient air monitoring shed. Alicat Whisper flow calibrator pictured at bottom left.

Flow calibrating a gas analyzer can consume your entire day. Most dreaded are low flow calibrations, which can take an hour or two alone. You watch the bubble or piston slowly rise to the top of the positive displacement tube at a pace of 10 sccm over the course of several minutes, and cheer when it finally reaches the top. Do that another nine times for a single averaged data point, unless the bubble sticks or the piston stutters along the way. There must be a better way.

Every analyzer technician knows that maintaining a constant sample gas flow rate is critical to ensuring the measurement accuracy of the analyzer. Gas analyzers are used in ambient air monitoring to identify the concentrations of greenhouse gases such as carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) that are found in the air around us. Process analyzers are used to monitor the concentrations of such gases in smokestack emissions. Unfortunately, the flow controllers that are built into these analyzers often do not live up to our expectations, and this necessitates the use of regular flow check, calibration and audit cycles to ensure the quality of the data.

Gas dilution calibrator, showing its program of calibration flow rates.

Gas dilution calibrator, showing its program of calibration flow rates.

Analyzer flow rates can be checked and calibrated directly using high-accuracy flow meters. When it comes to testing the compositional analyses of the gas analyzers, gas dilution calibrators are used, which themselves use two or three mass flow controllers to control the composition of their outlet gas streams. Gas dilution calibrators combine a large flow rate of unpolluted air with a smaller flow rate of the greenhouse gas being analyzed, for example CO2, to provide the CO2 gas analyzer with a sample of the pollutant gas at the desired concentration.The presence of flow controllers within these dilution calibrators again requires the use of high-accuracy flow meters to ensure their correct operation.

Faster Flow Calibration with Laminar Differential Pressure

Calibrations and audits of gas analyzer and dilution calibrator flow controllers have often been performed using positive displacement flow meters. “Wet” positive displacement flow meters measure the time it takes for air bubbles to pass through a tube coated in a soapy solution. “Dry” positive displacement meters measure the time it takes for air to push a piston up a tube. In both cases, a single flow measurement data point can take as little as a few seconds up to many minutes to complete, depending on the flow rate. Calibrating all the analyzers in an ambient air monitoring shed can easily take a full day with these slow measurement methods.

Alicat Whisper-series portable mass flow meter.

Alicat Whisper-series portable mass flow meter.

Alicat Scientific’s “Whisper” series flow meters measure extremely low differential pressure through a laminar flow element to present live readings of both actual volumetric flow and standardized mass flow. Flow readings are calculated a thousand times each second (1000 Hz), so even the lowest flow rates are visible instantly. Streaming and recording this data over RS-232 can reveal cyclical flow inconsistencies and help identify underperforming mass flow controllers. The wide flow ranges of Alicat’s NIST-traceable mass flow meters (200:1, or 0.5% to 100% of full scale) means you can calibrate analyzer mass flow controllers down to the bottoms of their ranges with confidence in the accuracy of their calibration. Using a Whisper, calibrations at the bottom of a mass flow controller’s flow range take just seconds instead of hours to obtain a robust average of flow measurements. Portable Whispers (MWB series) integrate a rechargeable battery for even greater convenience and efficiency while you are conducting your flow audits.

Whisper-series mass flow meters are available in common analyzer flow controller ranges of 20 slpm, 10 slpm, 200 sccm, 100 sccm and 5 sccm, or any other range of your choosing between 0.5 sccm and 1000 slpm. These flow meters are also well-suited to other applications, including:

  • Auditing the flow rates of indoor air sampling instruments used in industrial hygiene applications,
  • Setting the mechanical flow controllers used with air sampling canisters, and
  • Calibrating the flow controllers used in gas chromatographs and other analytical instruments.

Explore the Whisper product page to learn more about these fast flow meters.

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