What is gas flow calibration?
Gas flow calibration refers to the calibration of a flow sensing device such as a flow meter or flow controller by comparing its measurement against a flow measurement reference. Typically the device, or unit under test (UUT) is pneumatically connected in series with the flow reference so that they measure the same gas flow, then the indications of the two devices are compared.
Where is gas flow calibration used?
Flow meter/controller manufacturing and service
Manufacturers of flow meters and mass flow controllers (MFC) have a large volume of calibrations to perform and require a standard that is fast, accurate and can be automated. molbloc/molbox’s accurate real-time flow output is especially important for manual adjustment of mass flow meters and mass flow controllers. Many MFC service centers use molbloc/molbox to perform manual analog MFC calibrations.
Technicians use molbloc/molbox for calibration, testing and troubleshooting instruments and test systems from manufacturing to research applications. In these applications, the key requirements are often repeatability, speed and portability.
These and other processes involving coatings and gas deposition processes are heavily reliant on the performance and stability of mass flow controllers. Calibrating MFCs at the plant-level may save money over outsourcing the service, and having a reliable reference for troubleshooting can save hours of downtime. A molbloc can often be connected directly into a manifold to check MFC performance without uninstalling.
Gas flow rates into bio-reactors and fermenters can be critical, and molbloc/molbox is used to perform traceable MFC calibration in many pharmaceutical plants. Transportability is key for many in-situ calibration applications, and molbloc/molbox can deliver a portable solution for up to hundreds or even thousands of liters per minute with molbloc-S.
Accurate testing of both flow meters and controllers is key in research facilities developing petrochemical processes that can be later scaled up to production quantities. Fluke Calibration can provide or specify simple hardware setups to allow molbloc to test devices running at pressures of 3,000 psi or more.
Fuel cell testing
Testing fuel cell performance requires accurate measurement of hydrogen and other gas flows. molbloc/molbox is a fast and accurate way to calibrate MFCs in small or large fuel cell testing environments.
Dynamic gas blending is an alternative to weighing or other filling methods to more quickly create standard blends of gases. molbloc/molbox can provide traceability of the blending system flow devices, or in critical applications can be used directly in the blending control system.
molbloc/molbox is the first choice for calibration of ambient and toxic gas monitor “calibrators.” These devices control critical concentrations of pollutant gases for calibration of monitors. molblocs are easily connected to the calibrators in the lab or field for traceable calibrations. molbloc/molbox is used by state, local, national and international agencies to support their air quality monitoring instruments, and by instrument vendors who supply them.
Calibration of flow meters and testing of breathing air systems is a common and highly critical application for molbloc/molbox, both in production or recertification. Aircraft/safety medical.
Nuclear, gas mask, manufacturing
molbloc/molbox is used to test flow meters and orifices used for leak testing of various products, from gas masks to medical devices. molbloc/molbox is the choice for flow calibration of leak rate monitors used in nuclear utilities because the molbloc can be operated at the elevated pressure required for running upstream of the devices.
National Measurement Institutes (NMI)
Both molbloc/molbox and Fluke Calibration’s primary gravimetric flow standard, GFS2102 are used in premier NMI gas flow laboratories around the world. GFS is a true primary mass flow reference and molbloc/molbox provides the utmost precision as a flow transfer standard.
Additional reference materials
Mass flow vs. volume flowA frequent topic of discussion and confusion surrounding gas flow measurement is that of mass flow and volume flow. Flow meters and flow units used for flow measurements are inherently used to measure and express either the amount of volume of gas, or the amount of mass (or number or moles or molecules) passing through the device. When performing a gas flow calibration it is nearly always beneficial to use a mass flow reference measurement, because the mass flow rate stays constant throughout a flow system in steady state. Since gas is compressible, the volume flow rate varies at different locations in a flow system due to changes in density caused by changing temperature and pressure. molblocs are mass flow standards, which allow reliable comparisons to other flow devices. The molbox terminal is also able to calculate and express the flow rate in terms of volume flow at another point in the system to allow testing of volume-based devices.
Primary flow standard vs. transfer standardFluke Calibration offers both primary gas flow standards and secondary standards (or transfer standards) that deliver the utmost in measurement performance. Customers are sometimes faced with the question of whether they are best served by using a primary or secondary standard. Our advice is that customers should use a standard that provides the required measurement uncertainty and reliability and also compliments the needs of the calibration lab. Fluke Calibration’s GFS2102 dynamic gravimetric mass flow system is a primary mass flow standard offering the lowest possible measurement uncertainty. But for the vast majority of customers, it is more effective to use a molbloc/molbox flow calibration system. molblocs are calibrated by comparison to a calibration chain based on primary gravimetric flow measurements and produce highly repeatable and precise performance as a flow calibrator. Meanwhile, molbloc/molbox offers far greater speed, convenience and portability and is more cost effective than GFS.
molbloc/molbox has been selected by laboratories to replace many piston prover and bell prover systems in mass flow calibration and rotameter calibration applications. Piston provers and bell provers are widely considered primary standards, but it is important to note that they are primary volume flow measurement devices. Most often, they use a direct volume flow measurement along with pressure and temperature measurements and gas property data to calculate a mass flow rate that is used for calibrations.
An important characteristic of secondary flow standards is that they must undergo a rigorous flow comparison during each recalibration. Primary standards are not calibrated by comparison to another flow measurement standard, but are instead calibrated by only testing their measurement components individually (time, distance, mass, pressure, or temperature for example). Without regular proficiency testing and intercomparisons to a suitable flow reference, the user does not have assurance of the primary standard’s flow measurement performance. Most importantly, laboratories must select a flow calibrator that offers the best measurement reliability, and operational features.
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