Achieving Traceable Flow Measurement and Control of Gases in Scientific Research and Biopharmaceutical Systems

As more research in science and pharmaceutical development is dependent on understanding and controlling the growth of cells and complex biochemical reactions, accurate and traceable measurement and control of the gases used in these processes are needed. Measuring or controlling gases such as carbon dioxide, nitrogen, and air has typically been done using simple variable-area flowmeters, which have limitations in regard to accuracy, traceability, and maximum allowable pressure.

Figure 1 - Variable-area flowmeter.

These devices, which use differential pressure to suspend a ball in a conical tube and are typically calibrated for air as the primary reference at room temperature and pressure, must use correction factors for any variation in temperature, pressure, and gas used. By adding a metering valve at the inlet or outlet, the height of the ball can be adjusted and the flow of gas visually read against the reference scale on the tube (see Figure 1). This greatly limits the accuracy and traceability of the devices, and offers no method to capture or totalize data other than by operator reading and timed notation of a visual event. As such, the accuracy of these devices is at best between ±3% and ±5% of full scale, with repeatability of perhaps no more than ±0.25% of full scale. Measurement and control that are more accurate and traceable to a known standard are required.

Figure 2 - Thermal mass flow technology.

MFM200 thermal mass flowmeters and MFC202 thermal mass flow controllers (CONCOA, Virginia Beach, VA) offer higher accuracy and direct traceability of flow measurement or control that is not hampered and is minimally impacted by changes in pressure or temperature. As the name suggests, thermal mass flow devices use the heat transferred by a moving gas to determine the flow of that gas. As shown in Figure 2, the heat transfer effect is measured in a capillary stainless steel sensor tube with a heater at its center and two temperature sensors arranged symmetrically around the constant temperature heater.

As the gas flows, it causes the temperature of the downstream sensor to increase. The flow is measured by comparing the difference in the temperature sensors. Though the flowthrough of the sensor tube is very small—at most 10 mL/min—by pairing the sensor with a precise flow bypass or shunt that splits the flow in a known proportion, higher flow rates can be measured up to 20 L/min. After assembly of each device to the respective flow shunt, the entire unit is calibrated against a NIST reference so that a precise and traceable reference can be certified for each unit against the full scale of the device. Installation of a normally closed but highly accurate solenoid valve with a feedback loop creates a complete flow control device with the ability to regulate flow and record data.

The thermal mass flow technique renders a measurement that is essentially unaffected by the gas pressure or temperature. The certificate of NIST traceability is supplied with each unit for nitrogen with specific correction factors for air, carbon dioxide, hydrogen, and helium (other gases are available upon request). The accuracy of each device is ±1% full scale, and repeatability is ±0.05% of full scale. With a maximum inlet pressure rating of 500 psig, in comparison to a typical variable-area flowmeter of 100–200 psig at most, these units can be used in experiments and with systems that require higher pressure than simple flowmeters.

The readings are directly proportional to the 0–5 V return signal in the case of the mass flowmeter units, and the flow is controlled with the same input signal of 0–5 V against the full scale of the mass flow control units. Unlike variable-area flowmeters, there is no visual interpretation. With mass flowmeter and control units, all readings and control points are traceable to NIST by virtue of the certification supplied with each unit. They can be used with a wide range of gases and for most applications since the material of construction is 316L stainless steel, and with a broad range of flows from 0–10 mL to 0–20 L/min.

The way to choose between these two dramatically different technologies is determined by the degree to which the experiment or process needs to be automated, recorded, and traceable. If the process is fixed and requires less precision and repeatability, then variable-area flowmeters are a prudent and economical solution as long as the results do not require traceability.

If the process or experiment requires a highly precise measurement or control with virtually identical repeatability, then thermal mass flow is the technology that gives the highest precision with directly traceable results to a known and recordable standard. As should be expected, the technology comes at a cost that can be five- to tenfold higher than the simpler variable-area technology. But by offering thermal mass flowmeters and controllers that are versatile and designed to be easy to use even for beginners to this type of flow monitoring or control technology, the total cost can be justified against the precision and traceability of the devices.

This technology, which was previously limited to the arenas of manufacturing computer chips because of the units’ high cost balanced against the need for precise control of the varied gases used in that process, can now be employed in more routine processes.

A final important consideration is the ruggedness of the units. The flowmeters and controllers can withstand high RF backgrounds that may be found in process areas and university laboratories.

Mr. Gallagher is Specialty Gas Products Manager, CONCOA, 1501 Harpers Rd., Virginia Beach, VA 23454, U.S.A.; tel.: 800-225-0473; e-mail: lgallagher@concoa.com.

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