Mass Flow Meters

Mass flow meters are one of the most popular gas-measurement technologies in use today because mass flow measurement is often needed to determine and control the mass ratios in chemical reactions and other applications. Mass flow meters are typically used in laboratories for flow research, biomedical studies, toxicology studies, energy studies, industrial hygiene, occupational safety, and experimentation. Since it measures mass flow rather than volume flow, this device is relatively unaffected by fluctuations in gas pressure and temperature.

Mass flow meters are generally available in pipe sizes from 1/4 inch to 8 inches. The diameter is important when specifying mounting options. They can measure liquid flow from 0.0002 ml/min. to more than 4,000 LPM, and gas flow from 0.02 ml/min. to more than 500 m3/hour. Most mass flow meters have an analog or digital output signal to record the flow rate and have an accuracy of ±1.5% to 2% of full scale flow.

Thermal meters use a heated sensing element isolated from the fluid flow path. They are based on an operational principle that the rate of heat absorbed by a flow stream is directly proportional to its mass flow rate. There are three basic operating methods that are commonly used to excite the sensor: constant temperature thermal mass flow meters; constant power thermal mass flow meters; and calorimetric or energy balance thermal mass flow meters.

Constant temperature thermal mass flow meters require two active sensors that are operated in a balanced state. One acts as a temperature sensor reference; the other is the active heated sensor. These meters typically have a turndown ratio of 1000:1 when properly sized. Constant power thermal mass flow meters require three active elements: a constant current heating element coupled to a resistance temperature detector (RTD), which acts as a heat loss flow sensor, while a second RTD operates as an environmental temperature sensor. These meters typically have a turndown ratio of 100:1.

Calorimetric or energy balance thermal mass flow meters require one heating element and two temperature sensors. This design is usually limited to a maximum flow rate of 200 liters per minute and typically has a turndown ratio of 10:1.

One class of mass flow meters is based on the Coriolis effect. In a Coriolis meter, media passed through a small vibrating flow tube causes a twisting of the flow tube proportional to the mass flow rate of material. This meter directly measures the mass flow rate based on the Coriolis force generated. These flow meters are easy to size based on process requirements, tend to be much larger in size compared to other flow meters, and are higher priced. They typically have a turndown ratio of 10:1.

Important specifications to consider when selecting mass flow meters include mass flow rate range, operating pressure, fluid temperature range, type of material, and any special conditions such as particulates in the media and viscosity of the media, whether media conditions remain constant or vary, required flow range of media, special installation considerations, and accuracy.

Alicat Scientific
These portable mass flow meters utilize a standard 9Vdc battery to make metering gas flow, temperature and pressure convenient and reliable in the field or in the lab. All mass flow meters include: display of mass flow, volumetric flow, pressure and temperature; RS-232 I/O and analog I/O; and 30 gas select calibration. Options include: high accuracy calibration of +/- 0.4% of reading + .2% of full scale; custom gas mixture calibrations; alternate STPs; and an integrated totalizer. All instruments carry the CE mark and are NIST traceable.
www.alicatscientific.com

Sierra Instruments
The Cal=Trak™ XL is a powerful high flow primary gas calibrator designed to replace aging bell provers. It offers an impressive standardized accuracy of +/- 0.25% reading over the flow range of 5 to 500 SLPM. It is a primary standard piston prover that operates with the press of a button—assuring end users superior calibration accuracy, convenience, ease-of-use and productivity. The device is ideal for calibration of gas measuring instruments including mass flow meters and controllers. At 33” H x 24.6” W x 12”, it fits on any laboratory bench replacing large bell provers with one simple, integrated package. The device can be used in the manufacture, inspection, validation and recalibration of mass flow controllers, flow meters and many other varieties of flow equipment with flow rates too high for common primary standard devices.
www.sierrainstruments.com

Brooks Instrument
The B-Series mass flow controller is designed to be insensitive to fluctuations in pressure and temperature. Built on a standard 1.125-inch wide platform with a powerful user interface and local digital display, the series provides a compact flow measurement and control solution that eliminates the need for costly temperature and pressure control devices and systems. The series was designed to minimize all sources of gas flow inaccuracy, providing outstanding accuracy, extremely fast response, exceptional stability, and robust resistance to flow variation due to pressure and temperature fluctuations. Whether flowing gas to a CVD chamber, chemical research reactor, fuel cell test stand, or any other important flow control application, the quality of results is only as good as the performance of the mass flow controller.
www.brooksinstrument.com

TSI
The 4000/4100 series mass flowmeters are designed for a multitude of gas flow measurement applications. Whether measuring gas flows in a laboratory or manufacturing setting, these generalpurpose mass flowmeters provide accurate results with multiple data output options. The display versions come with accessories that make setup and operation fast and convenient. The flowmeter sensors are specified as 2% of reading. One mass flow sensor covers the same range as three or more “percent of full scale devices,” with better accuracy at most points. Fast 4-millisecond response ensures accuracy in fluctuation flows. The fast response time is ideal for closed-loop control systems and integrated volume measurements. Low pressure drop minimizes flow circuit back pressure and its impact on the system.
www.tsi.com

Tanuja Koppal, PhD, is a freelance science writer and consultant based in Randolph, N.J.

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