General purpose instruments with elastomeric seals have maximum leak-rate specifications ranging from about 1 x 10 -9 to 5 x 10 -9 atmosphere cubic centimeters per second of helium. The amount of gas leakage you can expect from seals on your mass flow meter or mass flow controller. The time it takes the flow meter to settle on the new flow rate after a sudden change in flow. Turndown or Rangeability is the range of flow (maximum to minimum flow rate) that a flow meter is able to measure in its specified accuracy. Example: different operators/engineers, facilities, time intervals, laboratories, etc. The flowmeter’s ability to produce similar results/readings when the conditions of measurement differ. Some manufacturers specify a repeatability of +/-0.2% of reading. Typically, the repeatability of general purpose MFMs and MFCs is +/-0.2% of full scale. The flow meter or controller’s ability to produce the same results/readings multiple times with no change in conditions (same flow rate, same operator/engineer, same laboratory, and short intervals of time, etc.). Divided range: X % of reading (≥ Y % of full scale) and X % of full scale (Combination: X % of reading + X % of full scale.Percent of full scale: X % of full scale.These are some examples of how manufacturers express the accuracy of their MFCs and MFMs in measuring mass flow rate: Typically, the accuracy of most general purpose MFMs and MFCs in measuring gas mass flow rate is 1% of full scale, including linearity, and at flow calibration conditions. Īn accuracy specification includes errors due to: (a) uncertainty in the flow calibration standard (b) any non-repeatability of the MFM or MFC under test (c) disagreement between the curve-fitting function and the actual flow response curve and (d) inability of the flow calibration facility to deliver a sufficiently constant flow rate. Officially, the American Society of Mechanical Engineers (ASME) defines the “accuracy” of flow meters as: the degree of freedom from error, or the degree of conformity of the indicated value of the instrument to the true value of the measured quantity. The accuracy statement on any spec sheet indicates how precise the flow meter/controller will measure flow within its flow ranges. NOTE: Most manufacturers make no distinction between mass flow meters and controllers and published specifications may vary from manufacturer to manufacturer. The amount of fluid a mass flow meter or controller is best suited to measure flow for.Įxample: the table below shows mass flow ranges of three typical flow body sizes of general purpose mass flow meters (MFM) and mass flow controllers (MFCs) for air at 0☌ and 1 atmosphere pressure: Whether you have been on the job for decades or just starting to purchase or specify a mass flow meter or controller to measure fluid flow in your application, there is key terminology to understand. Here are seven terms you should know to assure you are getting what you need to measure or control flow in your process effectively. 7 Need to Know Terms for Successful Specification of a Mass Flow Meter or Controller
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