Flow Measurement & Control - Matheson

1 Basic Flowmeter PrinciplesHow They WorkFlowmeters are used in fluid systems (liquid and gas) to indicatethe rate of flow of the fluid. They can also Control the rate of flow ifthey are equipped with a flow Control are a particular kind of flowmeter, based on thevariable area principle. They provide a simple, precise andeconomical means of indicating flow rates in fluid systems. This variable area principle consists of three basic elements: Auniformly tapered flow tube, a float, and a Measurement scale. Acontrol valve may be added if flow Control is also operation, the rotameter is positioned vertically in the fluidsystem with the smallest diameter end of the tapered flow tube atthe bottom.

2 This is the fluid inlet. The float, typically spherical, islocated inside the flow tube, and is engineered so that its diameteris nearly identical to the flow tube s inlet fluid gas or liquid is introduced into the tube, the floatis lifted from its initial position at the inlet, allowing the fluid topass between it and the tube wall. As the float rises, more andmore fluid flows by the float because the tapered tube s diameteris increasing. Ultimately, a point is reached where the flow area islarge enough to allow the entire volume of the fluid to flow pastthe float. This flow area is called the annular passage.

3 The float isnow stationary at that level within the tube, as its weight is beingsupported by the fluid forces which caused it to rise. This positioncorresponds to a point on the tube s Measurement scale andprovides an indication of the fluid s flow way to change the capacity, or flow range, of a rotameter is tochange the float material, and thus its density, while keeping theflow tube and float size constant. Floats which are made from lessdense materials will rise higher in the tube and therefore will yieldlower flow capacities for the same diameter flow tube. Floats made from more densematerials will rise less therebyyielding higher flow flow capacities forsome common float materialsare shown in Figure 1.

4 Another way to change thecapacity is to change thediameter of the flow tube andthe size of the The Right Flowmeter SizeThere are certain factors which affect the Measurement of a fluid sflow rate with a rotameter. The fluid s temperature, pressure andspecific gravity all impact gas flow capacities (ranges) for the flowmeters described in thiscatalog are given for air at standard conditions - psia ( Abs) and 70 F (21 C). Sizing a meter for a gas other than air,or for your specific application pressure and/or temperature,requires that you first determine the equivalent flow capacity in airat standard conditions.

5 Once determined, the flow capacity tablesin this catalog can be applied directly. Reference Scale tables can berequested for each flowmeter ordered which will provideconversion to your desired fluid or conditions. Note that for flowmeters calibrated at standard conditions with a valveon the inlet, readings on the tube are correct provided that the outletpressure is close to atmospheric. When the valve is on the outlet, readingsare correct if the inlet gas pressure is equal to the pressure for which thetube was your convenience,Table 1 provides correction factors for gasesother than air at standard conditions.

6 Call a customer servicerepresentative if you require further assistance in sizing arotameter for your particular Measurement & ControlRotameters Flow Measurement & Control IBasic Flowmeter Principles (continued)Flowmeter Measurement ScalesDepending upon the model, the flowmeter s Measurement scalecan be either direct reading or in reference scale reading tubes are straightforward. The Measurement scaleon each of these tubes reads actual flow at standard conditions in achoice of English or Metric units. Reference scale tubes, on the other hand, provide a uniformlycalibrated scale in arbitrary millimeter (mm) units.

7 Obtainingactual flow rates with these tubes requires the use of a referencescale flow correlation table (available from Matheson ) whichrelates the mm scale reading to an actual flow rate. Reference scaletubes are useful when measuring flow rates for gases other thanair, for non-standard conditions, or when conditions or the gasmay frequently change. Variable area flowmeters are often defined by their measurementscale length, that is, the distance between the zero and full scalemarking. Scale length is typically indicated in millimeter (mm)units. The FM-1050, for example, uses a 150mm tube and the FM-1000 uses a 65mm tube.

8 Note that this scale length has no relationshipat all with whether the flowmeter is a direct reading or a reference Measurement & Control Rotameters Flow Measurement & Control 1: Flow Rate Factors for Gases other than AirGasFactorGasFactorGasFactor (Natural Gas) Carbon other gases or for non-standard temperatures and pressures, call a customer service that for flowmeters calibrated at standard conditions with a valve on the inlet, readings on the tube are correct provided that the outletpressure is close to atmospheric. When the valve is on the outlet, readings are correct if the inlet gas pressure is equal to the pressure for which thetube was estimate which flow tube should be purchased when measuring the flow of a gas other than air, multiply the flowrate desired by its factor above to find the air flow equivalent.

9 The flow tube whose range (capacity) covers this flowrate should be the one purchased. Be sure to keep units consistent. Air Equivalent= Gas Flow Rate Desired x factors assume standard operating conditions. Temperature 70 F/21 C; pressure psia ( K Pa Abs). Basic Flowmeter Principles (continued)Flow Measurement & ControlRotameters Flow Measurement & Control Of Flowmeter ModelsMatheson offers a complete line of variable area flowmeters to meet your application requirements. Model Series Scale LengthScale Type1 Comments Page _____355FM-1050150mmReferenceHigh accuracy356FM-100065mmDirectHigh accuracy358FM-110070mmDirectHigh flow capacity360FM-1127127mmDirectHigh flow capacity360 Mixers150mmReference2, 3, and 4-tube mixers 362 Replacement Parts_____364PG-100050mmDirectEconomical Acrylic body with glass tube 365PM-100037mmDirectEconomical Acrylic block and tube3671 Note: Direct Reading scales are available in both English and Metric scales come with a flow correlation table for air at standard conditions.

10 Request a correlation table for the gas(es) or liquid(s) you will be Calibration and ServicesThere are many formulas available, which calculate the flow of afluid through a variable area flowmeter for which it is notcalibrated. Moreover, these equations are used to generatecorrection factors for correlating other fluid flows to some knowncalibration, as shown in Table has conducted extensive experiments to determine theaccuracy of these mathematical formulas. At best, calculatedvalues estimate flow rates to about 5% accuracy. If you requiregreater accuracy, it will be necessary to calibrate the flowmeterwith the actual gas, or at the particular conditions(temperature/pressure), in has a fully equipped laboratory and has developedmany special procedures for calibrating flowmeters to differentgases and/or the years, we have built an extensive library of flowcorrelation tables and curves for dozens of gases at numerousconditions.