How They Work
Flowmeters are used in fluid systems (liquid and gas) to indicate the rate of flow of the fluid. They can also control the rate of flow if they are equipped with a flow control valve.
Rotameters are a particular kind of flowmeter, based on the variable area principle. They provide a simple, precise and economical means of indicating flow rates in fluid systems.
This variable area principle consists of three basic elements: A uniformly tapered flow tube, a float, and a measurement scale. A control valve may be added if flow control is also desired.
In operation, the rotameter is positioned vertically in the fluid system with the smallest diameter end of the tapered flow tube at the bottom. This is the fluid inlet. The float, typically spherical, is located inside the flow tube, and is engineered so that its diameter is nearly identical to the flow tube's inlet diameter.
When fluid —gas or liquid — is introduced into the tube, the float is lifted from its initial position at the inlet, allowing the fluid to pass between it and the tube wall. As the float rises, more and more fluid flows by the float because the tapered tube's diameter is increasing. Ultimately, a point is reached where the flow area is large enough to allow the entire volume of the fluid to flow past the float. This flow area is called the annular passage. The float is now stationary at that level within the tube, as its weight is being supported by the fluid forces which caused it to rise. This position corresponds to a point on the tube's measurement scale and provides an indication of the fluid's flow rate.
	One way to change the capacity, or flow range, of a rotameter is to change the float material, and thus its density, while keeping the flow tube and float size constant. Floats which are made from less dense materials will rise higher in the tube and therefore will yield lower flow capacities for the same diameter flow tube. Floats made from more dense materials will rise less thereby yielding higher flow capacities. Relative flow capacities for some common float materials are shown in Figure 1.
	Another way to change the capacity is to change the diameter of the flow tube and the size of the float.

For other gases or for non-standard temperatures and pressures, call our Technical Service Line at (800) 544-1658

Note that flowmeters calibrated at standard conditions with a valve on the inlet, readings on the tube are correct provided that the outlet pressure 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 the tube was calibrated.

 

Flowmeter Measurement Scales

Depending upon the model, a flow-meter's measurement scale can be either direct reading or in reference scale units. Direct reading tubes are straightforward. The measurement scale on each of these tubes reads actual flow at standard conditions in a choice of English or Metric units. Tubes with direct reading scales include the following series' flowmeters: FM-1000; FM-1100; FM-1127; PG-1000; PM-1000; VM-1000 Reference scale tubes, on the other hand, provide a uniformly calibrated scale in arbitrary millimeter (mm) units. Obtaining actual flow rates with these tubes requires the use of a reference scale flow correlation table (available from Matheson) which relates the mm scale reading to an actual flow rate. Reference scale tubes are useful when measuring flow rates for gases other than air and/or for non-standard conditions. Tubes with reference scales include the following series' flowmeters: FM-1050; TF-1000; TF-1050; Shielded SF Series A sometimes confusing matter in flowmeter size terminology is that variable area flowmeters are often defined by their measurement scale length, that is, the distance between the zero and full scale marking. 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. Note that this scale length has no relationship at all with whether the flowmeter is a direct reading or a reference scale tube.

Flowmeter Calibration And Services

There are many formulas available, which calculate the flow of a fluid through a variable area flow-meter for which it is not calibrated. Moreover, these equations are used to generate correction factors for correlating other fluid flows to some known calibration, as shown in Table 1. Matheson has conducted extensive experiments to determine the accuracy of these mathematical formulas. At best, calculated values estimate flow rates to about ±5% accuracy. If you require greater accuracy, it will be necessary to calibrate the flowmeter with the actual gas, or at the particular conditions (temperature/pressure), in question. Matheson has a fully equipped laboratory and has developed many special procedures for calibrating flowmeters to different gases and/or conditions. Over the years, we have built an extensive library of flow correlation tables and curves for dozens of gases at numerous conditions. Utilizing these flow correlation tables with a Matheson flowmeter will yield the accuracy specification listed for each flowmeter model in this catalog. (Ex. ±5% for FM-1050 or FM-1000). Request the tables when you order your flowmeter. For accuracies better than ±5%, Matheson can directly calibrate your flowmeter(s) at a cost below what most laboratories would incur to do it themselves. To request a price quotation for your specific flowmeter application, please specify the gas, operating pressure and temperature, and flow rate or flowmeter model number to your nearest Matheson office, or call our Technical Service Line at 1-800-544-1658.

Overview of Flowmeter Models

Matheson offers a complete line of variable area flowmeters to meet your application requirements.

Model Series	Scale Length	Scale Type¹	Comments
FM-1050	150mm	Reference	High accuracy; glass tube
FM-1000	65mm	Direct	High accuracy; glass tube
FM-1100	70mm	Direct	High capacity; glass tube
FM-1127	127mm	Direct	High capacity; glass tube
TF-1000	65mm	Reference	Teflon®; glass tube
TF-1050	150mm	Reference	Teflon®; glass tube
PG-1000	50mm	Direct	Acrylic body with glass tube
PM-1000	37mm	Direct	Acrylic block and tube
VM-1000	45mm	Direct	Economical Plastic
Shielded	100mm	Reference	Software flow correlation
Flow Kits	150mm	Reference	Flowmeter assortment
Mixers	150mm	Reference	2, 3, and 4-tube mixers Replacement parts

Note 1:Direct Reading scales are available in both English and metric units.

Reference scales come with a flow correlation table for air at standard conditions. Request a correlation table for the gas(es) or liquid(s) you will be measuring.