The variable area rotameter has been the primary tool utilized for determination of flowrates passing through the air circuitry pathway of air sampling or air monitoring systems for many years.
In air sampling applications a pollutant is captured on a collection medium (filter paper, charcoal cartridge, etc.) and taken to a laboratory for determination of the quantity of pollutant on the collection medium.
In air monitoring applications a real time detection device is continuously making measurements of the pollutant quantity being collected on the collection medium and possibly correcting for background levels of pollutants. In either case, utilization of a rotameter affects the accuracy of the air sampling and the air monitoring results in the same manner.
Rotameter Technology Basics
Rotameters have the advantages of being simple in design, very economical to manufacture, small in size and easy repair or replace.
The variable area rotameter has a float made of plastic or metal which is raised in the variable area tube of the rotameter in proportion to the mass of air entering the rotameter. The scale on the rotameter is determined by altering the flowrate of air passing through the rotameter while maintaining the absolute pressure at the outlet of the rotameter and the temperature of the air passing through the rotameter constant. An absolute pressure of 14.7 PSI (1 ATM) is usually chosen as the reference pressure. Air temperatures of 15ºC, 20ºC, 21.1ºC or 25ºC are common references dependant upon the manufacturer’s target market.
The air monitoring specialist usually reads the widest diameter of the float, or other measurement point identified by the manufacturer of the rotameter and estimates the flow value on the given rotameter scale.
The observation made by the air monitoring specialist is generally referred to as indicated flow or actual flow. It is representative of the actual conditions of air temperature and absolute pressure at the outlet of the rotameter at the time the flowrate is observed assuming a properly performed observation of the float level and interpretation and interpolation of the scale whether the ball is steady or oscillating up and down.
In order to accurately compare the current flowrate measurement with a prior, or subsequent flowrate measurement, one must (1) measure the absolute pressure at the outlet of the rotameter, (2) measure the temperature of the air passing through the rotameter and (3) correct mathematically to the reference T and P values that the manufacturer’s scale is based upon, or to any other mutually acceptable industry standard reference conditions utilizing the formula specifically applicable to variable area rotameters.