Rotameters and Bubble Tube Purge for Level Measurement



Description of Application

One of the most widely used methods of monitoring / controlling liquid level in a tank is the use of Bubble Tubes with Pressure or Differential Pressure Transmitters. A small but uninterrupted flow of air or inert gas is forced down through a dip tube which extends to near the bottom of the tank. The back pressure of the introduced gas is a function of the liquid level or head in the tank. The pressure transmitter takes the back pressure and converts it to an analog signal that is sent to the control room. The plant operator will then be able to monitor the exact level in the tank.

Method of Operation

A small but uninterrupted flow of air (or inert gas such as nitrogen) is easily set and monitored by the use of a Purge Type Rotameter. The flow rate must be low to insure no increase in head back pressure due to pressure drop through the purge piping and dip tube. Conversely, the flow cannot be interrupted or the back pressure may decrease below that of the head giving an incorrect level reading and possibly allowing the process liquid to reflux back to the purge Rotameter and pressure transmitter. Note that controlling the exact flow rate is not critical. The flow rate must be low and uninterrupted. The purge supply gas pressure must exceed the maximum line pressure by about 10psi.

Model Selection

For most applications, the standard 2 SCFH air Rotameter range is suitable. We recommend you use the RAGL41-T0PP-PPSAA-K633G-GLBGN/R3. This meter has polypropylene connection material. If the customer requires stainless steel connection fittings then use RAGL41-T0SS-SSSAA-K633G-GLBGN/R3. For pressures over 250 PSI use model RAKD41-T6SS-31NNN-T80NNN/R3 

Related Products & Solutions


RAGK and RAGL are specifically designed for the measurement of low flows of liquids and gases. Their main application field is in purging, analyzer and gas sampling systems, laboratories and machinery equipment.
Measureable Flow Rates:
Water = 0.002 l/h to 630 l/h
Air = 0.1 l/h to 6300 l/h


The RAKD Rotameter is used for measurement of liquids and gases and is highly accurate, particularly for low flows and high pressure applications.

Rotameter - Variable Area Flowmeter

The Variable Area Flowmeter is one of the oldest and mature principles in flow measurement with its simple design: a float rises inside a conically shaped glass tube as the flow increases and its position on a scale can be read off as the flow rate. Since this measuring method is purely mechanic, it as simple as it is reliable.


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