The Best Way to Meter Low Flow Applications Part 1

Because mechanical flow meters cannot extract adequate energy for linear results, metering applications with low fluid flows can be a very trying task. By comparison, electronic flow meters often have problems with sensitivity, zero drift and response times. To understand how different technologies approach metering low flows, it is important to take a look at their benefits and disadvantages.

Variable Area Meters

Variable area meters consist of the ubiquitous ball in a tube flow device, commonly referred to as a Rotameter. Variable area meters have been the backbone of measuring low flows in laboratories for many years; unfortunately, they are not very accurate at lower flows, (typically ±5% for a given set of conditions). This meter type also suffers because changes in liquid properties will change the height the float attains, thus also the flow rate indication. Variable area flow meters typically do not offer an electronic output.

Low Flow Axial Turbines

Axial flow turbines do not work effectively at low flow because the energy required to spin the rotor becomes swamped by the drag from the system at low flow rates. Other factors, such as changes in fluid density or viscosity also affects the calibration. These devices provide the best results for metering turbulent flow, which is rarely seen with low flows. Some small polymer axial turbine devices have been quite successful in metering low flows, but changes in fluid properties are still an issue.

Pelton Wheel Turbines

These radial flow devices work better than their axial counterparts with lower flows, but still have similar problems. Titan and several other manufacturers try to reduce the drag that occurs with the transition from turbulent to laminar flow; however, the linearity still changes as the flow reduces. Bearing drag is also an issue, except with Pelton wheel turbines, as the system design does permit low friction bearings.

Titan’s first patent was for a radial flow turbine, which had no blades. Two plain discs on low friction bearings were placed 0.8mm apart and a 0.8mm jet of liquid was injected into the gap.

The fluid in the gap acted as the resistance to the incoming jet and rotated the discs. Having no blades to “stir” the fluid, the overall drag on the system was significantly reduced. Flows down to 0.5 ml per minute were recorded with this “bladeless” turbine; however, Titan no longer makes this product, since it was very expensive to produce and has been superseded by Titan’s ultrasonic technology.

To read further and learn more about metering low flow with varying devices, stay tuned for our next blog where we will cover five other meters in depth. JLC International is determined to assist those in any field with all of their measurement needs, including with carbon dioxide transmitters or even dew point meters. Do not hesitate to contact us at any time so we can properly assist you with inquiries or anything else. Call 1-888-358-2518 for more details.