Introduction of Microturbine Flow Meter

1. Working Principle

A microturbine flow meter operates based on a rotating turbine positioned within the flow path. As fluid passes through the meter, it causes the turbine blades to spin at a speed proportional to the velocity of the fluid. A sensor detects the rotational speed, typically using a magnetic pickup or optical sensor, and converts it into an electrical signal. This signal is then processed to determine the flow rate.

Example: McMillan 10X Series Microturbine Flow Sensor

McMillan’s 10X Series Microturbine Flow Sensors utilize a Pelton-type microturbine wheel to measure flow rates as low as 13 mL/min and as high as 50 L/min. The turbine wheel is supported on a sapphire shaft with maintenance-free bearings, allowing it to spin freely with minimal friction. 

As fluid flows through the sensor, it is directed onto the precision-machined nozzle, which controls the flow onto the turbine blades. The rotational speed of the turbine increases proportionally to the volumetric flow rate, and an infrared emitter and sensor detect the wheel’s movement, generating pulses that correspond to the flow rate.  

Figure 1 McMillan Microturbine Technology with Reflective Turbine

On some microturbine models, the Pelton wheel has alternating white and black sections evenly spaced on it’s surface. As the wheel rotates (as shown with green arrows), an infrared beam (as shown with red arrows) is reflected off each white section and directed to a phototransistor which detects each reflected beam and converts them into measured pulses. 

Increased flow causes the wheel to spin faster, increasing the pulse rate. When the wheel stops (under zero flow conditions), no pulses are generated. This eliminates the possibility of “zero drift” and the need for adjustments to the instrument’s zero reading. Processing circuitry provides analog and/or pulse outputs that are linearly proportional to the flow rate.

2. Accuracy & Precision

Accuracy: ±1.0% or better, depending on calibration. 

Precision: Highly repeatable measurements with deviations typically less than ±0.1%.

3. Flow Measurement Range

Designed for low to medium flow rates, ranging from milliliters per minute to liters per minute, depending on the model. For example, U706 model has a max. flow rate of 50L/min. 

Suitable for liquids and gases, with adjustments for viscosity and density variations.

4. Viscosity Range

Microturbine flow meters are best suited for low-viscosity fluids, typically ranging from 0.5 to 10 centipoise (cP).

Higher viscosity fluids can slow turbine response, reducing accuracy. Some models incorporate viscosity compensation to extend the range.

5. Temperature Range

Standard microturbine flow meters can operate within a temperature range of -450°F to 750°F (-268°C to 399°C), depending on bearing and sensor materials.

Extreme temperatures may require specialized materials to maintain performance.

6. Density & Pressure Range

Microturbine flow meters can handle fluid densities from 0.6 to 1.8 g/cm³, depending on calibration.

Operating pressures typically range from 5,000 PSIG, with custom configurations capable of up to 30,000 PSIG.

7. Sensitivity to Particles

Particles can obstruct turbine movement, leading to inaccurate readings. Thus,  filters or screens are recommended upstream to prevent clogging.

8. Significant Factors Affecting Measurement

Viscosity: Higher viscosity fluids slow turbine response, reducing accuracy.

Temperature: Extreme temperatures alter fluid properties and affect turbine rotation.

Density & Pressure: Changes in fluid density impact flow velocity and turbine responsiveness.

9. Applications & Best Use Cases

✔ Low-viscosity liquid applications

✔ Gas flow monitoring

✔ Medical or laboratory fluid measurements