Flow rate is very low, but what is the velocity in the duct? If the duct is small, the velocity could be large enough to be measured.

You may use an ultrasonic flow meter (just look for available models on the web) or (cheaper but may not be sensitive enough) a diaphragm and pressure difference.

Coriolis Mass Flow Meters

You can decrease pipe diameter to increase velocity and use a suitable animometer to measure velocity

Multiply Velocity and area to get volume flow rate

Multiply by density to obtain mass flow rate

Ultrasonic Flow Meters is also suitable for hot fluids with low flow rate without contacting the fluid

For more details about

ULTRASONIC LIQUID FLOW METERS

visit the following link

https://www.energoflow.com/ultrasonic-liquid-flowmeters

Respected Sirs

Thank you very much for your advice.

Welcome Amit Kumar

Dear Mr. Amit Kumar

There is a high quality rotameter but before select one you must to define your fluid and define each of viscosity and density because the flow rate depending on these properties when the temperature of fluid changed to the design temperature of rotameter, so there is a calibration equations for the fluid flow through rotameter are used when there is a changed in fluid temperature.

also there is a turbine flow meter its expensive but you can set it if you have fluid type ( gas or liquid ) also you can set the density and viscosity value or select directly the fluid type, but for all flow meters you must to select single phase flow for accurate. you can find the attachment file below .

the real problem the range of flow rate.

with my respect

Thank you Sir

You may use hot-film as used by a researcher mentioned below:

Hot-film and calorimetric thermal air flow sensors realized with printed board technology

July 2016Journal of Sensors and Sensor Systems 5(2):283-291

DOI: 10.5194/jsss-5-283-2016

Or using infrared cameras made for this purpose as below:

Infrared Thermography for Thermo-Fluid-Dynamics

January 2013

DOI: 10.1007/978-3-642-29508-9_1

In book: Infrared Thermography for Thermo-Fluid-Dynamics

Thank you sir