Non-Invasive Internal Flow Velocity Measurements 

Heat flux sensors have been proven to work as flow meters to non-invasively determine the velocity of internal fluid flow. With flow velocity and other flow characteristics, BTU metering information can also be calculated.


Using an Existing Internal Flow Temperature

The measurement arrangement involves having a PHFS heat flux sensor in conjunction with an already installed internal fluid temperature sensor within the pipe/duct.  The heat flux sensor is placed on the outer surface of the pipe/duct and simultaneously measures heat transfer from the pipe surface and as well as the surface temperature.


With additional internal flow temperature information, the velocity of the flow can be determined using the following calculations:


  1. Install the PHFS heat flux sensor integrated with the thermocouple temperature sensor.

  2. Install the Tflow temperature sensor or use the existing temperature sensor. Tflow the temperature of the fluid within the pipe/duct. This should be close to the location of the heat flux sensor.

  3. Determine the heat transfer coefficients of the heat flux sensor to air (R”out). The thermal resistance of the heat flux sensor (R”Sens) is provided in it’s datasheet.

  4. Calculate the heat transfer coefficient of the heat flux sensor to air (R”out) by using

  5. R”Flow = Heat Flux x (TFlow – Tout)^(-1) – R”out – R”Sens - R”Pipe)

  6. Determine the parameters a, b, and c of the setup. These parameters are needed for King’s Law

  7. RFlow = a + b x (VFlow)^c

  8. Calculate the velocity of the fluid by applying King’s Law

  9. The velocity of the flow is VFlow = ((R”Flow – a)/b)^(1/c)