Visit the Yaskawa Electric Europe web site

Flow sensors use thermal technology

Edited by the Engineeringtalk editorial team May 22, 2006

Flow averaging tubes provide accurate measurement of gas or air flows in large pipes and ducts, without the expense and complexity of traditional multipoint systems.

New from Flotech Solutions is the innovative Master-Touch FAT range of flow averaging tubes from Eldridge Products.

These Master-Touch FAT units provide accurate measurement of gas or air flows in large pipes and ducts, without the expense and complexity of traditional multipoint systems.

They introduce only a minimal obstruction to the flow in the duct.

These units offer a cost-effective solution for heating, ventilation, and air conditioning (HVAC) and variable air volume (VAV) applications.

The thermal technology used in the measurement sensor involves a solid state instrument using the principle of convective heat transfer to directly measure mass flow.

The Flotech Eldridge sensors consist of two resistance temperature detectors (RTDs).

A forced null Wheatstone bridge preferentially heats one RTD; the other RTD acts as the temperature reference.

The process gas flow dissipates heat from the first RTD, causing an increase in the power required to maintain a balance between the RTDs.

This increase is directly related to the rate of gas flow past the sensor.

The Eldridge sensors are temperature compensated and insensitive to pressure changes.

No additional instrumentation or calculations are required.

The output signal is a true mass flow signal which can be directly interfaced with your data acquisition system.

The Master-Touch FAT probes use a flow averaging tube to give a stable flow signal in applications where the flow profile is less than ideal, such as downstream of a bend, valve, tee or obstruction.

The flow averaging tube is sized to fit across the pipe or duct, and has a number of large diameter (0.125in) inlet ports along the length of the upstream impact surface.

The impact pressure from the flow at each inlet port produces an axial flow up the FAT tube, averaged inside the tube.

This is directed across the single flow sensor, to measure the total, averaged sensor flow.

The gas flow then passes back into the main flow stream via the downstream gas return ports located near the flow sensing element.

Because of the inherent flow averaging across the profile with the Flotech FAT sensor, the flow averaging tube technology significantly reduces the traditional requirements for straight, unobstructed upstream piping at the measurement point.

Depending on the piping configuration, the traditionally required upstream straight run can be 10, 20 or even 50 diameters.

By measuring the cumulative flow velocities within the probe across the whole duct, the Flow Averaging Tubes are far more tolerant of flow profile problems than other instrument technologies.

This allows the required straight run to be greatly reduced.

And, when coupled with flow straightening plates, the upstream requirement can be reduced to as little as three diameters.