Thermal mass flow sensor for gases - inline principle

How does a thermal mass flow sensor for gases work?

A thermal flow sensor uses the thermal properties of gases to measure their mass flow rate. To this end, heat is introduced into the flowing gas, and the (temperature) sensor measures how much heat is absorbed by the gas.

A traditional way of accurately measuring low gas flow rates is to use a mass flow meter with the bypass principle. When looking for a way to measure the low flow rate of gases - as well as liquids – that is less sensitive to humidity and contamination, the inline principle is highly favourable.

Constant Temperature Anemometry working principle

This inline principle uses CTA or Constant Temperature Anemometry. Another synonym for this inline measurement is ‘direct through-flow measurement’. And – as the name indicates – ‘inline’ means no bypass.

The Constant Temperature Anemometry - CTA - gas flow sensor consists of two stainless steel probes (‘metal pens’) which are positioned inline in the main gas flow channel, where gas is forced to flow along them. The first probe is a heater and the second one is a temperature sensor. A constant temperature difference (delta-T or ΔT) is created between the probes.

Regardless of the actual mass flow rate, Constant Temperature Anemometry aims to keep this temperature difference (ΔT) between both sensor probes at a constant level. The heater energy required to maintain this constant ΔT is proportional to the mass flow rate and is thus a measure of the mass flow of the gas. This proportionality factor depends on thermal properties of the gas such as its thermal conductivity and specific heat capacity, as well as its dynamic viscosity.

The higher the gas mass flow rate, the more energy that is required to maintain the chosen ΔT. The actual mass flow rate is calculated by measuring the variable power required to maintain this constant temperature difference as the gas flows along the sensor.

This principle stems from over a century ago, when the Canadian physicist and inventor of the hot-wire anemometer, Louis Vessot King, postulated the Law named after him (King’s Law) that describes heat loss of a hot-wire due to gas or liquid mass flow flowing along.

Amongst various flow measurement techniques, the thermal mass flow measurement based on the Constant Temperature Anemometry (CTA) principle is used for both gases and liquids. Mass flow meters based on the CTA principle cover a wide range of measurement and control applications in almost every industry sector.

Bronkhorst inline thermal mass flow meters for gases

An application with a slightly moist gas (humidity), or where lower accuracy but high repeatability and robustness is required, is a good application for flow controllers with a robust inline sensor in combination with a straight flow channel such as MASS-STREAM or MASS-VIEW devices. There are no narrow pipelines to clog with particles, so they can handle most gases.

Where are these ‘inline’ mass flow meters and controllers used?

Aeration in fish farming - A Norwegian distributor assisted a customer with Bronkhorst MASS-STREAM mass flow controllers to facilitate the oxygenation of salmon in open fish farming cages with an aeration solution containing automated and remotely controlled air flow controllers. Read more...
Calibration of air samplers - For microbiological monitoring of the air inside isolators that work under clean and sterile conditions for pharmaceutical preparations, air flows must be calibrated on a regular basis. Bronkhorst MASS-STREAM mass flow meters are used to this end. Read more...
Membrane filtration - Bronkhorst IN-FLOW thermal mass flow meters are used to accurately measure gaseous CO2 flows for pH control for re-mineralisation in a 'reverse osmosis' membrane desalination plant. Read more...
Beer brewing - A Bronkhorst MASS-STREAM mass flow controller is used to accurately dose the desired amount of gaseous oxygen into beer brewing vessels for the reproduction of yeast cells. Read more...