Top 5 reasons why to use flow meters with inline measurement

August 18, 2020 Nicolaus Dirscherl

In the world of flow measurement different techniques are used. A well-known technique is thermal mass flow measurement. Thermal mass flow meters and controllers can be divided into 3 different sensor principles: 

  1. Bypass principle
  2. Inline principle for gases (CTA)
  3. Inline principle for liquids (CTA/CPA)
In this blog I would like to share my insights on the inline principle for gases, also known as CTA or ‘Constant Temperature Anemometry’ or direct through-flow measurement.

Let me start by explaining this technique followed by my top 5 reasons why to use flow meters using this principle.   

Video explaining the principle of operation of the MASS-STREAM thermal mass flow controller

CTA, Constant Temperature Anemometry

The official name of this measurement technique is ‘Constant Temperature Anemometry’, however, is popularly known as ‘inline measurement’ or ‘direct through-flow measurement’. Amongst various flow measurement techniques, the thermal mass flow measurement based on the CTA principle can be 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.

Working principle CTA gas flow sensor
Laboratory tubes

Examples of applications are burner and furnace control, aeration processes, gas consumption measurement, leak rate tests and environmental air sampling at atmospheric conditions. Within the Bronkhorst® portfolio, these reasonably priced flow meters enlarge the scope of mass flow measurement solutions for higher flow rates (up to 10.000 In/min), for low pressure requirements and for conditions within an application and/or local work environment that would be unsuitable for another measurement principle such as thermal by-pass.

Top 5 key reasons why to use flow meters and controllers based on the CTA principle:

  1. It is the preferred thermal measurement solution for high flow rates of gases, where the technical efforts of a thermal by-pass measurement with capillary sensor and laminar flow element are exceeded. The inline CTA measurement is available from a few ml/min up to hundreds of thousands of m3/h and even more.
  2. Compared to traditional thermal mass flow meters and controller with by-pass, the construction of the direct measuring CTA devices is less sensitive to humidity and contamination.
  3. The compact and robust instrument design provides continuous mass flow measurement with an excellent repeatability. It is extremely versatile and is used within many different industries and applications.
  4. This CTA concept makes it possible to build and to calibrate an instrument with Air or Nitrogen and to then model it for almost any other gas or gas-mix.
  5. The pressure loss over the instruments is almost comparable to a straight length pipe and is thus usually negligible.

working principle CTA gas flow sensor
Working principle CTA gas flow sensor

Working principle of inline gas measurement

The Constant Temperature Anemometry – CTA – gas flow sensor consists of two stainless steel probes, the first being a heater and the second being a temperature sensor. A constant temperature difference (delta-T / dT) is created between the probes. Regardless of actual flow rate CTA is aiming to keep this delta-T or temperature difference (dT) between both sensor pins at a constant level. The flow rate and the heater energy required to maintain this constant dT are proportional and thus indicate the mass flow of the gas. The actual mass flow rate is calculated by measuring the variable power required to maintain this constant temperature difference as the gas flows across the sensor.

Check out our customer's story about using CTA instruments for 3D printing of metal products.

Would you like more information about CTA inline measurement?

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