Thermal Mass flow sensor for liquids

For thermal liquid flow measurement we distinguish two inline measuring principles:

  1. CTA, Constant Temperature Anemometry principle
  2. CPA, Constant Power Anemometry principle

CTA/CPA is the abbreviation of Constant Temperature Anemometry and Constant Power Anemometry, which is also named ‘direct through-flow measurement’ or ‘inline measurement’. Therefore, this principle has no bypass sensor.
Thermal mass flow sensor for liquids

Liquid flow sensor: CTA measurement principle
Figure A: Liquid flow sensor for CTA measurement principle

CTA (Constant Temperature Anemometry) principle, for liquids

Thermal mass flow sensors for liquids using the CTA principle do not have the probes you see at the sensors suitable for gases. In case of liquid Mass Flow Meters the sensor is built around a stainless steel tube without any moving part or obstructions.
The heater/sensor assembly is arranged around the tube and, by following CTA principle, a constant difference in temperature (ΔT) is created. In this case, the first element acts as temperature sensor, and the second element acts as a heater, as shown in figure A. The heater is heated to a certain constant temperature difference (ΔT) over the medium temperature. The actual mass flow rate is calculated by measuring the variable power required to maintain this constant temperature difference as the liquid flows passes the sensor.

For example the Bronkhorst LIQUI-FLOW™ Series L10/L20 uses this principle. Due to the benefits of the unique patented sensor, the fluid will be warmed to a maximum of 5°C, thereby making this Mass Flow Meter suitable for fluids with low boiling points.

Liquid flow sensor: CTA measurement principle
Figure B: Liquid flow sensor for CPA measurement principle

CPA (Constant Power Anemometry) principle, for liquids

The principle of CPA can be compared with CTA. However, in this case, the two elements are used both as heater and as temperature sensor, as shown in figure B. Both elements are provided with an equal amount of constant power, the temperature difference (ΔT) between them is a measure for the flow.

Liquid Flow Controllers

Liquid flow control can be achieved by integrating a control valve onto the body of the liquid Flow Meter (Liquid Flow Controller) or by applying a separate, close-coupled control valve. Bronkhorst liquid flow control valves have a purge connection to eliminate air or gas during start-up. The electronic control function forms part of the normal circuitry of Bronkhorst liquid Flow Meters.

Selection of thermal Mass Flow Meters & Controllers using the CTA/CPA principle for liquids:

Bronkhorst High-Tech B.V.

Nijverheidsstraat 1A
NL-7261 AK Ruurlo (NL)
Tel. +31 573 45 88 00
[email protected]

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