As is the case with gases, the inline principle is used to measure the mass flow rate of liquids. This ‘inline measurement’, also known as ‘direct through-flow measurement’, can be distinguished in two measurement principles: Constant Temperature Anemometry (CPA) and Constant Power Anemometry (CPA). As the measurement occurs by ‘sensing’ the flow in the main flow channel of the liquid flow rate measurement device, these principles have no bypass sensor.
Sensor Principle Liquids
How does a thermal mass flow sensor work for liquids?
A thermal mass flow sensor uses the thermal properties of liquids to measure their mass flow rate. To this end, as can be seen in Figure 1, heat is introduced into the streaming liquid by a heater, and the (temperature) sensor measures how much heat is absorbed by the liquid. In these thermal mass flow meters for liquids, the heater as well as the sensor are built around a stainless-steel main flow channel tube with no moving parts or obstructions inside the tube.
Constant Temperature Anemometry (CTA) principle for liquids
The heater/sensor assembly is arranged around the tube and, by following the CTA principle, a constant difference in temperature (ΔT) is created. In the current setup, the first element operates as a temperature sensor, and the second element (downstream) works as a heater, as shown in Figure 2. 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.
Figure A: liquid flow sensor for CTA measurement principle
Figure B: liquid flow sensor for CPA measurement principle
Constant Power Anemometry (CPA) principle for liquids
The principle of CPA is similar in some ways to CTA. In this case, however, the two elements are used both as heater and as temperature sensor, as shown in Figure 3. Both elements are provided with an equal amount of constant power. The temperature difference (ΔT) between them is a measure for the liquid mass flow rate.
Liquid mass flow controllers
Liquid flow control can be achieved by integrating a control valve onto the body of the liquid mass flow meter or by adding 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 is part of the normal circuitry of Bronkhorst thermal liquid mass flow meters.
Bronkhorst thermal mass flow meters for liquids
Examples of liquid mass flow meters with a robust inline sensor in combination with a straight flow channel are Bronkhorst LIQUI-FLOW thermal mass flow meters (and controllers), LIQUI-FLOW Industrial Style devices for use in industrial environments, or µ-FLOW devices for ultra-low flow rates.
For example, the Bronkhorst LIQUI-FLOW Series L10/L20 uses the CTA principle. Due to the benefits of the unique sensor, the fluid will be warmed by a maximum of 5 °C, thereby making this mass flow meter suitable for fluids with low boiling points.
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