Ric Besseling
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In different kinds of applications like aging processes, validation testing and/or in research on plant growth, often a specified flow of moist air is needed to achieve specific ambient conditions in a test chamber. Nowadays, we have multiple solutions for these kinds of applications, one of them with the help of controlled evaporation and mixing systems. Let me explain what the benefits are of these systems in comparison with the more conventional bubbler systems.

How does a Bubbler System work?

Small concentrations of moist air can be created using a bubbler system. This conventional method requires optimal pressure and temperature control of the bubbler system. A complete bubbler level measurement system therefore consists of a source of compressed air, an air flow restrictor, sensing tube, and pressure controller. The latter converts the back pressure to provide output to a controller, which calculates the liquid level. The quality of the moist air fully depends on the theoretical calculation of the degree of saturation of the air flowing through the liquid and the accuracy of pressure and temperature control. With this conventional approach it is difficult to achieve a specific air moisture content.

Set-up Conventional Bubbler System

Figure 1. Set-up Conventional Bubbler System

Bronkhorst evaporation systems

In addition to this approach, Bronkhorst developed a CEM-system, based on Controlled Evaporation and Mixing, which can be used for moist air applications. This CEM-system is an innovative vapor delivery solution, based on a liquid flow controller (LIQUI-FLOW or mini CORI-FLOW), a gas flow controller and a temperature controlled mixing and evaporating device.

Compared to the more conventional bubbler system, a CEM-system offers a more direct approach. The method is very straightforward, and theoretically any concentration can be made in a matter of seconds with high accuracy and repeatability. Moreover, it’s possible to adjust a relative humidity between 5 and 95 percent.

Set-up Bronkhorst CEM System

Figure 2. Set-up Bronkhorst CEM System

The moisture content is accurately controlled by the liquid flow controller and the amount of air flow can be adjusted by the gas flow controller. On top of the CEM a mixing valve allows for a correct atomization of water in the air flow. Because of the relatively low pressure ratio of the water mist in the air flow, the water can be evaporated at a low temperature in the spiralized heater tube at the outlet of the mixing valve.

CEM insights

The set-up of a CEM-system basically consists of:

  1. A Mass Flow Controller for gases for measurement and control of the carrier gas flow (e.g. EL-FLOW Select series).
  2. Mass Flow Meter for Liquids for measurement of the liquid source flow (e.g. LIQUI-FLOW series, mini CORI-FLOW series).
  3. Temperature controlled mixing and evaporating device (CEM) for control of the liquid source flow and mixing the liquid with the carrier gas flow resulting in total evaporation; complete with the Temperature Controlled Heat-Exchanger to add heat to the mixture; Basic Bronkhorst CEM-systems are available as a complete solution, including control electronics, offering total flexibility in realizing a vaporizing solution in virtually any situation.

Do you want to learn more about CEM technology? Visit the Bronkhorst Vapour Flow Control section on our website and read all about our different products and applications in vapour control.

James Walton
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Thermal mass flow instruments that make use of a bypass (capillary bypass or bypass sensor) are what most people have in mind when they think of thermal mass flow instruments. What are the differences?

In instruments based on the thermal principle, power is applied to heat the sensor tube. Accordingly the temperature of the tube is measured at two points. With no flow measured, the temperature differential between the two points will be zero.

When the flow increases, the temperature at the first measuring point will decrease, as fluid carries away the heat. At the same time the temperature at the second measuring point will increase as the fluid carries heat to it. More flow will result in a greater temperature differential and this temperature differential is proportional to the mass flow.

Another technology used to measure mass flow is CTA (Constant Temperature Anemometry). In a CTA (through flow, straight tube) instrument there are two measurement “probes” inserted into a straight tube flow path. The first “probe” both heats and measures the temperature of the fluid, as the second “probe” measures the temperature of the fluid.

Again, as the gas flow increases the gas will carry heat from the first measuring point to the second one. In a CTA, however, the power is varied to keep the temperature between the two measuring points constant, and it is this power level that is proportional to the mass flow.

Each technology has its advantages and disadvantages which generally are application specific.

A clean, dry gas application where higher accuracy is as important as repeatability, may be a better application for a bypass instrument like the Bronkhorst EL-FLOW series.

An application with a dirty or slightly moist gas, or where lower accuracy but high repeatability and robustness is required, may be a better application for a CTA instrument like the Bronkhorst MASS-STREAM™ series.

Curious about using a thermal Mass Flow Meter or Controller? Or the top 5 reasons why we use Mass Flow Meters with CTA measurement?.

For more information, please visit our website

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