Gas flow meters vs. liquid flow meters: What can I flow through my device?

Gas flow meters vs. liquid flow meters: What can I flow through my device?

Alicat devices can be customized to flow a wide range of fluids, however each device has its own unique capabilities. Taking a look at gas flow meters vs. liquid flow meters can help you get the most out of your instruments while minimizing risk of damage and measurement error.

What is each device capable of safely, accurately flowing? If a device measures both volumetric flow rate and mass flow rate, does that mean it can flow both liquid and gas? The answers to these questions depend on both underlying flow technology and what the device was specifically built to do. Here we discuss the flow capabilities and limitations of our flow devices.

Laminar differential pressure instruments

Most of our devices operate using laminar differential pressure technology. These devices contain a laminar flow element, which converts turbulent flow into smooth, laminar flow. Internal sensors then measure gas temperature and pressure of the smoothly flowing gas, as well as pressure drop across the flow element.

When using a liquid meter, a volumetric flow rate is then calculated using temperature measurements. A gas flow meter requires additional pressure information to make its final flow calculation, due to the highly temperature and pressure dependent compressibility, viscosity, and density of gas.

Due to large differences between gas and liquid viscosities, one device will not be able to effectively flow both liquids and gases. As an example, water is about 50x more viscous than air and requires a much larger flow body for comparable flow ranges.

Why can a gas device flow almost any gas, whereas a liquid device only flows one liquid?

While our liquid units can only be calibrated to flow one specific liquid, our gas units are calibrated to easily switch between flowing 98+ gases. This boils down to a matter of practicality. A liquid unit could technically be configured to flow multiple liquids, however it is challenging to produce a flow body with the proper size and chemical compatibility to effectively do so.

Because physical properties of different gas types are less variable, our gas models can be used to effectively flow multiple gas types. Gas instruments are still calibrated for optimal flow of a specific gas or two, but there is much greater flexibility and you can easily switch between gas calibrations.

What happens if liquid is flown through a gas meter, or if a liquid meter contains bubbles?

Accurate flow rates require clean, single-phase fluid flow. However, a gas meter may occasionally come into contact with liquids due to a splash, humidity, or backflow. While it is never advised to intentionally flow liquid through a gas device, our gas units can handle small amounts of liquid contamination without resulting in damage to internal electronics.

Liquid flow meters (and anti-corrosive gas flow meters) tend to be a bit more robust as they are built to be resistant to corrosion, however liquid units are not calibrated to provide accurate gas readings. So what happens if your liquid flow contains gas bubbles? These interfere with the sensors and create inaccurate readings. To minimize bubbles interfering with readings, our liquid devices are equipped with bleed ports that remove the gas from the flow stream.

Coriolis mass flow instruments

Most of our units operate using differential pressure technology, however we also make Coriolis mass flow instruments. These devices measure mass flow rates with no dependence on temperature or pressure measurements. They instead use sensors to measure the deflection of the tube through which the fluid flows. This deflection is directly proportional to the mass flow rate.

A strength of this technology is that it measures mass flow rates regardless of fluid properties or composition. One Coriolis can therefore measure mass flow of either gases or liquids, with no need for recalibration. Like our differential pressure devices, these do have the limitation of requiring single-phase fluid flow.

Thermal instruments

Our Basis OEM thermal flow controllers operate by measuring the amount of current required to maintain a fixed temperature across a heating element. Faster flows require higher current and vice versa. Because liquids have significantly higher specific heats than gases, a much higher current is required to heat a liquid flowing through an instrument than for a gas. This means thermal meters for gases cannot get wet or the current will get too high and permanently damage the internal sensor.

Applications requiring liquid and gas flow meters

Microfluidics: Differing roles for liquid and gas meters

Microfluidic liquid dispensing applications can utilize liquid controllers to directly move liquids through the system. Alternatively, a combination of pressure and flow control can be used to control headspace pressure. The ideal solution will depend on requirements for flow precision and also on fluid compatibility. For example, liquids which are highly viscous or contain particulates may not be suitable for flowing through a liquid controller.

Liquid product dispensing: Fluid incompatibility

Liquid flow controllers enable precision dispensing of fluids with changing flow rates and short dispensing periods. This may be an ideal solution for highly dilute medicines with viscosities similar to water. However, highly viscous liquids such as paint are incompatible with most liquid flow meters. This means some liquids must instead be dispensed indirectly using gas mass flow and pressure control.

Flow characterization and leak checking: Measuring liquid flows directly

Mass flow and pressure devices can be used to leak check cooling systems and characterize pumps. However, liquid flow meters offer a simpler solution especially for fluids with viscosities similar to that of water.