What type of fluid can be flowed through an Alicat flow instrument?
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 vs. liquid flow devices 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 meters and controllers 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 or controller, a volumetric flow rate is then calculated using temperature measurements. A gas flow device requires additional pressure information to make its final mass 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 device, or if a liquid device contains bubbles?
Accurate flow rates require clean, single-phase fluid flow. However, a gas instrument 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 instruments (and anti-corrosive gas meters and controllers) 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
We also make mass flow meters and controllers that use Coriolis technology. 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.
Our Basis thermal mass 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.