Volumetric vs. standard volumetric vs. true mass
“Measuring flow rates” of gases and other fluids means different things, depending:
- volumetric rate: the space the material consumes, seen as “volume unit per unit of time” (e.g., liters per minute)
- molar mass flow rate: widely abbreviated to “mass flow” this is the number of molecules per volume under specific conditions, expressed as “standardized volume units per unit of time—under normalized conditions” (e.g., standard liters per minute, or liters normal per minute)
- true mass flow: measuring the mass of the molecules in the flow, which is reported as units of mass per unit of time (e.g., grams per minute).
Each of these types have their applications, but each is more technically challenging than the one before, because of added levels of complexity in the measurements. The most difficult one, resulting in the priciest solutions, is true mass flow.
So it’ll come as a significant advantage to our customers that Alicat now offers selectable engineering units on our instruments, including true mass, standard (molar) mass, and volumetric flow units—essentially extending our multivariate instruments into multi-purpose measurement and control devices.
Why doesn’t everyone offer true mass?
Measuring mass itself is always an indirect process. It means measuring momentum, weight or thermal capacity…there are several means. Some methods work with flowing fluids—for example, coriolis devices use a vibrating channel, and measure the variation on that vibrational momentum, which is caused by mass flow of gas or liquids through the channel. They are unaffected by changes in the fluid, or even chunky and dirty flows. The catch is, they’re expensive—several times the price of an Alicat instrument. And they will give you mass, but not any information about how many molecules are flowing, because they are ignorant about the media that’s flowing. (Find out more about measurement technologies in the “Types of Gas Mass Flow Meters” section of the How it Works page).
With updated firmware, an Alicat flow meter or controller can now measure true mass for your fluids—in units of your choosing—for any of the fluids on the unit’s Gas Select list, or any custom mix you build from those fluids. It’s true, you can order other mass flow metering instruments that have been programmed to provide true mass flow data for a single, known, pre-determined fluid. They’re programmed in the factory before shipment, but that fluid type can not be changed without a factory reprogram because the instrument is not capable of adjusting for changes in viscosity. The unit loses its traceable ties between its calibration and changed conditions.
With Alicat instruments, when you change gas types, you change the viscosity look-up of the gas itself — effectively re-calibrating the instrument to the new gas—all the necessary information is present to calculate true mass flow. For the same reason, we don’t have an accuracy modifier, which is common to other instruments, on our measurements when you change gases in our selection table. You can decide in the field to measure true mass, then change gas types (changing your selected gas setting, too), and stay as accurate as ever. You get true mass measurements, without paying the coriolis price.
Who needs true mass flow measurements?
In bioreactors, systems engineers may be interested in measuring out-gas mass (for example, a fuel gas like methane) from the biomass. Metering this in kilograms can be more useful than liters. Conversely, regulating reactions in a chamber may be best measured in mass terms, for calculating consumption rates and predicting production output. Hypothetically, “I’ve got 4 kg of biomass, and it will require 2 kg of CO2 in the nutrient mix to keep it productive over the next 24 hours.”
In pharmaceuticals, gases and fluids are used in many reactions. A common process is hydrogenation, especially to provide coatings. Knowing the intended thickness, density, mass and area of coverage, one can calculate the required mass that would be added, and therefore using true mass dispensing of reactive gases into the process would be easier to calculate than converting standard volume into mass.
The point is, it’s easy
While some users will benefit from true mass units, others will find an advantage in the volumetric, standard volume or pressure control capabilities that are already built into our instrument’s smart sensing. Nearly any flow system can benefit from the fact that the change between units is easy—a few clicks through the display menus, or a few instructions through the serial connection. This capability allows for easy repurposing of the flow instrument, or realigning units to correlate better with other calculations.