Why is my variable area flow meter not matching up with the Alicat?

A Rotameter is a variable area flowmeter (VAF). It provides a flow measurement by using the pressure of a flow stream to float a ball within a conically shaped tube and provides the mass flow measurement of a gas or liquid, when operating in its standard fluid, temperature and pressure environment.

When ordering a Rotameter, one must choose a specific gas, standard temperature, and pressure that it will be specifically calibrated for. If one parameter deviates from these specifications in actual operating conditions, the accuracy will suffer. The Rotameter will not automatically adjust its readings and is virtually blind to any sort of changing conditions. Even though the Rotameter is marked in “standard” flow (for example “Standard Cubic Feet per Minute”), you must remember that the measurement lines are a fixed distance apart, and if your operating conditions deviate from the STP (or you are running a different species of gas), the scale doesn’t adjust. You’re no longer measuring SCFM!

By contrast, Alicat mass flow meters are multi parameter devices that provide standard and volumetric flow readings, while reading process pressure and temperature conditions. Because the Alicat is measuring all the variables, it is able to adjust in real time to changes in operating conditions. Alicats always give the standard mass flow reading. It’s NIST-traceable.

Customers who have Rotameters and receive Mass Flow Meters from Alicat often ask how they can calibrate their Rotameter using an Alicat. We have to resort to the old saw, “comparing apples to oranges,” when looking for correlation between these two different types of flow meters. Although similar in purpose, they are different technology, measuring with different underlying assumptions about conditions. Correlation is not impossible, just difficult—you can compare apples with apples, if the variables are aligned.

A Rotameter calibrated to three gases: 0xygen, N20 and Air.

The conditions within the Rotameter must be controlled to the calibration conditions it was built for, to converge on a correlation. In a calibration lab, the tech will flow the specified gas, bring the test bench to the proper temperature, and use a pressure control system to put the Rotameter in its STP conditions. This requires some significant investment in equipment. Once the Rotameter is in its STP environment, the meter can be adjusted to read the same as the calibration standard, thus calibrating the Rotameter.

I don’t have a calibration lab. How do I match a Rotameter to an Alicat?

Temperature has some effect on the flow reading, but pressure changes, whether caused by process conditions, barometric or altitude differences will have a stronger effect (unless the temperature change is large). Try to adjust internal temperature and pressure to match the Rotameter’s STP. This will require some sensors. In the field, you could use an Alicat as near as possible to the Rotameter in the process, to give you some readings. If internal pressure can be controlled within the Rotameter to match the fixed STP values, then there is cause for a legitimate comparison from the Alicat to VAF.

Ensure that the Rotameter is flowing out to its intended pressure. Let’s assume venting pressure is 30 PSIA for this example. Then, plumb an Alicat Mass Flow Controller with a downstream valve configuration inline and downstream of the Rotameter. Set it to our “Closed Loop Pressure” control mode. If the ranges and gases allow, use an Alicat Whisper product, so that the pressure drop induced by the Alicat doesn’t skew the Rotameter conditions. Ensure the gas selection and the STP setting on the Alicat are identical to the Rotameter’s. (Gas selection and STP are adjustable on Alicats.)

Then set a backpressure control point to equal the rotameter’s calibrated venting pressure, let’s say 30 PSIA. This means the Alicat is controlling the most important condition within the Rotameter, pressure. Now that you have standardized the Alicat to the Rotameter conditions, introduce a flow, and they should be comparable.

We now have apples all around. (Eat fruit, it’s good for you.)