Variable area flow meter readings not matching Alicat readings

Why are my variable area flow meter readings not matching my Alicat readings?

Operating principle of variable area flow meter overlaid on an apple and Alicat flow meter overlaid on an orange

Variable area flow meters, commonly known as rotameters, use the pressure of a flow stream to determine a volumetric flow measurement. The pressure of the flow stream causes a ball situated within a conical tube to float. The height of the ball is directly correlated with the flow rate of the gas or liquid. When operating in standard fluid, temperature, and pressure conditions, the volumetric flow equals the mass flow of the fluid.

When ordering a variable area flow meter, you must choose a specific gas, standard temperature, and standard pressure to which it will be specifically calibrated – and the accuracy of the flow reading will suffer if the ambient environmental conditions different from the calibration conditions. Unlike mass flow meters, which are able to adjust to and compensate for environmental conditions, rotameters are “blind” to changing conditions and unable to adjust their readings. The further the environmental conditions stray from the calibration conditions, the lower the accuracy of the measurement will be.

Apples to apples: calibrating a variable area flow meter with an Alicat

We often work with people looking to use Alicat mass flow meters to calibrate their variable area flow meters – but in doing so, we need to ensure we’re comparing apples to apples.

While variable area flow meters and Alicat differential pressure mass flow meters are similar in purpose, they use different technologies. More importantly, they include different underlying assumptions about the environmental conditions in which they’re operating. Rotameters are designed to operate in environments without changing conditions, while Alicats are design to compensate for any environmental changes, and are particularly useful in environments with changing conditions.

So to calibrate a the variable area flow meter, it’s important to ensure the variables are aligned. The best way to do this is to ensure that the conditions in which the rotameter is operating or being calibrated match its original calibration conditions. In a calibration lab, this is simple: the calibration technical can flow the specified gas, bring the test bench to the proper temperature, and use a pressure control system.

Once the rotameter is at its standard temperature and pressure conditions, it can be calibrated using the calibration standard.

If the conditions do not match the conditions at which the rotameter was calibrated, we have apples and oranges: the Alicat mass flow meter will compensate for environmental changes, but the rotameter will not.

I don’t have a calibration lab. How do I avoid comparing apples to oranges?

The process described above for a calibration lab is equipment-intensive. To ensure your calibration has the highest-possible accuracy, let’s break it down to look separately at temperature and pressure.

Temperature is unlikely to fluctuate too significantly from the calibration conditions. While a large temperature change will effect the accuracy of the flow reading, most industrial environments are within a few degrees of one another. You can use the thermostat in the room to get the temperature closer to calibration conditions, or accept that the inaccuracies will be minimal as long as the temperature change remains small.

Large pressure changes, however, are more common. Local weather patterns can cause barometric differences, and if the calibration is occurring at an altitude significantly different than the one at which the rotameter was originally calibrated, the pressure differences can be significant. Measuring the local pressure can be done simply with standard sensors, so you can understand the extent of the pressure difference and the impact it is likely to have on the accuracy of your rotameters.

Using an Alicat to calibrate a rotameter

If you are able to adjust the ambient temperature and pressure to match the standard conditions of the variable area flow meter, it becomes much simpler to compare its readings to those of an Alicat flow meter. Putting the Alicat as physically close as possible to the variable area flow meter will also help in the comparison.

To calibrate, ensure that the outlet pressure of the variable area flow meter is known. Plumb an Alicat mass flow controller with a downstream valve configuration inline, and downstream of the rotameter, using the “Closed Loop Pressure” control mode.

If the ranges and gases allow, use a low pressure drop flow controller so that the pressure drop induced by the Alicat doesn’t skew the conditions of the variable area flow meter. Ensure the gas selection and STP settings on the Alicat are identical to the calibration conditions of the variable area flow meter.

Then set the back pressure control point on the Alicat to equal the outlet pressure of the rotameter. This creates a condition in which the Alicat is controlling the rotameter’s pressure, standardizing the Alicat to the variable area flow meter.

At this point, we have apples all around (eat fruit, it’s good for you!). You can introduce a flow and use it to calibrate the variable area flow meter.