How to Correct Flow Data after Choosing the Wrong Gas

Correct your flow data after choosing the wrong gas in Gas Select

Like most flow meter technologies, Alicat mass flow meters need to know which gases are flowing through them for their flow readings to be accurate. Our differential pressure-based technology uses the known absolute viscosity of the gas or gas mixture (stored in Gas Select), combined with the measured absolute pressure and temperature to produce flow readings.

If you accidentally selected a preloaded gas calibration that was different from the gas you actually flowed through the instrument, your flow data will be corrupted by approximately the ratio of the viscosities of the two gases. This article shows you how to correct your data by multiplying each reading by the ratio of the selected gas’s viscosity to the desired gas’s viscosity.

How to correct your flow data if you accidentally selected a different gas in Gas Select


Let’s start by defining a few variables:

Qindicated = Flow rate recorded by your device
ηselected = Absolute viscosity of the gas you incorrectly selected
ηactual = Absolute viscosity of the gas that actually passed through your device
Qactual = Actual (corrected) flow rate of the gas that passed through your device

To get to the corrected flow rate of the gas flowing through the unit (Qactual), multiply the indicated flow rate by the ratio of the gas viscosities:

Qactual = Qindicated x (ηselectedactual)

Your Alicat product manual lists the absolute viscosity (η) of each gas that is part of Gas Select, at reference temperatures of both 0 °C and 25 °C. If your process operates near room temperature, the viscosity at 25 °C is a good choice, but note that viscosity does change with temperature.


You flowed a natural gas mixture of 95% CH4/3% C2H6/1% N2/1% CO2 (Gas Select #190, Nat Gas-2), but you accidentally had Gas Select set to air. Your gas was close to room temperature, and you recorded a flow rate of 1.00 slpm.

Qindicated = 1.00 slpm
ηselected = 184.8989 (absolute viscosity of air)
ηactual = 111.5557 (absolute viscosity of Nat Gas-2)

Using the equation above, we come up with the following corrected flow value:

Qactual = 1.00 x (184.8989/111.5557) = 1.6575 slpm


Learn more


WordPress Video Lightbox Plugin