Correcting flow data after choosing the wrong gas in Gas Select™
Selecting the correct pre-loaded gas calibration is an important step in obtaining accurate mass flow data. If you do notice you have been collecting data with the wrong gas selected, there is no need to panic. Data can easily be corrected using the instructions laid out in this article.
Choosing the correct gas using Gas Select™
The Gas Select™ feature is built into the firmware of all mass flow devices, enabling users to choose from a list of pre-loaded gas calibrations. To correct flow data and calibrate the instrument to the correct gas, simply scroll through Gas Select™ and choose the right gas from the list.
For example, suppose air is selected on a mass flow controller, but the actual gas flowing is Nat gas-2 (95% CH4 + 3% C2H6 + 1% N2 + 1% CO2). As long as you have already defined the gas mixture using Composer™, you can select this Nat gas-2 instead of air and get the correct flow data.
How is this correction made?
This correction is made by simply multiplying the incorrect reading by the ratio of the selected gas’s viscosity and the desired gas’s viscosity. See the equation below:
Qindicated = Recorded flow rate; ηselected = Absolute viscosity of incorrect gas; ηactual = Absolute viscosity of the correct gas; Qactual = Actual (corrected) flow rate
Revisiting the above example, imagine you are flowing Nat gas-2 at 25°C and 1 SLPM. You accidentally have air selected, but realize it and switch to Nat gas-2. The gas tables indicate that at 25°C, the viscosity of air is 184.8989 and the viscosity of Nat gas-2 is 111.55570.
The device make the correction as follows:
Flowing a gas that is not on the standard gas list
Imagine you are flowing a gas that is not available* in the standard gas list, such as water vapor**.
Simply choose a gas that is available, such as air, and use the following equation to determine the proper setpoint to give the device:
Qinput = Commanded setpoint; ηselected = Absolute viscosity of selected gas (i.e. air); ηactual = Absolute viscosity of flowing gas (i.e. water vapor); Qdesired = Desired setpoint
Water vapor has a viscosity at 25°C of 98.71700. For a setpoint of 25 SLPM of water vapor on a device that is set for air, calculate the input setpoint as follows:
*If a gas is not available in the gas select menu on the screen, it might not be compatible with the wetted materials. Please contact Alicat with any compatibility questions.
**Water vapor is available as a custom software that our service department can program into our anti-corrosive devices.