## I Accidentally Had the Wrong Gas Selected. What Can I Do?

Posted by:

In brief:  Both the mass flow rate and volumetric flow rate depend on the viscosity of the gas. If a different gas is selected than that which is flowing through the device, the flow rates will be off by approximately the ratio of the viscosities.  Simply multiply your reading by the ratio of the selected gas’s viscosity to the desired gas’s viscosity.

With Gas Select and COMPOSER, you need never have to do the calculations in this FAQ answer. They are more accurate than you are likely to be using this solution, since the instrument is equipped to measure temperature and pressure and account for their effects.
Gas Select is a feature built into the firmware of our meters and controllers which lets the user choose from a list of installed gas calibrations. Using Gas Select, you would simply access the list of gases through the menus in the display (or through serial commands, if you’re operating remotely), choose the gas you are using, and your instrument is fully calibrated to that gas. Using COMPOSER, you can also make mixtures of the Gas Select gases and save them.
For example, suppose air is selected on a mass flow controller, but the actual gas flowing is natural gas made up of 95% CH4 + 3% C2H6 + 1% N2 + 1% CO2, or as we call it, Nat Gas-2. Due to the age of the device, this is not a selectable gas. If the serial number is greater than 80,000, it can be upgraded to the newest 6v firmware to increase the number of selectable gases and add many new features. (See 6v blog post).

### Discussion and example:

Both the mass flow rate and volumetric flow rate depend on the viscosity of the gas. If a different gas is selected than that which is flowing through the device, the flow rates will be off by approximately the ratio of the viscosities.

##### The variables

Let’s start by defining a few variables. Suppose your selected gas has a viscosity of   and the device shows a flow rate of  . Similarly, the gas that is actually flowing through the device has a viscosity of  .  To get to the corrected flow rate of the gas flowing through the unit,  , you can multiply by the ratio of viscosities as follows.

The viscosities of all the gases available on your device are listed in the manual both at 0 °C and at 25 °C, with the most common gases listed on the back cover. Usually, the viscosity at 25 °C is a good choice, but it is important to note that the viscosity is a function of temperature.

For the sake of illustration, let’s say you decide to correct the numbers by hand.  The gases are approximately at room temperature and the flow indicated on the device is 1 slpm.  From the gas tables available in our manual at http://www.alicat.com/knowledge/documents-resources/, the viscosity of air at 25°C is 184.8989 and the viscosity of Nat GAS-1 at 25°C is 111.55570.  Using the equation above,

If the gas cannot be selected, it may be useful to determine an equivalent setpoint. This is extremely similar to the above. Let’s call the set point desired  and what must be given to the controller in the gas selected as  .

Suppose that you are controlling the flow rate of water vapor. It’s not on our standard gas list, but it is available as a custom software that can be programmed into our anti-corrosive series in our service department. After taking into consideration the compatibility, the device on hand is determined a good fit. Water vapor has a viscosity at 25°C of 98.71700.  This isn’t in the gas tables in the manual, but we’re happy to help find viscosity data for you. If a setpoint of 25 SLPM of water vapor is desired on a device that is set for air, the input setpoint can be calculated as follows.