Correcting flow data after choosing the wrong gas in Gas Select

Correcting flow data after choosing the wrong gas in Gas Select

Correcting flow data after choosing the wrong gas is fairly straightforward. As both the mass flow rate and volumetric flow rate depend on gas viscosity, having the wrong gas selected leads to readings that are approximately off by the ratio of the viscosities. This can be fixed by simply multiplying your reading by the ratio of the selected gas’s viscosity to the desired gas’s viscosity – and your device can make this correction for you.

Choosing the correct gas using Gas Select

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. To correct flow data, use Gas Select to scroll through the list of preloaded gases and choose the correct gas. Your instrument will then be fully calibrated to the correct gas.

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. As long as you have already defined the gas mixture using COMPOSER, you can select this gas instead of air and get the correct flow data!

My gas isn't in Gas Select. What do I do?
If a gas is not available in the gas select menu on the screen, it might not be compatible with the wetted materials. We are constantly adding new gases to our repertoire, and our service department may be able to upgrade your device firmware to flow the gas you need. Please contact Alicat with any compatibility questions.

How is this correction made?

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, you can multiply Qindicated 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.

Example correction

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, you can see that the viscosity of air at 25°C is 184.8989 and the viscosity of Nat GAS-2 at 25°C is 111.55570.  Using the equation above,

Making a correction when the correct gas isn’t in Gas Select

If the gas cannot be selected, it may be useful to determine an equivalent setpoint. To do this, use the altered equation below where the desired setpoint is represented by Qdesired.

Suppose you are controlling the flow rate of water vapor. While it is not on our standard gas list, it is available as a custom software that our service department can program into our anti-corrosive devices. After considering the compatibility, the device on hand is determined a good fit.

Example correction

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.