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.
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.
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.
The gas table in the manual includes all gases available on both our standard and anti-corrosive series. Take care to note which are available on your specific device. 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. We are constantly adding new gases to our repertoire. In some cases, a firmware upgrade or custom firmware can be added to your device in our service department to add the gas that you need to your device.
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.
If you’d like to add a custom gas to your device, or upgrade your software, or if you need help determining a viscosity, please contact Alicat.
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