I am looking for a simple way to detect nitrogen gas flow through a tube. I don't even need any sort of precision, just really need to know if flow exists or not.
What are some options I could pursue to easily measure this with the arduino?
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You can use a pressure sensor for gas and measure the pressure differential between two points and if there is a large pressure differential then you know a flow exists.
What you are looking for is a mass flow sensor (MAF), or something along those lines as you mention wanting to know if there is flow, this gives you the 'speed' of the flow. There are various types available according to size, flow rate, type of element, and accuracy. Just search for mass flow sensor, you can even hack them off cars.
If you just want to detect only the presence of the N2 as you say then you can split your pipe somewhere,and then add a T-piece to that. You then put a gas sensor in the T part, and seal it off so there is no leaks. You would then hook that up to the Arduino board and interpret the signal.
Can't recommend any specific sensors though.
Here is an image for the idea: