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I understand that there are quite a few fairly simple and cheap methods to measure the flow of gas in terms of volume, allowing you to get a flow rate in terms of litres per minute for example.

However, what would be the cheapest method to measure the mass of gas flowing along a pipe, so that differences in pressure etc. don't impact the results taken?

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    $\begingroup$ Auto makers are pretty good at doing things inexpensively; and every car with fuel injection has a gas mass flow sensor. Those I know of are "hot wires" , they correlate the air mass to temperature change ( with compensation for air temperature). $\endgroup$
    – blacksmith37
    Commented Apr 10, 2019 at 22:03
  • $\begingroup$ I agree with blacksmith. A hot wire mass airflow sensor is exactly what you need. That or if you already have a volumetric flow sensor, just add a pressure sensor and do a bit of math. $\endgroup$
    – Drew ♦
    Commented May 17, 2019 at 4:49

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By knowing the volume flow rate of a gas , you can also find the mass passing through a cross section area for certain thermodynamic conditions.

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It depends what you call "cheap" but a mass flow meter/controller, such as these ones, will measure the flow rate, and also control it to a set point regardless of pressure variations (for the mass flow controller). They rely on a thermal bypass to measure the flow rate:

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If there is no control of the flow rate, and you're just measuring it, then any pressure change will result in a change of flow rate, it's the underlying physics and you can't avoid that.

If you just want a rough idea, putting the end of the pipe into an upturned cylinder full of water, and measuring how much volume/level the water has come down over a certain period of time, does work pretty well and gives you a good idea. Obviously, this isn't an "online" measurement, it's just a check, and it doesn't have the same level of accuracy as a mass flow meter. Again, as with any measurement technique, this will depend on the pressure.

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  • $\begingroup$ Thanks for your answer. Another option I thought of was to simply measure the volume, but obviously this would only be able to give an accurate measure of the mass if the pressure is kept constant - are there any devices that could be added to a pipe to compress the gas in such a way that the "output" pressure is kept constant, regardless of the pressure of the gas at the "input"? $\endgroup$
    – PhysicsGuy123
    Commented Apr 10, 2019 at 17:35
  • $\begingroup$ Actually, I will edit my answer later because it is slightly incorrect: if you are not controlling the flow but simply measuring it, any change in pressure will result in a change of flow rate, it is the laws of physics and any measurement technique will reflect that, you can';t avoid it. If however, you are controlling the flow, then there are ways to make sure that changes in pressure don't affect the flow rate - the mass flow controllers I suggested will take care of that. $\endgroup$
    – am304
    Commented Apr 11, 2019 at 10:57

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