# a quick question about gas flow rate and volume

Discussion in 'Physics & Math' started by kira, Dec 12, 2011.

1. ### kiraValued Senior Member

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Say I have a suction pump which is supposed to collect 80 mL/min gases from ambient air (1 bar). Running the pump for 1 min will result in 80 mL of gases, right...?

If I now connect this pump to a tube, and connect this tube to a nitrogen tank, and the pressure of the nitrogen gas is set to be 0.5 bar (so now the actual pressure is 1.5 bar, right?), how much nitrogen gases is collected in 1 min? Without the pump, the nitrogen should flow anyway, but I kept the pump running as well.

How to calculate this??

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3. ### wlminexBannedBanned

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. . . calculate? . . . something to do with Ideal Gas Law . . . . PV=nRT, or a related equation.

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5. ### James RJust this guy, you know?Staff Member

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PV=nRT is right. From that we get:

$P_i V_i = P_f V_f$

or

$V_f = V_i (P_i/P_f) = V_i (1/1.5) = (2/3)V_i$

In other words, if your tank is at 1.5 bar then the volume of the gas will decrease to 2/3 or its volume as you pump it in.

So, 80 mL becomes about 53 mL once it has been compressed into the tank.

Of course, as you keep adding gas to the tank, the tank pressure will keep increasing and the gas will keep compressing.

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7. ### kiraValued Senior Member

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Thank you for your replies. However, I think I didn't make myself clear. I actually would like to know, how to measure a flow rate of a gas, if I know the pressure and the cross section area, but without using any flow meter.

I am using an analytical instrument, this instrument is equipped with a built-in suction pump, and can draw-in gas sample with a flow rate of about 80 ml/min.

However, I modified the sample introduction system of this instrument. Instead of directly draws in gas sample from the environment, I connected the sample inlet into a nitrogen tank, and then I injected my gas sample manually (so now my gas sample is flushed by the nitrogen, and together with the nitrogen entered the analytical instrument). My gas sample (which I injected manually) is only 0.5 ml. The nitrogen tank is set at 0.5 bar. The pump is kept running at 80 ml/min for 30 s. But now instead of drawing in 80 ml/min of sample, the pump draws in nitrogen, plus 0.5 ml sample.

I tried to measure the flow rate of the nitrogen entering the system by directly placing a flowmeter on the mouth of the nitrogen tank, however my flowmeter can't read this, because the max flow that can be read by the flowmeter is 500 ml/min. So I don't know how much nitrogen entering the sytem. However, the system has an exhaust pipe, so I tried to measure the flow from the exhaust, because I figured what goes in should be the same with what goes out. What I confused is, whether I connect my instrument into the nitrogen tank or not, the flow rate at the exhaust is the same, i.e. 76-78 ml/min.

How much nitrogen actually entering my system?? Because I need to calculate the dilution factor of my sample. Since the measurement at the exhaust show that the output flow rate did not change, does it mean that the nitrogen entering the system is 80 ml/min (regulated by the pump), despite my flowmeter showed that the nitrogen flow rate (before I connect it to the instrument) is over 500 ml/min??

8. ### originHeading towards oblivionValued Senior Member

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I think you need to supply more information.

1. What is the size of the chamber you are entering?
2. Is there a continuous flow through the chamber?
3. Is this a positve displacement pump?
4. When you say the flow rate is >500 ml/min is that simply venting the tank to the atmosphere?
5. Where do you inject your sample (upstream of the pump)?

9. ### kiraValued Senior Member

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1. It is only an analytical sensor with an overall dimension of 248 mm x 97 mm x 134 mm. It looks similar to this: http://www.sionex.com/products/index.htm
2. No, it's batch. I program the pump to run for 30 seconds and then the pump (and a valve) will close.
3. There is no information about it in the instrument specification and also in the manual, but I see your point. If it is a positive displ. pump, the flow rate should be constant. I am going to ask the supplier for the pump specification. But I think it is a positive displacement pump, because my flow meter showed constant flow rate.
4. Yes. I forgot to mention that the nitrogen was not directly supplied from the tank. The nitrogen tank is placed in gas storage room, the nitrogen is distributed through taps. On each tap there is a pressure regulator. I set the pressure first at 0.5 bar. I connect the gas tap to the flow meter, then I open the valve and read the value displayed by the flowmeter.
5. Through a T-joint, upstream of the pump:
http://sciforums.com/showpost.php?p=2838477&postcount=3

Sorry for the long time it took me to reply back. I was occupied with some work...

10. ### kiraValued Senior Member

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Here is the experimental set-up:

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As you can see, on the front side of the instrument there are 2 things: the left one is an exhaust, the right one is the inlet. This inlet is connected to nitrogen supply through a teflon tube, you can see the pressure regulator above the instrument. Thanks in advance...

11. ### originHeading towards oblivionValued Senior Member

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I don't see the picture.

12. ### kiraValued Senior Member

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I don't know what happened, I (still) can see it

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. Anyway, here I upload it again, but in a different image hosting web:

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13. ### wlminexBannedBanned

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Kira: you might Google some info on flow rate-measurement instrumentation and air-flow calculations . . I use such calculations frequently . . I can provide links if you need them . . .