# gas flow measurement in ml/min and m/s

#### ep.hobbyiest

Joined Aug 26, 2014
201
I want to use gas flow meter. I checked for some sensors which have given measurement in ml/min and some has in ml/min.
I tried to convert the ml/min to m/s on google but got the result in m3/s.
I am not sure about the flow measurement and units used in the datasheets.
Is there any explanation article or something I can refer to?

#### drc_567

Joined Dec 29, 2008
1,156
$$Q (m^3/sec)=A(m^2)V(m/sec)$$
... where Q is volume/sec flow rate
A is cross section area of the tube or pipe
V is flow velocity

#### Boatman47

Joined Jan 22, 2021
11
I want to use gas flow meter. I checked for some sensors which have given measurement in ml/min and some has in ml/min.
I tried to convert the ml/min to m/s on google but got the result in m3/s.
I am not sure about the flow measurement and units used in the datasheets.
Is there any explanation article or something I can refer to?
These are two different types of flow measurements. The first is about “flow volumes”; the second, “flow rate or speed” . To relate the two requires the cross-sectional area of the flow tube, hence the supplied formulae provided.

#### ep.hobbyiest

Joined Aug 26, 2014
201
Thanks for making it clear. I want to check the flow rate or speed of gas.

#### drc_567

Joined Dec 29, 2008
1,156
... If necessary to employ the formula above, use the inside dimension of the pipe or tube to find the cross section area, rather than the outside diameter dimension.

#### Papabravo

Joined Feb 24, 2006
19,829
Thanks for making it clear. I want to check the flow rate or speed of gas.
Speed of a fluid in a pipe is not uniform. There is in fact a velocity profile which shows that the velocity at the boundary is zero and the velocity in the center is a maximum. IMHO the volume flow rate is the more useful measurement.

#### Boatman47

Joined Jan 22, 2021
11
Speed of a fluid in a pipe is not uniform. There is in fact a velocity profile which shows that the velocity at the boundary is zero and the velocity in the center is a maximum. IMHO the volume flow rate is the more useful measurement.
This is an important observation in terms of flow linearity but does not change the general math equation. The flow (in ml/min) divided by the cross-sectional area of the tube (in square centimeters - remember that one milliliter = one cubic centimeter ) gives the "average" speed (in centimeters per minute). Divide by 60 to get centimeters per second and divide by 100 to get meters/sec.

#### wayneh

Joined Sep 9, 2010
17,201
... but does not change the general math equation.
True, the bulk velocity is the mass-average. We chemical engineers call it the bucket-and-stopwatch value: literally what you capture in the bucket over a timed interval.

The bulk flow can be a useful number, and it can be a useLESS number depending on the context. For instance if you need a chemical reaction to occur in a period of time defined by the residence time in the pipe, it matters a LOT if a fraction of the bulk is coming out in half the calculated time. Having a velocity distribution also means you have to be very careful how and where you measure that velocity. Hence the bucket and stopwatch method.

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#### Papabravo

Joined Feb 24, 2006
19,829
This is an important observation in terms of flow linearity but does not change the general math equation. The flow (in ml/min) divided by the cross-sectional area of the tube (in square centimeters - remember that one milliliter = one cubic centimeter ) gives the "average" speed (in centimeters per minute). Divide by 60 to get centimeters per second and divide by 100 to get meters/sec.
I'm not familiar with the term "flow linearity"; can you elaborate?

#### Boatman47

Joined Jan 22, 2021
11
I'm not familiar with the term "flow linearity"; can you elaborate?
I used the term to accept the fact that flow within a tube for gases and liquids are not uniform (should have used that term....) across the cross-sectional area. Thanks for the correcting clarification.