# Measuring flow rate (Doppler device)

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1. Mar 18, 2016

### mpotocni

Hello,
I am new in this field, so maybe my questions will be stupid :) sorry for that :)

In the company where I work, they measure flow rates of wastewater using Flow meter (Doppler device). The problem is, that the flow rate is very low, so this device is not submerged completely in the wastewater.

Then they put some obstacle at the end of the pipe, that the water level is higher and that this flow meter is submerged in the wastewater.

Is it correct to measure the flow rates this way?

Does anybody know how to calculate the flow in this circular pipe with obstacle at the bottom of the pipe (the pipe is half closed from the bottom up)? Is it even possible?

Does anybody know how to test this flow meter easily on-site, if it shows results correctly? It is not possible to get there huge testing maschines.

Thank you very much for help! And sorry for my bad english.

2. Mar 18, 2016

### Simon Bridge

Welcome to PF;
There are no stupid questions ... except maybe the one about the custard, but that's a trick.

As long as the meter is in the flow, yes.
If I understand that correctly, water can still flow out of the pipe, it's just that the exit is partially blocked.
The water will flow faster through the smaller opening, but the volume flow rate will be the same.

Yes, and yes.

Better to test the meter's performance in an experiment rather than in the field.
Get a long pipe - put the meter in it - pour water down the pipe at a constant rate.
You can time how long it takes for 1 litre of water to come out the other side (use a litre jug) ... compare with what the meter says.

Your English is fine, don't worry.

3. Mar 18, 2016

### mpotocni

Thank you very much for answer.
If we can calculate the flow in this blocked pipe, how can we do it?
Can you (or someone) please give me some link to equations or example? I tried to search, but didn't find any similar case.
The pipe shape should be circular.

Thank you for help!

4. Mar 18, 2016

### Simon Bridge

Easy: the equation is "flow out = flow in". The volume flow rate is given by $\phi = V/T$ where V is the volume passing a position in time T.
The mass flow rate is $\Phi = \rho\phi$ where $\rho$ is the density of the liquid.

You need to say what parameters you want to do the calculation from.
For instance, the volume flow rate will be $\phi = \pi r^2 v$ for a liquid flowing at speed v through a circular crossection pipe with inner radius r.
If the pipe is blocked so the opening in a smaller area $A$, then the liquid escapes with speed u given by $R^2v = Au$.

More generally see:
http://hyperphysics.phy-astr.gsu.edu/hbase/pber.html

5. Mar 22, 2016

### mpotocni

Simon, thank you for your answer! I hope that we will calculate properly now. If problems occur, I will ask for help again.

Bye!