# Flow Rate from Tank Level

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#### Himanshu

We have a tank having petroleum product in it. There is a level transmitter whose value is coming in a PLC. The level is increasing or decreasing based on receipt or dispatch. We need to calculate flow as liter per minute or m3/hour from this level which is changing based on the operation.

what will be formulae for this which can be executed in PLC?

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#### Vedry

Hi,

At least we need to know volume (V) and level of tank (l).

Assuming uniform area of tank, you can use formula max V / max l, to get volume per cm. For example. Now measure change of level, for example, every minute and you have Q in m3/min.

To calculation to work correctly, you can't fill and empty tank simultaneously.

#### David_2

I hope this is a school assignment because flow metering a valuable petroleum product without a flow meter is a silly idea.

Level is good for making volume based decisions like, "is there sufficient product to make the next batch" or is there "sufficient empty volume to accept a load of certain size". Level is a poor value to use in a flow rate calculation.

The problem with calculating flow rate from level is that it is a the calculation is a rate, a derivative function, dy/dt. A fixed time base is might be and is probably achieveable in a PLC so that the dt factor is a constant, but dy is an issue.

The problem with a using the change in level for a calculation is that most tanks of any size spread a given volume over a huge area to a very, very tiny depth. The charge or discharge rates generally do not create a useable change in level value with which to calculate a viable flow rate because those changes are very, very tiny changes in depth (level) which fall into the noise region of level-inferring instrumentation.

Given all that, the rate calc needs a holding register to hold the 'previous' level value, the ability to subtract, and a register to hold the difference between the previous value and the current value. Dividing the difference value by the time interval between previous and current samples gives you a rate, which can be normalized to seconds or minutes by a factor.

Don't be surprised if some readings appear to going in the wrong direction -filling rather than emptying - because noise is noise; some is positive, some is negative. and you're subtracting noise from noise.

If this is real, not school work, give serious consideration to a flow meter.

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#### RoyMatson

As others have said a flow meter, I would think a coreolis meter should be able to read center zero and integrate in both directions.