How to calculate flow with level using ladder

[caracena]

Hi guys. First post

We are working on a project for my company where we get data from some analog and digital transmitters (pressure, on/off switch and level). The level of a tank is given by a 4-20 mA HART Siemens radar and we need to calculate how much is going into the tank (flow rate) in liters per minute or whatever.

We are trying using a blink that grabs the initial level and after X seconds grabs the final value, calculates and then starts over. Waiting is not a problem. We can wait up to a minute to get the refreshed new flow rate. The problem is that it is acting kinda weird. Never trusted blink so much.

The real question is... Is there a way to calculate the flow rate with a ramp or something like that? What would be the best solution?

Thanks in advance!

 

[JesperMP]

Whats a "blink" ? And what do you mean by "kinda weird" ?

I am assuming that you know how to convert the level value to a volume by the tanks geometry.
The problem with a pressure sensor as a level sensor, is that filling or emptying will generate waves and turbulence that may affect the pressure signal.
So you may be looking at dampening the pressure signal to get a reliable flow value. That again may mean that the flow value has a significant dampening. It will not be near-instantaneous like an electromagnetic flowmeter.

 

[caracena]

Hi Jesper, thanks for your reply.

Yes, I have the coefficient of liters per centimeter so calculating the volume is not a problem and the level is acquired using a level radar from the top of the vessel, not a pressure sensor at the bottom.

Blink is a module CODESYS lets me insert into my program. It's a counter with an on/off signal. It is used for example to make a sound alarm go beep beep beep (intermittent) instead of just having a long beeeeeeeeep (continuous).

I haven't find a timer module so I'm using the blink with 1 second waves and X waves in total as a timer. The weird result I'm having is that at the end of each "loop", it throws all kind of weird numbers for some 5 or 6 seconds as if it were showing the calculations I make at the end of each loop. I know it must be some mistake I made at the program but still, before I correct the mistakes, I rather find a better way of getting the flow rate.

I'm guessing that calculating the ramp of the volume should not be hard because I already have a continuous signal coming in but I would like to know if that ramp could be used to instantly display the flow rate.

 

[JesperMP]

Your description of the "blink" sounds to me as an alternating off-on bit with a symmetric period.
If you are calculating the flow using this bit you would continuously make the calculations for many program scans when the bit is on. I think you want to calculate the flow cyclically with a fixed update time.
You should generate impulses by edge-detection (aka "one-shot") of the blink bit. Then trigger the calculation by the bit with edge-detection.

 

[boneless]

I would make a shift register of say 61 values. And shift in a new level every second.

To calculate approximate flow you could take the first and last value of your shift register every second. Problem will be a delay and the fact that you don't take into account what happens in that minute.

You can sample more often and have a shorter register to make it more accurate. Say 11 samples every second or 100 ms.

Let us know how it works for you. Looks like a fun little project to get working

 

[Jimbojones]

This is not directly related to your question, but I had some information that my be useful. My first controls project was somewhat similar....using a SIEMENS SITRANS ultrasonic probe to measure fluid level for fuel storage tanks.

These fuel tanks were refilled every month and had a very low usage. I inherited the SITRANS probe from a previous engineer but I had a tremendous amount of issues with erroneous spikes in the readings that would set off alarms, send emails, etc.

We looked at noise issues in the 4-20mA control line, replaced the SITRANS units, etc. The distributor at the time said to spray Rain-X on the transducer, which was not a feasible solution as these tanks were ~20-40' high.

In the end, I believe I placed some logic to suppress the alarms unless a certain time elapsed. After further consideration, the problem may of been caused by the tank geometry, and how close the sensors were placed to the edges.

 

[Tom Jenkins]

Codesys is supposed to be IEC 61131-3 compatible - I can't believe it doesn't have timers.