water level computing

i need some sugestions for calculating a medium value of a water shaft ... the level is oscilating because of waves with aprox. 40 cm up or down .Any ideas?
plc for aquisition of levels is cpu 317 pn from siemens .

catalin78 said:

i need some sugestions for calculating a medium value of a water shaft ... the level is oscilating because of waves with aprox. 40 cm up or down .Any ideas?
plc for aquisition of levels is cpu 317 pn from siemens .

Click to expand...

I'm not sure I understand what you want to do...Do you want to measure the level and disregard the oscillation caused by the waves, I assume???

I suppose you could average a set number of samples each cycle and eliminate the high and low sample, but I don't think that would be very accurate. Probably best to program a lag-filter on the input and use the filtered result as your number.

What is the objective of this measurement? For Control or Monitoring.

For monitoring propose there is no problem to use lag filter.
But for Control propose you should be careful!
If you use PID method for control you most use low gain and more time for Integral to make an averaging control. By this way you automaticaly compensate the wave effect.
If you use On-Off Control you can define a suitable deadband or Gap to mask the wave.

Extreme estimation. Notice I didn't say filtering.

I like Dr Watson's idea, but I would take it to the limit.

If I knew the wave action was sinusoidal then I could subtract out the sinewave leaving only the offset or water level to be filtered.

Amplitude = (max-min)/2
Period = time between maximums or time between minimums.

Current reading = amplitude*sin(2*PI*t/period)+ mean water level
or
mean water level = current read - amplitude*sin(2*PI*t/period)

Now one can estimate the mean water level for any time t between the waves.

Now I would filter mean water level to get an average.
The filtered average water level will be more accurate and will update faster.

I bet I can find some algorithms that do what I propose but I doubt they would run on a PLC. A S7-317 using ST may work if the waves are very slow. How does one fit a sine wave to a set of points?

Hey, did any catch the fact that when time t = 0 the value of the sine is 0. It would be better to use a cosine so one could count the time from the highest point of the wave. I am just checking to make sure you guys are paying attention.

I've had good success using digital filtering like the attached. I don't have it handy in Siemens, but you should be able to do the conversion without too much trouble. It just uses simple four function math.

Couldn't you also use an internal pressure sensor? It seems like that would provide level sensing with less noise (presuming a constant density of your medium).

Why not measure the water level in a tube? Stand an open bottom tube or pipe in the reservoir to isolate the wave action and measure the water level inside the pipe.

Thanks folks for your help . The project it's about measuring levels of water in a pipe . reading is done with two methods hidrostatic and encoder with a rotative system . I know the waves are about max 40 cm high but i don't know what variation will be in the measuring tube , the measuring points are set up on "Dunarea" stream in a harbour lock .I just arrived at the computer and i am reading your answers now .
Thanks and Merry Christmas everybody

The condition to operate the gates are :
the difference between the level in upstream (downstream ) and "sas" (chamber in harbour lock) must be max 30 cm , so i have to compare this levels to see if the condition to operate the gates it's fullfilled.
Sorry for bad english if there are more details needeed i'll try to post it.

Thanks .

I'm trying to get the picture here;

This is a lock system, presumeably to pass watercraft from one level to another. You are trying to measure the water level in a horizonal pipe between the locks. However there are waves in the pipe that are up to 40cm high, and you need to control the water level between the two locks with a maximum of 30cm difference between the two levels. In other words the waves are higher then the differance in the water levels.

If I understand this properly, under these conditions my first thought would be to measure the water level by installing pressure sensors at the bottom of each lock (measure depth by pressure). I'm not sure why one would measure water level in the pipe, execpt to maybe calculate flow, (speed x level = GPM). However flow could be calculated by the rate of rise or lowering of the water in the locks, again using the pressure sensors.

We have some pretty good size locks here in Seattle. Not as large as those in the Panama Canal but,,,,,,. Anyway they operate with an average tidal difference of 14 feet or so and a maximum of 20 feet several times a year. The lake level is approx 20 feet above the mean level of Puget Sound.

What I would like to see is when you open the gate with difference of 30 cm in water height. Ought toe be real interesting on the "downhill" side.

Think I would have a tighter tolerance.

With wave action height of 40 cm -- you need to dampen this out for sure.

Like Mike mentioned the level sensors (sea side) need to be isolated to reduce the magnitude of oscillations from wave action. Tidal guages here are set inside a pipe to acomplish this - does not eliminate but does reduce it. Sort of like a breakwater.

I would think to control the gates a running 5 or 10 minute average should suffice. You could take a reading every 5 or ten seconds and then average them.

What is your ultimate goal here
- to fully automate the locks?
- or to make operation easier for the operators?

Dan Bentler

It would seem that a pipe with constant diameter was lowered into a body of water which is experiencing waves, there whould be level changes at the top of the water in the pipe exactly coorelating with the waves. Now if a lower section of the pipe was restricted in diameter this would functions as a sort of low pass filter. But this is a pure guess.

An open ended pipe would reduce the magnitude of oscillation - just like a breakwater but would not eliminate it.

I agree Bernie necking the pipe would be just like putting a throttle valve to dampen gage oscillation.

Dan

I don't see the problem

What isn't clear is the orientation of the pipe. I now think this pipe is used to equalize the water levels. One waits for the flow in the pipe to be close to or reach zero or below before opening the locks. I don't think this requires filtering. If the flow goes back the other way from the Black Sea to the canal then it is just wave action. The exact level isn't needed.

Check out the canals at Constanta using Google Earth.
I can't add a Google Earth .kmz file. Phil? I had to zip it.
catalin78 did I mark the right canal?

pipes

Peter Nachtwey said:

What isn't clear is the orientation of the pipe. I now think this pipe is used to equalize the water levels. One waits for the flow in the pipe to be close to or reach zero or below before opening the locks. I don't think this requires filtering. If the flow goes back the other way from the Black Sea to the canal then it is just wave action. The exact level isn't needed.

Check out the canals at Constanta using Google Earth.
I can't add a Google Earth .kmz file. Phil? I had to zip it.
catalin78 did I mark the right canal?

Click to expand...

I am installing google to look at that peter , but this is a new project and the pipes will be installed next week and are for level measurement to see when it's permitted to open the gate , and as i said earlier the project includes a hidrostatic pressure measurement mounted at the bottom of pipes and a sistem with encoder wich is moved by a chain with a floater wich goes into the pipe , i am sorry but i don't have the engineerig projects now i'll have it after christmas and if you guys are still willing to make an opinion about this i will scan them and post it here in the the thread . So there are two measurement sistems for each pipe for safe guard . i thought i could work with an average between pressure sensor and encoder or in fault mode when one goes down just with one sensor .