Tidal level measurements...

Ultrasonic (and radar) is used for level measurements in tanks. Your tube idea is a good one -- I'd try looking on ebay for surplus level measurement equipment (ultrasonic or not) and get what I could afford.

Or I'd try to build one, just for the fun of it....

I'd consider a "pressure transducer". The water level above the transducer is directly proportional to the pressure, (small things like temperature and density being excluded). Ken

How about submerging a pressure sensor BELOW the low tide level? Avg pressure would indicate depth, but no wave action down a couple feet.

OK, thanks to all for the suggestions. As of the pressure transducer approach, don't you think I will get problems with algaes etc growing..? Also it needs to be absolutely watertight..

If anyone else have other suggestions or more concrete info on models and / or suppliers I'd appreciate that ;-)

Thanks again

Geir

Seal the transducer in a flexible plastic bag. Because the pressure in the bag and the ambient water pressure are equal, there is no pressure differential on the seals.

If a big fat fish comes and sits on your bag, well all bets are off :-). Maybe put it in a box? Ken

Typically you use bubblers - a small tube with forced air escaping out the bottom and the top is sealed with a pressure transducer on the end.

The transducer is above the waterline; the only submerged part is the tube.

You can build one for < US$50; the transducer is $15 or so, tube nearly free, add a basic reliable compressor or source of air.

--Yan

"Geir Holmavatn" skrev i en meddelelse news: snipped-for-privacy@mid.individual.net...

The bubble tube: The tube is below the lovest level you need to measure and pressurized air flow through so that the air bubble up at the end. The pressure level in the tube is the same as that at the end - allowing the sensor to sit in a dry, accessible place.

I've done some experimenting with that.

I built one, based on an idea from Greg Fretwell, using a PTO. It uses a fixed coil and a moveable core, with a colpitts. The varying permeability (as the core rises in the coil, or the coil rises with respect to the core) tunes the colpitts frequency. At 2M it would not be practical as designed - the coil and core would each need to be 2M long.

An unexplored idea was measuring the angle of a float with respect to a fixed point. The float arm is connected to a potientometer shaft, so as the tide rises and falls, the resistance changes.

|o|=======Float | | ~~~~~~~~~~~~ | |

Lateral mechanical stress on the float arm pivot point is a problem.

One other idea: equal weights suspended by a pulley, with one weight supported by a float. The weights would ride inside PVC tubes so that water flow would not displace the weights. The pulley would move as the tide changes, and could drive gearing to move a pot or other kind of sensor.

For what it's worth, the conclusion I reached is what someone already posted: a pressure sensor is the best idea. I haven't found a cheap one that would work - if you do, please post it!

Ed

You got quite a few suggestions already, but here is a simple question: how many bits of accuracy do you need? I was thinking of something like what the automobiles use to detect the presence of water on the windshield to turn on the wipers. The devices uses IR emiters and sensors. Somehow it detects the presence of water by a change in Snells law. You could build a tube two meters long and place sensors every so often to detect the water level. Every wuarter meter would mean 8 sensors.

What about waves?

Here is a link on one such water detector:

If you have a tube, how about some sort of float arrangement? Maybe a floating magnet.

Robert

Why not just get it from your local weather station? In So. Cal, they routinely announce the tides on the weather news.

Good Luck! Rich

A proven method for water levels is a little membrane compressor with a tube leading into the water. A pressure sensor reads the pressure required to produce bubbles.

Rene

I have worked for a few mobs that have designed bubblers, they are a pain. The most common is pressure senor.

But, the easiast way is to use a float sensor. If you ae worried about waves then you place it inside a big plastic tube ,big enough so that algal growth wont affect the level. Like all devices it will still need to be cleaned periodically.

Seawater cruds things up astonishingly fast, and optical sensors are surface-sensitive. That sort of sensor wouldn't last a week in seawater.

Cheers,

Phil Hobbs

Should work. Most tidal stations have to deal with waves, and it's common to use a 50-gallon drum with a pinhole as a 'low pass' filter.