Tide Monitor

Tide Gauge Back Up

2011-03-13T15:41:00.002-04:00

Some maintenance required, new battery installed. It should be up and running again.

Do you need to measure Tides, or other water levels?

2010-05-19T11:08:00.000-04:00

Shortly after starting this blog I was contacted by the folks at ioBridge.com. If you've been reading this blog for a while (or care to go back through history), you'll see my design was based on the ioBridge sensor gateway.

Using their easy to use internet sensor interface, they built a tide gauge which they are considering turning into a product. They asked me if I'd beta test it. Here's a photo:

ioBridge.com Beta Test

We've been running the their tide gauge next to mine for a few weeks. Their packaging is slick, includes temperature compensation (which is important as the speed of sound in air changes with temp), and is very much weather proof. It uses a Zigbee radio to connect back to the ioBridge sensor gateway.

The temperature sensor is in the white PVC pipe (for sun shield), and the acoustic transducer is pointed down at the end of the arm reaching out from the piling.

They have also produced a beta quality web site to display the output:

www.iobridge.com/apps/GreenPondTide

The web site is still under construction, but it does display the tide level accurately, along with a couple of weeks of history.

I've been helping them with this. If you think you might be interested in getting one of these, please let us know emailing sales@iobridge.com.

Although we've focused on tides, other water level monitoring applications might be interesting, such as irrigation, river flood levels, reservoir levels, etc.

Still Running, total history to date

2010-05-08T09:02:00.000-04:00

More Log Data

2010-04-24T07:18:00.000-04:00

Here's a plot to date.

Full moon was late March, new moon was 4/14, we are approaching full moon again at the end of the month.

Maximum negative declination was around 4/4, the moon was on the equator at about 4/12, and maximum positive declination was around 4/19.

Here's a potential simplified explanation of what's going on:

There was a big storm with NE winds late in March, along with the full moon. Large tidal ranges resulted.

In early April, the large negative (below equator) lunar declination resulted in a larger diurnal component. Smaller tidal range caused by lunar phase.

Around 4/12 the tide looks more semi-diurnal, presumably as the moon is at the equator. I am surprised not to see more tidal range at the new moon around 4/14 (I cannot remember what the wind was doing).

Looks like the diurnal component increases again as we approach 4/19, and the lunar declination is at its maximum above the equator.

I am curious what is happening to the running average tide level. It seems to bounce around a bit. Some of this might be wind (for example the strong NE winds in late March), and maybe some is astronomical.

Over 3 weeks of data

2010-04-19T18:03:00.000-04:00

If you were expecting a regular sine wave, not here.

Tide Log to Date

2010-04-12T07:44:00.000-04:00

Here's the tide log to date:

The Tide Log To date

2010-04-08T08:59:00.000-04:00

The Tide Monitor has been up and running for nearly 2 weeks now. Here's a plot of the data to date:

The full moon was around 4/1. A coastal storm with strong easterlies occurred at the end of March. Note that in April, we seem to be measuring a mixed tide (with components of diurnal and semidiurnal).

A. C. Redfield

2010-04-06T10:10:00.005-04:00

I'm in the process of reading The Tides of the Waters of New England and New York. Woods Hole

Oceanographic Institution. Taunton, Massachusetts: William S.

Sullwold Publishing, Inc., by A. C. Redfield.

Yes, the double tops (plateaus) on the high tides are real, and described in this book with reference to Falmouth Harbor which is near by.

Redfield's book can be found on line here:

https://darchive.mblwhoilibrary.org/handle/1912/1136

A great read.

Here's an engineer's (not oceanographer) summary. The tidal range at Falmouth in Vineyard sound is very small (around 1.5 feet). As the tidal rise (wave) moves toward the coast, it gets into the sound through 2 major openings, one around Monomoy, the other near Cuttyhunk. These waves destructively interfere (a node) around Falmouth. Thus the small tidal range in Falmouth becomes progressively larger as you proceed east or west.

The tide consists of multiple harmonics. Some are semidiurnal, and some are diurnal. Semi are roughly 12 hr, diurnal are roughly 24 hr. The harmonics are causing the double tops.

The primary harmonics are:

M2: the moon which is semidiurnal - 12.42 hr period
S2: the sun which is semidiurnal - 12.00 hr period
K1: a lunar diurnal component - 23.94 hr period
O1: a lunar diurnal component - 25.83 hr period
P1: a solar diurnal component - 24.06 hr period
K1, O1, and P1 are related to declination.

M2 and S2 are the big components, but Vineyard Sound is tuned to reduce these components. These tide components propagate from Cuttyhunk and Monomoy and nearly cancel near Falmouth. The K1, O1, and P1 harmonics are smaller but are different period, so the Sound is not tuned to cancel them at Falmouth. Their small amplitude appears relatively larger as M2 and S2 are reduced, resulting in the strangely shaped waves.

Any readers of this blog that are tide experts, please comment so I get this right!!

Remote LCD Monitor Packaging

2010-04-02T09:36:00.000-04:00

Not beautiful, but better than exposed circuit boards, LCD display, and wires.

Filtering Artifacts and the Impact of Waves

2010-04-02T08:26:00.002-04:00

Initially, when debugging the tide sensor, there were artifacts:

Each plotted tide level is an average of 10 samples taken at 1 second intervals in an attempt to average out wave action. Something was causing the downward slices in the waveform.

The tide sensor is in a bay where strong winds can deliver up to a 1 foot chop. When these strong winds occur, the data would exhibit the artifacts. On an async terminal, I can look at the data stream and see the raw individual samples before averaging. The individual samples where usually sensible; however, with high winds and wave action some samples would be 2X the others, or just very large numbers.

I believe the 2X numbers can be explained as follows:

When the ultrasound pulse hits a wave front, it's reflection can be directed by a wave under the dock. The resultant return pulse hits the bottom of the dock, back down to the water, and back to the transducer. The result is 2X the expected distance.

Very large numbers can be explained by a total miss. The wave front deflects the pulse off into space (away from the dock), and the sensor never sees the return.

A filtering algorithm that has been running for the last week removes these artifacts out by tossing out samples that don't make sense. The sensibility test involves looking at past tide heights, and assuming a very large change in a 15 minute interval is not possible (unless the moon explodes or something like that!).

Here's the filtered data which I've been taking for the last week or so:

I am curious if the plateaus at high tide are real, or some other artifact of the sensor. I'll be looking at that next.

The Storm

2010-03-31T07:54:00.000-04:00

Here's another tide plot from the log at ioBridge.com.

Yesterday, we had a very strong easterly wind associated with the large storm headed up the east coast. Can we attribute the higher low, and higher high tides to the east wind? See the last 2 hills and the valley between. Note this AM's valley is much lower. Barely a ripple on the water this AM.

A Few Days of Log Data

2010-03-30T07:42:00.000-04:00

Here's a few days off the log. We are having a good storm now with SE winds and lots of rain with flooding inland. So far so good.

Data Log

2010-03-28T09:09:00.005-04:00

IObridge.com provides an on line logging service. Every 15 minutes, we are posting tide height in inches to the log. Here is a graph of recent log output:

The tide level is measured in inches vs. a bottom reference at the measurement location. Time is on the X-axis.

I'll be watching the tide in this coastal estuary on Cape Cod vs. reference tide information in Boston.

Some additional signal processing is required to average out wave action. I'll review more of this in later posts.

Updated Block Diagram

2010-03-27T08:55:00.002-04:00

The system now consists of 3 elements:

1. A tide sensor on the dock.
2. An internet interface.
3. A remote tide level monitor with an LCD screen.

The interconnect is based on Freewave 900 Mhz spread spectrum serial radios. They are running in multipoint mode.

TIDE SENSOR:

The tide sensor is powered by a 12 volt battery charged by a solar panel. The sensor is based on a Parallax Ping (parallax.com) ultrasonic transducer, connected to a Arduino microcontroller (www.arduino.cc) with a custom program (sketch) developed by the author. The program measures the distance from the Ping acoustic transducer to the water. In oder to calculate depth, vs. a reference, the user enters an offset (distance from the transducer to the bottom) in inches through the serial interface on the Arduino through a simple user interface.

The Arduino can be set to take continuous samples or samples every 15 minutes. Continuous sampling mode is used during initial installation to make sure everything is working. Each sample consists of 10 acoustic pings taken at 1 second intervals and averaged. The averaging is an attempt to remove wave action from the tide calculation. Samples are transmitted on the serial interface to the Freewave Radio (freewave.com).

INTERNET INTERFACE:

The internet interface is based on an IObridge IO-204 with a Serial Smart Board SB-5 (see IObridge.com). Another Freewave radio receives the serial data stream from the TIDE SENSOR forwards it to the internet. IObridge has excellent internet logging services, and the ability to tweet and/or email this information. I'm tweeting tide levels now on twitter account Capecodda.

TIDE LEVEL MONITOR:

The tide level monitor is an Arduino micro-controller with an LCD interface connected to a 3rd Freewave Radio. The serial data stream is parsed by the Arduino and a tide level is displayed. A short custom program (sketch) that parses the serial data stream and displays tide height was developed by the author.
Next step is packaging.

Up and running

2010-03-25T14:51:00.000-04:00

The remote sensor is housed in an electrical box. The solar panel is attached to keep the battery charged.

Here's what's inside the box:

The Freewave Radio, 12 volt battery, Arduino with a proto shield, battery charger, and ping transducer.

Hope it is water proof enough, we'll find out shortly since its going to rain tonight.

Original Block Diagram

2010-02-07T08:17:00.001-05:00

The wireless interface will be based on Freewave 900Mhz spread spectrum radios with serial interfaces (Freewave.com). The internet interface will be based on IOBridge (IObridge.com).

Tide Monitor - first concept

2010-01-17T14:01:00.001-05:00

OK, the new project will be a tide monitor. It will look directly at the tide level in a coastal pond in Falmouth, MA.

The system design will consist of the following:

1. Arduino controller with a Parallax Ping ultrasonic range finder to measure water height from a fixed dock.
2. A wireless async connection using TBD technology.
3. An IObridge to connect to the Internet
4. This web site to monitor the actual tide height.