Hello oceanmark,
Your new question is certainly an intriguing one and it congers up
images of you bobbing around out there on the water with all kinds of
ropes, antennas, cameras, fins, weather vanes, cylinders, baffles,
snorkels...
INCLINOMETER:
I'm happy to report that I found a web page that describes the
construction of an inclinometer which a person can use to measure
currents.
http://www.sccwrp.org/pubs/annrpt/92-93/ar-04.htm
I'm also happy to report that my son is home from university and that
he offered to give me a hand with your question.
Home-made inclinometers and other pendulum-like instruments actually
have a lot of physics behind them. The materials used in the
construction, and the shape of the final product will determine at
what angle the inclinometer will displace to in a current. The
inclinometer must first be calibrated in water moving at a known
velocity (or by moving the inclinometer through the water at a known
velocity). Perhaps you could fix the inclinometer to the front of your
camera in such a way that it would be visible through the lens of the
camera. The inclinometer would be attached to a sort of swivel head,
so that the direction of the current could also be measured (if you
know what direction the camera is pointing).
PENDULUM:
In addition to this kind of an inclinometer, we have played with the
idea of using a pendulum. In other words, attach a pendulum (with
known attributes) to the camera so that it is able to sway out into
the current. The angle at which the pendulum bob displaces depends
upon many factors.
1) The density of the sea water approx 1000 meters down (~ 1.028 g/cm
cubed).
2) The density of the pendulum bob (to be determined by dividing its
weight by its volume).
3) The radius of the bob (it is easier if it is spherical).
4) The viscosity of the water (approx. 0.015 poise for 5 degree salt
water).
5) The velocity of the current (to be determined).
Since 1-4 can be measured easily, we may be able to approximate the
velocity of the current.
The force that would push the bob away from a horizontal position
would be equal to the drag that that occurs when the bob and the water
interact. This is dependent upon the viscosity of the water, and the
area and shape of the bob. To simplify things, (I think) the equation
to calculate the drag coefficient would be F(drag)= - 0.2827rV where r
is the radius of the sphere, and V is the velocity of the water.
The force of the drag (Fdrag) would be equal and opposite to the
horizontal force incurred by gravity acting upon the bob. Since
gravity would have less affect on the bob in water due to the density
of water, so the weight of the bob in salt water is equal to it's mass
minus the buoyancy of the water on the bob. If I read correctly, the
buoyancy force is equal to the volume of the bob multiplied by the
density of the water and the force of gravity. So F(buoyancy)= V(bob)
* 1.028 * 9.8.
This means that the weight of the bob in water would be
M(bob)*9.8 - V(bob)*1.028*9.8.
Next, to calculate the velocity of the water using the angle at which
the bob resides from verticle, you can use an approximation of
horizontal force. Weight and horizontal force are in approximate
equilirium (only when horizontal displacement is less than 10% of of
the length of the pendulum) when F(drag)/weight(bob) = sin (angle to
horizontal).
Adding all of the equations together, you get
V = [sin(theta) * M*9.8 - v *1.028*9.8]/0.2827433r
where V = velocity of the current
M = mass of bob
v = volume of the bob
r = radius of the bob
theta = angle pendulum forms with the vertical
These calculations should be taken with a grain of salt, and are meant
more to stimulate your own thinking on the subject than anything else.
We would really recommend that you check with an expert (we're not
oceanographers). These are the types of things you may face if you
were to try and measure current with a pendulum. Apparently, in 1901,
pendulum current meters were in use. See this link for an example.
http://www.photolib.noaa.gov/ships/ship4034.htm
ADDITIONAL IDEAS, LINKS, THOUGHTS:
Description of the viscosity of ocean water:
http://www2.ocean.washington.edu/oc540/lec02-24/
Description of the density of ocean water
http://www.windows.ucar.edu/tour/link=/earth/Water/density.html
Description of equations necessary to calculate the drag coefficient
of an object. Either one could assume that the viscosity of salt water
was close to what is published and that the shape of the object was a
perfect sphere, or one could calculate the drag coefficient of an
object by oserving the amount of time that the weighted object took to
fall through water of known density and of known depth.
http://www.rwc.uc.edu/koehler/biophys/2d.html
Electronic:
A simple screw type current meter that you could attach to a line and
sinker. It does not say how deep it is able to go, and its minimum
velocity rating is 4 cm/s (just above your minimum). It also requires
an electronic counter.
http://www.hydrologic-h2i.com/cu010.htm
...and another. This site mentions that is can be used in tidal water,
but it does not mention how deep it can be used.
http://www.valeport.co.uk/openchannelflowmeters.htm#model001002
Manufacturers of non-acoustic type current meters.
The 105 and 106 models have a depth rating of 500m:
http://www.valeport.co.uk/indexlayers.htm
http://www.valeport.co.uk/pdf/model105.pdf
A seller of impeller type current meters:
http://www.aquadyne.no/topframe/oseanografisk.htm
A current meter rated at 1500 meters, the deepest impeller type meter
we have seen. It looks very expensive, though:
http://www.seatronics-group.com/oce/308.htm
Historical photos:
Various photos of old mechanical current meters.
Included is a model based upon an anemometer:
http://www.photolib.noaa.gov/ships/shind24.htm
We trust that there is enough here to keep you busy for awhile. You've
taken us on an interesting journey and we'd like to wish you a happy,
fruitful and safe experiment.
Sincerely,
hummers
Search Terms:
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"horizontal force"
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etc |
