Laser proximity sensors

"Shop season" resumes here about 1 November. It's time to go fishin'!

Always does!

Yep, 'cept now you have a non linear-reading barometer. The ideal is an "infinitely large" reservoir, so the height of the reservoir mercury rises unmeasurably with changes in the column height. With a U-tube, the height changes are usually equal, and the vacuum side now has to contend with both the ambient pressure AND the weight of the mercury in the other column.

It would still work, and over a short distance it would be reasonably linear, but you'd have to do some table-lookup compensations or math to get the readings out.

LLoyd

It would be linear, merely exhibit half as much deflection for given pressure. The difference in height between the surface under vacuum and the surface at given pressure is proportional to pressure difference between them. In a "straight" barometer the pool height is assumed to be constant so the column height is the height difference. In a U-tube of constant diameter, one side goes down as much as the other does up so the difference is half the deflection of either side. On the atmosphere side of a U-tube, the height of the air column (distance to surface) would be proportional to atmospheric pressure, and this air distance (or air volume in the tube) could be sensed acoustically.

Yeah... when I think about it again, that seems right.

Now... I _really_ like the idea of using the open tube as a resonator, but one must be careful with it. If the length is too long, the sound is in the audible frequencies. If the length is short enough to be inaudible, but the signal is too strong, all the dogs in the neighborhood go into convulsions.

LLoyd

Perhaps it has already been suggested, but a standard way to do this is to fit a brass tube around the glass column and measure the capacitance between the mercury inside the tube and the brass tube. This is very accurate and linear.

If one wants to be able to also see the mercury, cut a long window in the brass tube. If one uses a slightly undersize tube, it will hold the glass column snugly, and a dab of varnish will prevent sliding.

Use a piece of iron wire to make the connection to the mercury pool. (If the wire surface is black, sand it bright.) Copper will dissolve in mercury, and disappear. Likewise brass.

There are lots of ways to measure capacitance, and someone (Analog Devices? National?) recently announced an IC for use with capacitor-based sensors, turning the varying capacitance into a varying DC voltage.

Joe Gwinn

I once made a sampan detector using this principle. Riverine sampans without engines (or not using engines) had no heat signature and nearly no metal so they evaded detection at night by available sensors.

While duck hunting one morning, enroute by boat to the blind on glassy smooth water I noted what we dubbed a "precursor wave" from the bow. It was very low amplitude (a few mm) and of very long period (several meters at least). Never would have seen it except for the oblique angle of the rising sun on the flat surface of the water.

We later consulted some marine architechts and engineers both practicing and in academia, and were told flatly and authoritatively that there ain't no such thing. Oh darn! So we abandoned the idea. NOT!

I made a capacitive water level sensor disguised as a reed, and devised filtering elex so it would not respond to wind-produced waves or ripples but only the very long period and low amplitude of a "precursor wave" so stealthy it doesn't exist according to cognizant authority.

We tried it with a variety of small boats under various surface conditions -- windy, calm, choppy, etc. It was flat impossible to get past it undetected with a canoe or small sailboat at ranges of less than 50 meters under any condition we tried it, and it never false alarmed.

We were feted as heroes and became stinking rich. NOT!

About the time we had a demo version ready, MacNamara's DOD lost interest in sampans. Oh well!

In defense of (some) marine architects, my dad, who was one, told us about this as we were canoeing; the wave was easily visible at pretty much right angles to the bow on a section of vertical rock that formed the boundary of the lake at that point. He called it a "displacement wave". No surface disturbance, just a rise in the water level.