Laser proximity sensors

Hi all,
Does anyone here know much about laser proximity sensors? I'm thinking of the sort of thing which is used in the Leica total stations for
surveying, but I don't know a great deal more about them.
I'm looking for something with a range of 1000 mm, a resolution of 0.1 mm, preferably with a digital output and the ability to measure off a reflective metal surface.
The application is a mercury barometer. I've always wanted to build one, and after the thread about boys playing with mercury in West Virginia, I asked a few people about acquiring some. The result is that I've been offered a 76 lb steel flask of mercury for 30, which sounds like a good deal.
No doubt some people will think I should avoid the mercury. It's interesting the way that mercury ignites such strong opinions. There seem to be two camps of people. Those who think mercury is cool, and those who think mercury is to be avoided at all costs. I'm in the former camp, although I'm aware that I need to be careful with it.
I'd like to build a real showpiece of a barometer, and the digital measuring was just a thought I had. Does anyone know if suitable proximity sensors are available and what they might cost?
Or I guess I could steal a DRO from a mill...
Best wishes,
Chris
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wrote:

Or, you could go old-school and use its conductive properties. Cover the back part of the inside of the tube with a resistive compound, and set up an analog volt guage with the face re-calibrated in mm-Hg, etc.
Admittedly not as cool as laser measurement, but retro is cool, too.
Dave
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On Apr 13, 7:13 pm, snipped-for-privacy@yahoo.com wrote:

This did occur to me, and it is a cool idea, but I'm doubtful that I'd get the sub-millimetre precision I'm looking for without a struggle. What do people think?
Best wishes,
Chris
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wrote:

This did occur to me, and it is a cool idea, but I'm doubtful that I'd get the sub-millimetre precision I'm looking for without a struggle. What do people think?
Best wishes,
Chris
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This did occur to me, and it is a cool idea, but I'm doubtful that I'd get the sub-millimetre precision I'm looking for without a struggle. What do people think?
I _think_ I remember that nichrome wire is not wetted by mercury. Were that the case, and if the mercury is scrupulously clean (perhaps re-distilled before use), then you could string a thin gauge NiCr strand the whole length of the tube. You could even make a "dun-metal" seal at the top, where the wire exited.
Then you'd be able to measure the resistance of the wire. Even if it wasn't perfectly linear, it would be repeatable, because there'd be no wear on the wire, and the same point would get touched each time the mercury rose to that level. Knowing those things, a little "smarts" in the reader assembly could keep corrections in table form to report the correct height from a given resistance.
I do believe you could get fractions of millimeters of resolution from such a system, which is essentially "stepless".
LLoyd
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wrote:

Lloyd, what do you mean by "wetted" in this context?
Best wishes,
Chris
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On 13 Apr 2007 11:24:54 -0700, "Christopher Tidy"

Noting your inclination to use a laser device and other good ideas re resistance of an immersed wire ... I had another idea today. This may seem a bit whacky but it might be fun and could be very accurate. You can sense the surface (meniscus) of the Hg nonintrusively and very accurately with a photoemitter, a small aperture or slit as a field stop, (maybe 0.2 mm, perhaps less) and a photodiode. It could be either reflective or transmissive. Such a sensor would only sense presence or absence of the Hg column -- but if it were mounted on a motor-driven leadscrew then it could "follow" the Hg as a servo. The elex would drive the leadscrew to raise the sensor until its output goes from zero (trasmissive) or max (reflective) to about 1/2 of max when it's lookin' right at the boundary between Hg and vacuum in the tube. It'd stop there, and thenceforth follow it as barometric pressure changes. I'd use two nuts with a spring between them to eliminate backlash. This servoed Hg-level follower could then either directly drive an inexpensive digital caliper (or dial indicator of sufficient range), or the same (dirt simple) servo drive could operate another (identical) motor and leadscrew that is located somewhere else to drive a readout device. If remoted, little stepper motors might be best because they'd stay in sync if not overloaded. Small stepper motors are available very inexpensively ($5 or less) as surplus and are found free in discarded stuff like old scanners and printers. A (common) 1.8 deg/step motor driving a 1mm pitch leadscrew (perhaps M6 x 1) would move sensor and readout 0.005mm per step so you certainly wouldn't need any belts or gear reduction. You'll find very nice and quite precise steppermotor-driven linear drive mechanisms (might be toothed belt rather than leadscrew) free in discarded printers and scanners.
You wouldn't even need the digital caliper or dial indicator, could use an up-down counter driven by motor pulses. But digital calipers are very inexpensive, have digital readouts, and read directly in mm over a completely sufficient range of 150 mm.
You could have 1000 mm range but you don't need it. Range from normal barometer to full vacuum (gasp!) is only 760 mm. Barometric variation at or near sea level from high-pressure to imminent tornado or hurricane is only about 100 mm Hg (130 millibars). A minimum tare of 700 mm or so is constant and need not be sensed. If the barometer ever goes below 700 mm Hg it's time to either evacuate or hang on and start praying.
If the mount and housing for the barometer is artfully contrived and crafted, the servoed reader could be concealed behind the barometer tube behind a strip of plastic that passes near infrared but looks black. The photoemitter would then be an IR LED as found in TV remote controls and otherwise readily available for a buck or so. Any silicon phototransistor works fine with these. You can even buy a "slot sensor" containing an IR photo emitter with .010" (.25mm) aperture stop and a phototransistor for about 10 bux. Saw it in half and you have the separate parts. The barometer could then be housed in a nicely-finished bit of cabinetry with a meter stick beside it, with the electronic digital readout being a litte secret.
Potential for embellishment: many cheapo digital calipers have digital outputs (details on websites) and the steppermotor pulses and direction are also digital output, so one could make this a recording barometer sending data to a PC for logging. A recording barometer is far more useful and informative than one that just registers the reading of the moment.
I like this scheme because it does not violate the integrity of the basic and fundamental Hg barometer. It requires no wires with hermetic seals, uses a dirt-simple sensor to directly and quite accurately observe the height of the Hg column and it reads directly rather than inferentially as by resistance, capacitance, laser triangulation or other surrogate of Hg level. It's also quite inexpensive and might be fun.
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That's a very interesting idea, Don. It has the advantage of using fairly cheap components, too. The downside is that it would be harder to make it look good, and the electronics required to get an output in millimetres of mercury might be more complicated (although I haven't thought this through fully yet). The nice thing about those laser measurement devices is that you could have two perfectly unobstructed columns, which would be a good starting point for an artistic barometer, but they are pretty pricey.
I'm pretty sure I'm going to go back and buy the flask of mercury. But actually building the barometer may need to wait until I have a mill. This seems like a project which demands a mill.
Thanks for the thoughts.
Best wishes,
Chris
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On 16 Apr 2007 16:30:51 -0700, "Christopher Tidy"

I think the "works" could be completely concealed behind a 1/4" wide inset vertical strip of IR transmissive plastic that looks black. A servoed digital caliper would read directly in mm -- the elecronics are already in the caliper. Perhaps you set it up so it is "zeroed" at 700 mm and the 7 is understood or permanently marked. It would then read mm above 700 mm of Hg.
The remaining elex could be very simple: a couple of comparators and a couple of pulse generators (555 timers). If sensor is below the Hg surface send "up" pulses, if sensor is above the Hg surface send "down" pulses, if it is between above and below send no pulses.
It could also be a single 14-pin $3.00 microcontroller (PIC) if one had the means to program it. I do, and I'd be happy to assist with a project like this.
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That's a very kind offer, Don. I'll bear you in mind, but it could be a while before I get started on this one. I've found that since I've started reading r.c.m, I've had more ideas for projects than I have time to complete them!
Did Mary have a good trip to England? Hope so...
All the best,
Chris
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On 18 Apr 2007 04:49:50 -0700, Christopher Tidy

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

Always does!
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Christopher Tidy wrote:

Save money, place a linear CCD array on one side of the tube and a diffuse lighted target on the other side. Can probably salvage the parts from a flatbed scanner. Should have plenty of resolution if applied properly.
Pete C.
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<snip>

An interesting idea. Do you think it would be possible to do this in such a way that it didn't look like I stole the parts from a scanner? My idea was to build a real showpiece.
Best wishes,
Chris
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Christopher Tidy wrote:

Machine and anodize a complete optical block out of aluminum to house the components around the glass tube. Everything looks good in anodized aluminum.
Pete C.
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Hi all, I'd like to build a real showpiece of a barometer, and the digital measuring was just a thought I had. Does anyone know if suitable proximity sensors are available and what they might cost?
I think you'll have trouble measuring the height of the column with such a sensor. They are designed to pick up a "fixed point" target (sure, the target can be moving, but the beam is usually fixed).
One way to do it is to float a mirror on top of the column of mercury, and to reflect a beam from the mirror at an angle, such that any vertical movement causes a deflection of the beam... which can then be measured. That would require either a flat-sided tube, or to have the beam originate inside the vacuum portion of the tube. Neither is very "clean".
Another way would be to float a magnet, and sense the magnet's position with Hall-effect sensors.
I think I would float a magnet, then allow the magnet to move a scale or pickup from a DRO (or digital caliper), external to the tube.
The system would need to be set up for zero friction, and unless very light, would also need to be counter-balanced so that its weight didn't throw the column height off too far. Any weight added to the top of the column will require compensation.
LLoyd
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wrote:

Can you explain this problem in a little more detail? Perhaps it's me, but I've lost you there. The beam in the barometer would be fixed, and I'm talking about the total stations which use a "reflectorless" EDM.

I don't think that's true, although I'm still thinking it through fully. Surely anything floating in the mercury, provided that it doesn't block the tube and act like a piston, merely displaces its own mass in mercury, and the rest of the column sees no change, so the level will remain the same?
I have wondered about having a ruler next to the mercury column supported by an aluminium float in the mercury reservoir. That way it automatically adjusts the zero level. But ideally I'd like to get better precision than a simple ruler offers.
Thanks for the ideas, Lloyd. You've made some interesting suggestions there.
Best wishes,
Chris
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I was thinking of a different type of sensor that only sensed presence, not distance.
LLoyd
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Consider that the magnet floating in the mercury - for purposes of "support" - becomes part of the column. As such, anything less dense than mercury will lessen the average density of the column. Although it's predictable, it's not linear, since a short column would have its average density reduced more than a tall column, etc...
LLoyd
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wrote:

No, because only part of the magnet is submerged. Just enough of the magnet is submerged that the mass of the magnet divided by the submerged volume is equal to the density of mercury.
Anyone agree with me?
Chris
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But the total weight of the column - including the floating bit - is what you're supporting with outside air pressure. If the weight of the column is less than the same volume of just mercury, then the height (total, including floater) capable of being supported by the air will be greater than the height without the floating bit.
The actual meniscus of the mercury column would be just a snip lower than it would otherwise, and the top of the magnet would be higher than would a bare column.... if I've done the thought experiment due attention.
LLoyd

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