Laser proximity sensors

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

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.

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

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

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

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

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.

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

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

I was thinking of a different type of sensor that only sensed presence, not distance.

LLoyd

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

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

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

BTW... that would be an advantage to using a nichrome resistance wire as the sensor... the same percentage of volume of the column that would be displaced by the wire would remain constant at any height. So it wouldn't be any different than just using a slightly smaller diameter column. And especially with something like #58AWG wire, it wouldn't be appreciable, anyway. You'd want the smallest wire you could reasonably work with, so there was a larger resistance change per millimeter. If you were really, really good at glass blowing, you might even use super-thin wire, and only sense in the "active" region, rather than the whole of the tube. That way, you'd get much larger percentage changes in resistance over the couple of inches the real-world pressure will move the column. You're not trying to measure the pressure inside a tornado.

Y'know... something in the back of my head says I didn't think the wire thing up, impromptu... I think I've _seen_ such an arrangement somewhere. Can't jog the old grey (turning grey-er) cells.

LLoyd

But the total weight of the mercury column and the floating bit is exactly the same as the weight of a mercury column with the same meniscus height, but without a floating bit. Right?

Best wishes,

Chris

Best laser rangefinder I can find that's not part of a laser transit is 1mm accuracy. Of course there are laser micrometers down to 6um, but I think the range and $$ might make that un-appetizing.

I'm thinking run the tube against a close-fitting 1/2-tube metal backshell and do a capacitance measurement via a tuned circuit.

How about using the nichrome wire to vary the voltage to a motor that spins a disk with a strobe and at diff RPMs diff numbers are visible?

Or turn the top (evacuated) part into a mercury-vapor discharge tube and measure the current? How can one not want to use a 10KV source to build a barometer?

Ok, now I'm getting silly, but this has the potential for a great steampunk project.

Dave

The Hg will short out the immersed part, so the only resistance that registers is the length of wire that is not immersed.

"Christopher Tidy" wrote in news: snipped-for-privacy@n59g2000hsh.googlegroups.com:

Chris, Check bannerengineering.com, Keyence.com, turck.com and balluff.com

Cost will be above $100 probably in the $140-200 range for that accuracy.

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

How about if you suspend the barometer tube from a strain gauge and effectively "weigh" the mercury column?

Jerry