Laser proximity sensors

I might be missing something, but I can't see how this could work. Either you'd have to weigh the whole barometer, in which case the weight wouldn't change, or you'd have to have a sealed, sliding, friction-free joint where the glass tube was attached, in which case you'd only be weighing the tube.

Best wishes,

Chris

Lloyd, what do you mean by "wetted" in this context?

Best wishes,

Chris

A laser can do that without issue. You need to get with a maker and get tech support. The real issue - software - theirs with any luck!

Or if you have a team - then some kernel libraries....

Martin

Martin H. Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net=

TSRA, Life; NRA LOH & Endowment Member, Golden Eagle, Patriot"s Medal. NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member.

Christ> Hi all,

Think the glass tube with mercury is suspended in an open bath of mercury. Karl

Indeed. But all the forces acting on the tube as a result of the pressure are horizontal and cancel each other out, so the force required to suspend the tube won't change with the mercury level. The weight of the mercury is supported by the vertical force caused by the atmospheric pressure at the bottom of the tube.

Best wishes,

Chris

why don't you do this with capacitance - deposit a strip of conductive material along the back half of the barometer tube, and a second strip that does inside to where the mercury is - as the mercury moves up and down the capacitance will change, change the frequency of a simple oscilator that you build using a PLL circuit (such s 567), put a voltmeter on the error signal and you have your readout.

Want more precision, deposit bands of couductive material along outside, single band along inside and again use capacitive coupling,but this time it's digital like you wanted - wire the strips to be a binary counter and just see which one is the "lowest" one that is coupled to by the mercury. Or you can put a linear photosensor array across the topof the tube, shine laser at slight angle so rising mercury causes it to scan across the array - less precision than previous one.

Or you can use an acoustic oscillator with the air column above the mercury setting resonance, and use freq counter.

or with a prism, create an interferrometer and measure to about 500 angstroms (with a green light source)

Or if you have a team - then some kernel libraries....

Martin

Martin H. Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net TSRA, Life; NRA LOH & Endowment Member, Golden Eagle, Patriot"s Medal. NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member.

Christ> Hi all,

"Christopher Tidy" wrote in news: snipped-for-privacy@q75g2000hsh.googlegroups.com:

I use quite a few of these things in the automation I design and build.

Take note that the prices I quoted are what my company pays, don't hold me to those numbers as we are a very large multi-national corporation.

Except that you don't want the inside strip - the mercury will the inside "plate". With an inside plated strip, the capacitance wouldn't change at all. Bob

On 13 Apr 2007 11:24:54 -0700, "Christopher Tidy" wrote:

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.

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

Now there's an interesting idea. And with mercury being so dense, the sound wouldn't cause the mercury to oscillate much at all. Do you think it would be possible to get decent accuracy with this system?

Best wishes,

Chris

Yes, but I'm sure he meant having the mercury in a U-tube and using the side which was at atmospheric pressure as the resonant chamber.

Best wishes,

Chris

Only vacuum above mercury

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.

It is possible, but it isn't trivially simple. I have designed and worked with industrial acoustic distance-measuring devices.

It would be possible to attain very good repeatability with a lower-Q cavity by using sophisticated spectral signal processing, but that would require a PC or DSP and some rather fancy software.

Either way, such a system would be both non-linear and highly inferential so it would need numbercrunching and calibration to convert a frequency to a barometer reading. It's nonlinear because your sensor is responding to 1/x (and some second-order terms) while you are interested in x. It would not be a primary sensor because its accuracy would depend entirely upon the accuracy of the instrument used to calibrate it.

Acoustic resonance is a standard undergarduate method for determining the density of gases, I seem to dimly recall....

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

bzzzzt barometer = torricelian vacuum above the mercury not a column of air

how about a float sitting in the reservoir with an arm to a variable pot. forget about the column it is in side a glass tube.

Stealth Pilot