meassure height (0-2 meter) / barometric sensor?

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dear all.
Im looking for a altitude/height/barometric sensor that I can use for height measurments in the range 0-2 meters (from the floor (0), and to the height of a person (~2 meter)).
I like to connect this sensor to an AVR for an easy to use height sensor. laser, IR and ultrasonic will not do in this particular case, cause I will not like to be dependent on reflecting surfuses and similar. I would find it neat to be able to close it all up in a box with just a display outside.
I havce looked around on barometric sensors, that seems promissing, however, I can not figure out if
are they sensitive enought (a resolution of about 5cm would be ok, but not more)?
Relative height is ok, intending that I can always 'set' the 0-height at the start, then lifting my sensor up to the height I like to measure (a person, a shelf, a table).
Will a barometric sonsor do this, or shall I look into other sensors?
I like a low-cost sensor for this project, since its a 'for fun' project. IDeas of sensors, I have searched the web without good results....
Many thanks.
-Erik

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Quote:

I can not figure out if
are they sensitive enough

Surely not? When I was flying we were told that barometric pressure varies by something like 37 millibars per 1,000 feet. So to measure 2 meters (call it 6 feet) it would need to sense a difference of only 0.22 miilibars and to actually measure a division of that range it'd need far more accuracy. Unless things have changed these days I doubt you'll find anything with that kind of accuracy.

Now I did have a Casio watch at some stage which included a barometric sensor so could measure altitude. I could get a difference in readings by taking the lift up a ten storey building. Maybe this was Casio discarding some accuracy but I doubt it.

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Sparkfun have a SCP1000 which claims to be good to 9cm but in practice I would say you would be good to get 50cm reliably. I use it on my bike computer and am happy with 1m resolution for which it dose a good job.

5 cm resolution I think is just asking too much if you had a sensor good for that you would still get stange effects with doors opening and closing Aircon going on and off etc. On the bike I can sometimes see the effect of a big truck overtaking me with 1m resolution.

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I would use an ultrasonic range finder. MaxBotix makes some nice ones. They are sold through hobby channels.

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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Intersema make barometer chips which work to about 30cm, but the noise figure is five times that; you need to integrate a lot of measurements. I've got loads of code to talk to one if you need it, but I wouldn't use it to measure a person.

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Here is one that claims to have a resolution of 20cm:

http://www.intersema.ch/products/guide/calibrated/ms5607b/

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Quote:
which work to about 30cm,

Quote:

have a resolution of 20cm

That's just a little under the difference in height between Michael Jordan (1.88m) and Kylie Minogue (1.52m)! Not much use for measuring human beings (if that's the intention here)

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I have made an altmeter based on Intersema + AVR. It has a resolution of 0.2 m (~2 Pa = 0.02 mbar). You can measure heights with it with that accuracy, IF the pressure is that constant and temperature is also constat. This is a big if, since air conditioning, wind etc easily change the pressure by more than 10 Pa (~1 m). Thus it is not a question of sensor accuracy, but the actual physics.

Also the pressure change with altitude depends on air density, which depends on pressure and temperature. These are easy to take into account in software.

Measuring altitude differences needs calibration for days weather AND you will get different results inside the building and outside, since the density of air is different due to different temperature.

I think I designed a rather clever calibration system that I haven't seen in any of the watches with altmeter. I input current altitude and outside temperature at current altitude for calibration. I have checked the accuracy to be as good as I can get from known altitudes. E.g. it gave exactly the same altitude difference as given for a 280 m alpine slope.

You can use pressure to measure height of a person accurately, but you need to measure the pressure difference, not the absolute pressure. One way to do that would be using a tube full of water and having a pressure sensor measuring the pressure difference between air and the floor end of the tube. The other end of the tube is open and the water level is lifted to the height to be measured. The pressure will be (rho_water-rho_air)*g*h, thus about 20 kPa for 2 m.

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So I guess the Paroscientific DigiQuartz would be out of the question? :lol:

Cheers,

Ross

Ross McKenzie ValuSoft Melbourne Australia

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The intersema chips have some very subtle corrections built in to auto calibrate; they also return temperature. Basically, they return absolute pressure and provide the sums and chip-specific calibration data for your processor to turn that into altitude.

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For those who have asked, I've stuck the Sema documents and the working C code in the tutorial section: https://www.avrfreaks.net/index.p...

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I guess only the military can resolve height that accurately with GPS ? Just a thought.

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You can get the position difference of two GPS with ~1 mm accuracy using RTK receivers, but likely not inside with limited signal. Even military can not measure better than ~1 m with just one receiver.

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I can go down to the hardware store and purchase a mass produced "electronic" tape measure which uses ultrasound and is much more accurate than 5 cm. It looks very much like the OP's request, a box with a display and a measure button. Hold it at the heigth of the person's head, aimed at the floor, push the button, read the display.

Inexpensive, reasonably precise, ...

JC

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Even if you find a sensor that can resolve the pressure change, there is still the problem that the lokal preasure is changing due to wind and wether changes.
With two sensors to compensate you have to send the signal from one unit to the other. Than something like ultrasonic delay is simpler than that.

If the measurement time is short enough, it could also work using inertial navigation. Just integrate acceleration.

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This is getting into the realms of:

- drop the device from the height of the person being measured and see how long it takes to hit the ground
- tie a bit of string to it, the same length as the person being measured is high, and time the duration of the pendulum thus created
- "if you tell me how tall you are, I'll give you this fine barometer..."

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Kleinstein wrote:
If the measurement time is short enough, it could also work using inertial navigation. Just integrate acceleration.

Not for for grade of sensors you can get from sparkfun. No chance!

-- Damien

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At my work, I've got a friend who just used a TI 24-bit ADC and a pressure sensor to get resolution that's sufficient to measure his height (1m76), or even a standard desk height (~75cm). I've seen it work and it's quite impressive. If you're still interested, I can find out what he's using. One caveat: it wasn't cheap.

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hi kubark42, sorry for late reply... but I have been on holiday. That sounds indeed interesting.Could you please provide me with more info? many thanks.

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It's the MPX 6115. However, be careful, because that's kind of an umbrella term at TI. You'll want to check because some of the sensors in that line are far more sensitive than others.

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thanks!