Measuring volume of large container by way of a hemholtz resonator

[Beau Schwabe]

Suppose I wanted to measure the volume inside of a 50 cubic foot container with 1 cubic foot resolution?

From my calculations using a very small "neck" (0.5 inch) an empty container (50 cubic feet of air space) should resonate at about 178Hz while a full container (5 Cubic feet of air space) at about 555Hz.
Initially each volume reduction of 1 cubic foot shifts the resonant frequency by about 1.78Hz. Towards the end as the volume decreases, the resonant frequency shift by 8.43Hz for each volume reduction of 1 cubic foot.

My problem is looking for a method to measure the frequency.

Think something similar to an ice box... I believe there is enough acoustic energy from opening or closing the door that I can measure the resonance of that event to determine the volume inside.

 

[alec_t]

Perhaps use a swept frequency audio source and detect when a mic response peaks?
Simple job for an Arduino, even allowing for change in speed of sound with temperature?

 

[gophert]

Assuming you can detect the audio with a microphone, then wither use an old-school voltage-to-frequency converter (which also can convert frequency to voltage). Unfortunately, the old-school parts I have require a +/-bipolar power supply (+/-12 or +/-15v). Otherwise, simply count rising edges with a microcontroller or use the onboard timer to measure the period of the wave.

At 555Hz you have 1802 microseconds and 1.78Hz less is 1807 microseconds - a difference that should be easily measured with a microcontroller - especially with a crystal time base on the microcontroller.

One problem is how clean your resonant frequency is. You may get some secondary resonances based on the shape of your container.

 

[tomizett]

In order to differentiate between the Holmholtz resonance and any dimentional resonances between the walls, it might be preferable to measure the velocity in the "neck" rather than the pressure in the chamber. Or possibly measure multiple points within the chamber simultainiously?
I can't help feeling that with a very small neck and a large volume you're going to get a very low Q which will make it hard to spot any tones in the decay from a transient... Whichever way, it'll be a nontrivial task to extract useable data from the noise - deffinately a DSP job.

It's an interesting idea though - you should give it a go. Stick a mic in your chamber and clap your hands, take an FFT of the result and see what's there!

 

[unclejed613]

find an old Polaroid autofocus camera and use the "sonar" module from it. i once worked for a company that made automated test equipment, and one of the things we built while i was there was an automated tester for the sonar modules... it had a "start" input which came from the shutter switch, and a control output that went to the lens motor. it started the motor when the shutter switch was pressed, and at the same instant it sent out a ping. when the ping returned, it stopped the motor. the motor turned at a constant speed, so the dwell time was equivalent to the distance the ping traveled, and the lens would be in focus for that distance. there were a lot of those modules on the hobby market years ago after film cameras went by the wayside, but i don't know if there are still any around. in any case, the motor output would be "on" from the time of the ping to the first strong return, and that amount of time would be easily measured with a frequency counter in "time" mode. or the gate time could be measured by a microcontroller and spit out the actual distance.

 

[gophert]

Suppose I wanted to measure the volume inside of a 50 cubic foot container with 1 cubic foot resolution?

From my calculations using a very small "neck" (0.5 inch) an empty container (50 cubic feet of air space) should resonate at about 178Hz while a full container (5 Cubic feet of air space) at about 555Hz.
Initially each volume reduction of 1 cubic foot shifts the resonant frequency by about 1.78Hz. Towards the end as the volume decreases, the resonant frequency shift by 8.43Hz for each volume reduction of 1 cubic foot.

My problem is looking for a method to measure the frequency.

Think something similar to an ice box... I believe there is enough acoustic energy from opening or closing the door that I can measure the resonance of that event to determine the volume inside.

what is in the container (besides air)? Powder, granules, liquid, ...? Dust explosion hazard, flammable liquid, aqueous solution,, aqueous dispersion, ...?

 

[Beau Schwabe]

A "ping" style ultrasonic distance sensor will not work in this case as you would just get an immediate reflection off of the back wall.

We can't use any kind of strain gauge force sensor to measure the weight, although I wish we could. That would make things so much easier.
We also can't put any measuring equipment inside of the cavity. It must be done from the opening.
There is no danger of explosion, but think food grade in terms of measuring/detecting interaction.

The cavity basically has air in the volume void, where inside there are containers of liquid that could be frozen that are added to the cavity.
Our goal is to determine the number of containers that are inside the cavity. or at least a rough estimate.

 

[rjenkinsgb]

A piston plus manometer / sensitive gauge system?
eg. If you pull (or add) a known volume of air from a known size container, the pressure will change slightly depending on the free air volume.
If the free space is reduced, the pressure change will be larger.

Or a vacuum pump or compressor plus a flow meter to measure the volume of air added or removed. You only need a minuscule pressure change if the gauge is sensitive enough, such as an altimeter type barometric sensor used with drones etc.

That should work for long enough to allow a stable reading beyond the duration of any echoes or reverberation, even allowing for slow leakage.

 

[gophert]

We can't use any kind of strain gauge force sensor to measure the weight, although I wish we could. That would make things so much easier.

can I ask why load cells are not acceptable? We used these hockey pucks all the time when I was in the chemical industry.
we'd just shim surrounding equipment to the new height if there were alignment issues (cut out the concrete and drop the load cell slightly below floor surface.

now I would add an esp8266 and send the weight by WiFi. (If it was not safety critical)

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otherwise, you could consider measuring the guzintas and gozattas to keep a running tally of the inventory.

 

[unclejed613]

use the airspace as an acoustic feedback element in an oscillator. use a speaker at the end of a tube, and a microphone at the end of another tube (you want the speaker and microphone isolated from each other but want to couple each to the airspace inside the container), and the air volume inside the container becomes the feedback element. as the air volume changes, the "oscillator" frequency changes. electrically the microphone feeds the amplifier the speaker is connected to.

 

[gophert]

A "ping" style ultrasonic distance sensor will not work in this case as you would just get an immediate reflection off of the back wall.

a 45° baffle works well as a reflector with ultrasonic sensors - assuming you have room for a baffle.

 

[shortbus=]

Our goal is to determine the number of containers that are inside the cavity. or at least a rough estimate.

Going to yet again show my stupidity. How does a company load a container not knowing how many things are in it? Any where I worked they had a "bill of lading" for a shipment of anything. How do you charge the people you're selling the containers contents to? You should know the cubic size of the items being packed, and the cubic size of the container. This seems to be a no brainer if the shipping department is doing their job.

 

[gophert]

Going to yet again show my stupidity. How does a company load a container not knowing how many things are in it? Any where I worked they had a "bill of lading" for a shipment of anything. How do you charge the people you're selling the containers contents to? You should know the cubic size of the items being packed, and the cubic size of the container. This seems to be a no brainer if the shipping department is doing their job.

the only thing I can think of is blocks of dry ice - they disappear whether you use them or not an then people get "angie" if the bin is empty. But, most everything else that might be in that box should be possible to measure the ins and outs to get a net.

 

[Beau Schwabe]

can I ask why load cells are not acceptable?

The use of load cells infringe on a patent for this project. It's stupid, more things are crippled from moving forward because of stuff like this, when you could simply apply fundamentals to get the job done and be done with it.

Going to yet again show my stupidity. How does a company load a container not knowing how many things are in it? ..... This seems to be a no brainer if the shipping department is doing their job.

The measurement is to prevent theft, and has nothing to do with what the shipping department is able to account for during proper loading/unloading

the only thing I can think of is blocks of dry ice - they disappear whether you use them or not ...

Very close ... and will work for any continued analogy or reference.

 

[shortbus=]

The measurement is to prevent theft, and has nothing to do with what the shipping department is able to account for during proper loading/unloading

That is what the seal is for on a shipping container or even a over the road truck. If the seal is removed you know it has been opened. And for me it would only take one time and the carrier would not be used again.

How does the measurement it's self prevent theft?

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[Beau Schwabe]

How does the measurement it's self prevent theft?

This is a kiosk the customer will interact with. It basicilly is to keep honest people honest and "minimize" theft. (nothing is 100%). The kiosk is in constant communication with a dedicated monitoring service.