Measuring your body volume without getting wet

[sophiecentaur]

TL;DR

Air displacement plethysmography uses the displacement of air, rather than water. I just thought I'd share it because it's a cool method.

Air displacement plethysmography
I saw a tv programme many years ago which shows this method of measuring the volume of a patient's body. Air displacement plethysmography was a procedure that I saw being used and, tbh, I never asked myself how it works, until now. It's smart, imo. You put someone in a sealed box full of air and then you determine the volume of air with the body in it. You just take a measured volume of air out of the container and measure the drop in pressure. Boyle's Law allows you to calculate the original volume of air (= containter volume - body volume).
There will be a number of practicalities, including keeping the air temperature constant (or accounting for any change when a warm patient is put in. Claustrophobia could be problem for some people - even with a large window.

 

[Baluncore]

With ADP, the volume of air in the lungs, along with any gas in the GI tract, are effectively measured as being external.

 

[sophiecentaur]

With ADP, the volume of air in the lungs, along with any gas in the GI tract, are effectively measured as being external.

Not too hard to compensate for that, though. One might ask for what purpose is body volume of interest - you could argue the the topology of the body puts the air in the lungs as 'external' in any case. The same could be said about the gut and its contents.

 

[Baluncore]

Not too hard to compensate for that, though.

ADP separates the volume of gas from the volume of liquid and solid. The weight of the subject, divided by their ADP volume, gives density. Since bone, blood and muscle volumes tend to track, the density computed gives an estimate of the low density fat.

How or why might you need to compensate for internal gasses ?

 

[anorlunda]

Cool. Could you use the same method to measure the volume of liquid in a tank; even if the tank shape was irregular?

 

[Baluncore]

Could you use the same method to measure the volume of liquid in a tank; even if the tank shape was irregular?

Yes. It would be an ullage-ometer.
Imagine a closed tank, connect it to a cylinder with a piston that effectively cycles the volume of the tank by a fixed displacement. Record the tank pressure, synchronously detect the amplitude of the pressure in the tank. From that, compute the ullage.

Unfortunately, fuel tanks are vented. How do you vent the tank, while still cycling the internal gas volume and pressure?

 

[sophiecentaur]

How or why might you need to compensate for internal gasses ?

Medically, probably not but the inclusive body volume might be relevant in, say, space suit design?

Unfortunately, fuel tanks are vented. How do you vent the tank, while still cycling the internal gas volume and pressure?

Temporarily close off the vent, perhaps. It would only involve an additional valve 'normally open'. That could be a problem when filling or emptying the tank quickly - which is when the venting could be important.

 

[anorlunda]

Unfortunately, fuel tanks are vented. How do you vent the tank, while still cycling the internal gas volume and pressure?

The flow rate in that fuel tank vent is very small. I was thinking of using the peak DP across a venturi in the vent to indicate the pressure in the tank, and indirectly the tank level. The piston could also force the air in via the same vent line. I want to avoid additional penetrations of the tank wall.

On second thought, an integrated unit installed in the vent line could do the whole thing in sequence
1 block the vent
2 stroke the piston
3 measure the pressure
4 calculate and report the tank level
5 unblock the vent

If it had bluetooth and a phone app, then the whole sequence could be done only when triggered by the phone app. The gadget could self calibrate the pressure versus ullage curve during tank filling.

 

[Baluncore]

Steps 1 and 2 can be merged as the vent port could be uncovered by the piston only when in the maximum volume position. But then only positive pressure would be measured. The piston must always end the cycle and halt in that safe position, and the piston force would need to be limited so as not to fatigue or burst an overfilled tank.

 

[sophiecentaur]

Steps 1 and 2 can be merged as the vent port could be uncovered by the piston only when in the maximum volume position. But then only positive pressure would be measured. The piston must always end the cycle and halt in that safe position, and the piston force would need to be limited so as not to fatigue or burst an overfilled tank.

You could get really smart and have the vent open all the time and compensate for it by modifying the peak to peak pressure excursion readings. I'm sure my iPhone could use an app to do that.

 

[Baluncore]

You could get really smart and have the vent open all the time

The problem with a calibrated orifice is that it must be small enough to partly obstruct the flow generated by the piston, while it must never get blocked by biological growth, contaminants, hydroxide soaps, or corrosion products.

The advantage of a port uncovered by the piston is that it can be relatively large and can be self cleaning as it is swept by the piston.

If the vent is allowed to be exercised by the tidal piston stroke, then over time, a fuel tank will draw in moisture with the air, and will vent volatile fuel components.

 

[tech99]

Maybe you could measure the mechanical susceptance (or springiness) of the air in the container, with and without an object, by applying a sound wave. In other words, in the manner of a Helmholtz resonator. This would yield the volume. If there is a vent, this can be part of the resonator, as it will add inertia.

 

[sophiecentaur]

A Helmholtz Resonator but @Baluncore points out that any vent would need to be large. This calls for specialist knowledge of this particular problem, I guess and we are giving it the PF treatment.

 

[anorlunda]

Rather than an analog pressure measurement, it could have a pressure limit switch. Then the device need only count the number of piston strokes until the limit switch triggers. That count would be proportional to the ullage. If it transmitted a pulse each stroke, then the counting could be done in the phone.
If the sequence was triggered locally, then the pulse could be sent by winking LED, or audio, simplifying the onboard circuits.

To avoid arguments with the owners, the diameter of the device when inactive should be no less than the I.D. of the original vent. On my boat, the vent line was a standard 1/4 inch fuel hose.