Submitted by That_Lego_Guy_Jack t3_zv5qux in askscience
If I want to measure the density of the material a sponge is made of, how do I go about that? For liquids I imagine you put it in a vacuum chamber on a scale. For metals I imagine you melt it down and do the same. Am I right?
Archimedes is also iffy above a certain porosity where the fluid will start to penetrate the sample. You will get the wrong measurement. You can smear some grease or coating on the outside to reduce interaction with the liquid.
Wouldn't that have more to do with permeability than porosity?
I've used nitrogen for pycnometry instead of helium; some really porous samples I've tested would cause the instrument to drift and never really settle down, but nitrogen (being heavier AND diatomic) gave "better" (faster) numbers.
Very different, but also interesting- BET surface area analysis.
Yeah, depends what kind of resolution is required for analysis
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Is this like what happens when you add 50 cL of alcohol to 50 cL of water and get less than 100 cL overall?
Take you dry sponge, put it on a scale, you have its mass. Take a graduated glass recipient with exactly 1L of water in it, put the sponge in it, read the new volume indicated, substract 1L to it, you have the volume of your sponge. Divide the mass by the volume and you have the volumic mass density of the sponge. Divide it again by the volumic mass of pure water (1000kg/m3 or 1kg/L) and you obtain its density.
Edit: corrected as I didn't get the correct english terms.
> Divide the mass by the volume and you have the volume mass of the sponge.
This calculates the sponge’s density. Mass divided by volume. Nothing further needed.
> Divide it again by the volumic mass of pure water (1000kg/m3 or 1kg/L) and you obtain its density.
This is totally useless and wrong.
Could be a language barrier. The second step calculates Specific Gravity in English.
You nailed it! And now I know just a little bit more than I did yesterday, thanks!
I made the mistake of not checking the exact english terms. In my language we differentiate density and relative density with two different terms, litteraly volumic mass and density. I thought op wanted the relative density
Awesome! Yeah I could tell you knew what you were talking about but that last line made me go whaaa? Makes sense!
I'm glad your took it that way after my explanation, in opposite to the last sentence of your initial response which I found a bit agressive :)
Why do you divide it by the density of water? Isn't the mass divided by the volume the density? What is "volumic mass"?
Density is an absolute ratio, where the volumic mass of the water is the reference (water density =1). By dividing the volumic mass of the sponge by the water's you compare both, and will easily know if it's denser (desnity > 1) than water or not.
Does this account for the sponge absorbing the water?
That is why you use the water, it's a tool in this case to measure the sponge's cavities. By using a known metric (water) and volume (1L) and calculating the differences before/after the absorption you can determine the quantity of water in the sponge. If you know how much water is in there, and you know how much space water takes, you know how much space is empty in the sponge.
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The air wouldn't contribute anything to the weight so it shouldn't matter
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The air is neutrally buoyant. It's not that it contributes a small amount to the weight, it contributes zero. For example if you had a cup filled with nothing but air and weighed it the air won't affect your reading at all because the weight of that air is 0 when you're in Earth's atmosphere.
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The buoyancy force would reduce the apparent mass of the cup by the mass of the displaced air. It may not be significant but if you want to be precise, you need to account for it.
Just like how a helium balloon isn't negative mass, it just appears to be because of the buoyancy force, it's important to account for the mass of the displaced air to be precise.
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In deed, you should try to press the sponge to evacuate all the air trapped inside.
Definitely completely porous is straightforward: Archimedes Principle will do that: weigh it, then immerse it in liquid and measure the volume change.
Where it's hard is semi-porous or mixed materials. Closed cell foam is an example of this - it's made of two very different materials, the plastic that makes up the foam, and the gas that makes up the voids. You can't non-destructively measure the density of the two parts together. The brutal approach is generally used - crush it to burst all the bubbles, then measure the density of the resulting lump.
This matters, because often porous materials contain trapped gas. So you measure the density as above and get quite different results from different samples. Think of immersing a sponge, then squeezing it - if bubbles of air come out you would have measured different densities before and after. How do you know that you have no closed cells containing gas? Answer: you crush it then measure it.
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there needs to be a lot more context to give you a good answer.
how much material do your have? do you mind if we destroy it as part of the measurement? how accurate (significant figures) do you want the result to be? what is the material made out of? how much money do you have to spend on this?
cheap simple solutions will work for some levels, but eg crushing may be to be done under vacuum to avoid trapped gas during the crush. you may even need to heat the sample to outgas it better before crushing. assuming that it can survive heating and vacuum conditions.
you could use a sectioning method with image analysis like microtomy to get a good answer, if your sample is soft enough, and you know the density of the two phases (sponge/pore). or you can go high tech, and use ct scanning to get the same answer, provided that your sample is xray transparent enough, and fits in the scanner.
i doubt you would want to try a crush method if the sample is a pu based foam!
Depends on how exact it should be. I did an exercise on bread/baked goods quality control in uni once. You an either use a laser scanner to get a 3D model of whatever youre measuring (this is nice, because you can also get Information about coulour, but the scanner is expensive, you can measure pretty much anything though) or submersion in plastic beads / seeds (measure the volume before and after) this is extremly cheap and reasonably exact.
If it's a foam like whipped cream or stuff like cereal puffs: use a beaker and weigh But be aware that this is not the real density, but bulk density. You can look up some constants about which geometries have which void space in bulk and get an approximation for the density of the bulk good from the bulk density.
If you want to measure the density of the material itself: try find out what material it is and then look the density up. Or weigh and measure it before foaming it up.
Look into void fractions or porosity. There are a number of methods depending on the material and what you’re goal is.
They revolve around different techniques to measure the volume of the pores/void spaces. Which then allows you to subtract it from the overall volume of the material when calculating density.
It depends on which density you want: bulk density or density of the solid fraction only, and of course how precisely you want to determine the value.
With solids or liquids, density is easy to determine, all you need to do is establish the mass (weight), which you use with a scale, and volume, which you can typically get by adding the material to a known volume of liquid in a measuring device like a graduated cylinder, and measure the volume change after addition. You could actually do this at the same time, by having the volumetric container on a scale when you add the liquid or solid. Get both volume and weight change in one move.
This will work with a sponge too, to get the density of the sponge material (assuming that you have total permeability, no blocked void spaces when immersed in liquid).
If you want a bulk density of something porous like a sponge, the best method is to measure mass (weight) dry and wet (saturated). The volume of the open space is defined by the mass of water contained within that open space (the change in mass from dry to wet is due to contained water only, and we have established water density to more precision than you will likely need for your purposes-you probably won't need to correct for temperature).
I don't do gas measurements, which generally has a pressure dependence and you would need to do a pressure-mass curve using a fixed volume container (a glass bulb of known volume, seems a good way to go). Empty the glass bulb of all air, measure its weight, then inject it with the target gas, and measure its weight. The change will be due solely to the added gas mass. Do that at a few different pressures to establish the pressure-density curve.
You could, conceivably, crush the porous solid to eliminate pore space and then get the powder mass and volume the same as with any solid. From that, you could establish the pore volume in the original sample (measure volume of the uncrushed sample and its weight and go from there with some simple math).
If you want to determine surface area of a porous solid, well, then you would need to go into more complicated or higher tech methods.
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ShxxH4ppens t1_j1nnxn7 wrote
You can use helium pycnometry to determine real densities of solids, essentially you calibrate a pressure vessel to determine it’s volume, then fill the volume with a known mass of sample, and then recheck the volume of the vessel, giving density of your sample, you can do archimedes method as well, much less accurate and requires attention to solubility