Using electronic scales in a pressure vessel

[Lachlan001]

We want to measure the rate of water absorption of rock samples placed in a flooded pressure vessel.
Traditionally to do this you would have to depressurise the chamber, open its top and weigh the sample while it is still suspended in the water. We are looking for a way to skip having to open the chamber by having the scales permanently inside the pressure chamber.

Does anyone know of electronic scales that function in pressure vessels? All of the suppliers I have contacted have been unable to help.
Ideally we would be looking for scales with a 2 kg capacity, 0.1 g resolution and able to function at 5 atmospheres of pressure.

Thanks in advance!

 

[GenB]

Hard topic. I'm just guessing, you have to make your own to get the weight, you will have to set the stone on top of the weight sensor and the wire or cable through the vessel wall to the reader, computer or whatever media you use to read the weight. Regards Genblr.

 

[marty007]

Any chance you could put the entire vessel on a scale like they do for chlorine storage? On second thought, probably not based on 0.1g resolution required...

 

[Compositepro]

Under water load cells seem to be commercially available.

http://www.sensing-systems.com/products/underwater_sensors/default.html

 

[Lachlan001]

Thanks for the suggestions.
Suspending the sample on a wire that passes through the wall of the pressure vessel would require some pretty good seals and I would be worried about what effect they would have on the recorded weight.

Weighing the whole pressure vessel introduces some interesting problems as well. The pressure vessel, water and sample would have a combined mass of 20-30 kg so as you say it might be hard to find some scales with 0.1 g resolution at that mass. It would also mean that I need to modify how I intend to pressurise the pressure vessel - I was going to use an air compressor to pump air into the flooded pressure vessel. That would mean that I would have a fixed volume of water so the total weight shouldn't change, the water would just be moving into the pores of the rock without entering/leaving the pressure chamber.

Underwater load cells is a great idea - I had only tried the digital scales manufactures. I have now contacted a few load cell suppliers and I am waiting for their response.

 

[gr2vessels]

My two cents;- How would the submerged 'rock' change weight by absorbing some of the surrounding water, assuming some change in volume and specific gravity? BTW, is it a rock expected to absorb water? It might be measurable when removed from the water, but a submerged 'rock'...?
Cheers,
gr2vessels

 

[GenB]

Supposedly you will drain the water off the vessel while weighting it. Else how you will deal with the vouyance. Wiring, you can fine electric pass.through connectors as common parts. I have use them for high pressure water and steam.

 

[Lachlan001]

The rocks are porous so they have lots of small connected holes which water can seep into. Before the rock is put in the water the pores contain air at either standard atmospheric pressure or at -0.9 atmospheres (we often partially evacuate the samples to speed up the rate at which they soak up water). When the rock is put in the water, the water slowly seeps into the pores, displacing the air. By replacing the air/vacuum, the water increases the weight of the sample without increasing its envelope volume. We weigh the sample while it is still in the water as this enables us to calculate the total volume of the sample (final soaked weight in air versus final soaked weight in water) and the volume of the pores (initial dry weight in air versus final soaked weight in air). From this information we can calculate the bulk density of the rock and its porosity.

 

[moltenmetal]

Sounds like what you're asking for is to be able to weigh the rocks while they're submerged...what you're interested in is the weight gain or buoyancy loss. Otherwise, you're talking about not only weighing in a pressure vessel, but also having a means to MOVE the rocks from the water to the pan of the weighscale while they're inside the pressure vessel- some non-trivial mechanism required in both cases. Then there's the drainage- the water that will invariably drip out of the pores that are large enough not to be overcome by capillarity. How long you weight and how much you shake the sample prior to re-weighing it will likely matter quite a bit.

Forget about suspending wires or rods passing through seals- weighscales are a force balance, and friction is the enemy. Friction such as that occurring through a pressure/vacuum seal will destroy the measurement.

It's possible to do weight measurements inside a pressure vessel- possible, but NOT EASY. You can't just pop a commercial weighscale into that environment and expect it to function properly. You do need to get power and signal in and out of the vessel through sealed connectors or wire passthroughs, but that's the easy part. You need to concern yourself about unbalanced forces on the loadcell parts etc. which are normally exposed to the atmosphere on all sides. Lots of work and suffering, none of it having been done for you by the weighscale vendors. We've done it, successfully, to high accuracy, inside a vessel pressurized with hydrogen to 600 psig, but it's a big job and not to be undertaken lightly.