Do plastic parts in salt water feel the full pressure from the chamber?

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I am trying to subject some plastic pieces to temperature and pressure testing while the parts are in a container of salt water. I have a pressure chamber that is pumped to a desired pressure with compressed air.

If I put the pieces in the container of salt water in the chamber and then pump it to 20psi, will the pressure felt by the parts be 20psi, or does the salt water affect the pressure the parts feel? Does that change if I seal the container with some air above the salt water?

Thanks.
 
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If you had a container filled with salt water 33' high the pressure at the bottom would be 14.7psi. If the container was open to the air, the pressure at the bottom would be 29.4 psi. Pumping in compressed air would result in a rise in pressure equal to the pressure of the air plus the 14.7 psi of the water.

If you pumped in 20psi of air, as you suggest, then the pressure at the bottom would be 34.7psi. Thats assuming a 33' water column. You can do the math to adjust the pressure the water adds if the column is less than 33'. For example, a water column half as high (16.5') would be (7.35psi).
 
If I get what you are saying, its like a ball of water in a box of air which you can pressurize?? Since water is incompressable, I think the water pressure would equal that of the air pressure assuming the 'ball' was made of flexible material.
 
The water exerts a pressure all by it's self, depending on the height of the water column. Any air pressuer you add on top of the water will add to the total pressure. As far as the container expanding, that is not necessary as the air is compressable.