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Calculating weight of items in seawater

Calculating weight of items in seawater


 I working in an ROV design project, where I want to calculate weights of its constituents(like motors,thrusters,electronic bottles...etc) accurately in order to findout total weight of my machine accurately. I am able to get weights of few items in seawater from my vendor but for some items I have to findout myself.
    Can somebody help me to findout weight of an object in water please. Say for an HPU, all I have is its weight in air and approximate volume (from solid works 3d model).

  I would be very much grateful, if somebody could help me.


RE: Calculating weight of items in seawater

Take the object's weight in air and subtract the weight of seawater displaced by the volume of the object when it is immersed. (Seawater weight = Volume_cf x 64 lbs/cf)

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

RE: Calculating weight of items in seawater

Don't forget that any volumes containing air (bouyancy compensators, attached balloons etc.) that will contract when exposed to higher water pressures with depth (a balloon is roughly half its volume at 33' depth, a third at 66' and 1/4 at 99') require special attention.

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

RE: Calculating weight of items in seawater

Quote (mkp520):

findout total weight of my machine accurately

I would suggest the only way to find out is to weigh it.  As you say you only have an approximate volume. Weight control calculations are nearly always wrong.  If you get it between ±5% I would say you have done well.

RE: Calculating weight of items in seawater

You question is tricky, you have not indicated where all the component will be?? is it all exposed to water or inside a capsule??

If the components are all enclosed in a capsule [like in most ROVs], your total machine weight in water would be the total weight of all the components in air minus the weight of water equivalent to volume of the capsule.

Narendranath R
Pipeline engineering is made easy with state of the art computer software, visit

RE: Calculating weight of items in seawater

Hi Nare,

Well, I understood what you mean.My ROV would be like a enclosed capsule.

  Right now I am more into finding weights of my individual parts(for our testing). I could calcualte weights of enclosed parts of defined geometry like electronic bottle...etc. But I am not able to asses same for purchased part like HPU, consisting of motor and pump.

  I am having solidmodel of it from my supplier, so the (outer)Volume of it. I am trying use this volume and mulitiply this with density of sea water(1030 KG/m3) and subtract that from weight of HPU from weight of HPU in air. The number I am getting is very far from that of supplier's number(the weight of HPU in sea water).
     I am doing this to cross check how accurate I can estimate weight in seawater so that might be helpful for me for some parts for which I cannot get that number from supplier.
  1.I am trying to find how this buoyancy concept going to work? when some part having unevn suface(like motor), cannot displace as much water as its volume is? If it can displce as much water as its volume, I should be able to calculate atleast to closer approximation.

  2. Is the volume solidworks showing is not (outer) volume that it can displace when taken into water?

Could some body clear my doubt plese.



RE: Calculating weight of items in seawater

Even or uneven surface has nothing to do with buoyancy, except makes the volume more difficult to calculate.

What you must be careful about is non-rigid containers that can change volume with water pressure, low strength containers that can collapse, and containers that could contain air/gas that is exposed to water pressure.  Those items will change their volume with changing water pressure.  For example, a plastic milk bottle full of 15 psia air will float up from a depth of 10 meters, but take it down to 50 meters depth and it collapses and sinks to the bottom.

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

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