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I have a probably very simple problem which I cant solve. I am weighing animals in seawater by placing them on a small PVC arm that is fixed on a balance and reaches into a beaker of seawater. On the arm, I place an organism and measure the specific weight of the animal. Now, the weight of the animal changes a lot between days which cannot be explained by physiological processes. I believe this might be due to the water temperature which I also collect alongside. Can anybody tell me how to correct for water temperature if I dont know the volume of the organism?

Sample data:

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  • $\begingroup$ There are two things that would help regarding an answer. Use consistent precision when reporting data (e.g., 0.16 should be reported as 0.160). Also, describe the details of how the organism is weighed, hopefully including a picture. $\endgroup$ – David White May 31 at 21:18
  • $\begingroup$ I have this idea that the odd drop of water trapped in or around the shell / organism could be sufficient given the size of the readings... $\endgroup$ – user207455 May 31 at 21:21
  • $\begingroup$ For the first animal, $w_{1}=[0.213\ 0.217\ 0.214\ 0.201],$ the mean=$0.21125$, variance=$0.049583$, std-deviation=$0.0070415$ - or roughly $0.211\pm 0.007$. The most common error is parallax. Also, is the water content consistent? Maybe take a sample of the water and weight it when you weigh the animals so you can determine if the density of water is changing significantly. And measure a drop water. $\endgroup$ – Cinaed Simson May 31 at 21:48
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It is highly unlikely the differences you are seeing are due to temperature unless this is occuring outdoors.

As you can see on this page, the density of water changes less than 1% between -20 and +20 C. If you are indoors, it is highly unlikely it's changing outside +15 to +25, which would be much less than 1%.

Even outdoors, even day/night changes would be unlikely to have a measurable effect, and a full winter/summer change would only be 2 or 3%.

What sort of differences are you actually measuring?

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  • $\begingroup$ I am weighing marine snails in seawater. I have a calibration of whole animal weight in seawater to dry shell weight. I am now weighing the snails every few days to see how treatments effect shell growth but for some reason the weight of all snails shifts systematically every time I weigh them. The systematic change made me believe that it must have something to do with the temperature. Do you have an idea what other systematic errors could impact the measurement? $\endgroup$ – Globoquadrina May 31 at 19:07
  • $\begingroup$ I suggest editing your original post (look for the "edit" button) and adding a sample table of actual results - date, temperature, measurement. Without data I don't think we can be much help. $\endgroup$ – Maury Markowitz May 31 at 19:11
  • $\begingroup$ Thank you for the suggestion, I added some sample data. $\endgroup$ – Globoquadrina May 31 at 19:20

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