Hydrometer sunk to bottom - no reading

I'm new to homebrewing - doing my first batch (Irish Ale kit with liquid malt extract). I did a partial boil (3 gallons) and then added the remaining water at the very end. Before pitching the yeast, I took a hydrometer reading, and it didn't float at all. It sunk straight to the bottom of the testing tube. I've since tested the hydrometer with just water and the hydrometer works. Any idea why I wasn't able to get any OG reading? Thanks.

You might need to add more wort to your testing tube if you can

When it sank straight to the bottom, what number was top of the wort at? I'm guessing the hydrometer wasn't fully submerged.

That was probably it. I filled it up maybe 2/3 full. Thanks for everyone who replied.

I took my tube, put my hydrometer in, then filled till the water about an 1/8th inch from the top. Then removed the hydrometer and made a sharpie mark at the water level, Now I fill to that level with my sample and measure.

IrregularPulse said:

I took my tube, put my hydrometer in, then filled till the water about an 1/8th inch from the top. Then removed the hydrometer and made a sharpie mark at the water level, Now I fill to that level with my sample and measure.

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Im not sure about the sharpie mark, you need to fill the sample tube enough so that the hydrometer floats. Make sure it's not stuck to the side by spinning it and take the reading.

Everyone knows this, but just as a review, as you disolve solids (sugar) into your soln the weight increases but the volume remains stable, i.e. it becomes more dense. The hydrometer will displace the volume of soln equal to it's weight at which point it will float (archimedes principle). At OG when there is a lot of sugar in the soln it will float higher since a smaller volume of displaced soln will equal it's weight. At FG much of the sugar has been converted to EtOH and CO2 so the soln is less dense so the Hydrometer sinks deeper. The difference equals the mass of expelled CO2 and allows you to calculate the ABV.