Kettle volume ghost?

[MrFinstad]

I seem to have a ghost messing around with my kettle volumes. Maybe someone can volunteer an exorcism, or maybe someone can explain this with..science?

I've noticed that when I am heating up my mash in my boil kettle that the volume increases as I approach 212 F. However, right when I start boiling, my sight gauge immediately drops about .4 gallons. It make sense that the volume increases with temp because the density of the mash is decreasing, but I can't figure out why the volume drops so fast when the boil starts. Is this some interplay of kinetic energy in the wort, surface tension, capillary action in the sight gauge? WTF. Or am I just haunted my a ghost?

My reason for asking is that I want an accurate pre-boil volume measure so I can dial in my O.G., adjusting up or down as needed with water or DME.

The details of my kettle: 25 gal megapot with a 3/8 OD sight gauge from bargain fittings. Maybe a larger diameter sight gauge would fix things?

 

[audger]

when you have a large column of water, boiling it can have several effects. the most straitforward is thermal expansion. the volume will increase slightly with temperature.

when water starts to boil though, you are creating gas pockets. in a small pot of water, they travel from the bottom to the surface in under a second, so the level change isnt very noticable. but when you have a 10 or 15 gallon column of water, it can take several seconds to escape. this adds apparent volume.

however- gas bubbles can also do some screwey things when you try to get an accurate weight of the liquid. (a site glass works by the laws of gravity, and, by extension, weight). the bubbles are trying to force their way upwards and are putting pressure upwards on the water, temporarily reducing the pressure at the bottom of the pot very slightly. now that the large body of water apparently weights slightly less than the small amount of water in the site glass, the level goes down.

to see this in action, kill the heat for a second durring your next boil. as soon as the bubbling stops, the levels should equilize, and then as soon as it starts again, the site glass should go back down. boiling doesnt change the actual volume (excluding evaporation), just the apparent volume. so if you want an accurate measurement, measure when there are no bubbles.

 

[MrFinstad]

awesome. science wins again.

 

[jmf143]

My reason for asking is that I want an accurate pre-boil volume measure so I can dial in my O.G., adjusting up or down as needed with water or DME.

So read your preboil volume before you start boiling your wort.

 

[MrFinstad]

So read your preboil volume before you start boiling our wort.

That's been the practice so far. Now I can just do it a little more confidently. I've been hitting my gravities every time without any adjusting, so I guess I was just looking for a better understanding of the weirdness.