Is steam temp correct?

[KatieWoe]

For phetsims/qa#298. Mostly a design/science question. I noticed that the steam temp is lower than the boiling water temp, and that it gets cooler after you stop heating the beaker. I would have expected it to be 100 degrees and stay that way when steam was present. If it slowly dropped I would expect that to be in the clear air? Am I misinterpreting the visuals?

[KatieWoe]

I did notice that the oil smoke/steam cools much faster (almost instantly)

[chrisklus]

I noticed that the steam temp is lower than the boiling water temp

The steam temperature is a function of the water temperature and distance away from the water level, so if you measure the steam right where it originates from the water, it is 100 degrees. I think it's plenty realistic that it's cooler than the water if you measure it a bit above the water.

and that it gets cooler after you stop heating the beaker.

That does seems a little odd to me, though it is subtle. We could probably hard code the steam temperature distance function to use 100 C instead of the water temperature, so that when the water is starting to cool but the steam is not yet gone, it still reads as 100. Or, we could change the threshold so that the steam stops sooner when the water temperature is falling from 100 C.

I'll leave that up to @arouinfar.

[arouinfar]

I noticed that the steam temp is lower than the boiling water temp.

The steam temperature is a function of the water temperature and distance away from the water level, so if you measure the steam right where it originates from the water, it is 100 degrees. I think it's plenty realistic that it's cooler than the water if you measure it a bit above the water.

That's correct @chrisklus.

and that it gets cooler after you stop heating the beaker.

That does seems a little odd to me, though it is subtle. We could probably hard code the steam temperature distance function to use 100 C instead of the water temperature, so that when the water is starting to cool but the steam is not yet gone, it still reads as 100. Or, we could change the threshold so that the steam stops sooner when the water temperature is falling from 100 C.

The current behavior in the sim is correct. As water is heated, the vapor pressure of the water increases as more water evaporates. When the vapor pressure reaches atmospheric pressure, the water will boil (which is why water boils at a lower temperature at elevation). However, steam can be visible before water boils -- think about a steaming cup of coffee. In the sim, the steam cutoff temperature is slightly below 100 C. The steam will appear just before reaching the boiling point, and conversely, the water will continue to steam as it cools down to the cutoff temp.

The physically correct behavior would be for the base of the steam to have the same temperature as the water. If we hard code the base of the steam to be 100 C, we'd run into the situation where the steam is hotter than the liquid, which would be problematic.

I don't think there's anything that needs to be done here, so I'll go ahead and close. Thanks for checking @KatieWoe!