Trouble with hydrometer readings

[TarVolon]

So when I take a hydrometer reading, I'm usually not that confident in my reading being super precise, so I just leave the hydrometer test tube out for a little while, then come back to check again to make sure I had the right reading. (I can be a little OCD about things like that, and as soon as I dump it out, I'm going to want another reading to confirm).

Anyways, the last several times I've done this, the hydrometer reading has been consistently higher after the sample had been sitting out for a couple hours than what it read when I took the sample in the first place.

Usually I do this for FG readings, and it'll be different by 1 or 2 points. This time, I brewed Saturday, took an OG reading of 1.065 Saturday night, and then forgot to dump it out before Sunday morning. On Sunday morning, it was reading 1.068. By Sunday afternoon, it read 1.069. (In this case I was dropping the house temperature for fermentation, but according to the hydrometer temperature adjustment tables, not nearly enough to make much of a change)

Two questions, the second more important than the first.

1. What is causing this?

2. Which one is to be believed?

I know it's not a HUGE different, but .004 is not insignificant, and even with a difference of .001 or .002, it makes it hard if I want to take hydrometer readings for three days to see if it's done.

 

[flars]

I can think of four, maybe more with some time, possibilities for inconsistent readings. Temperature change of the sample, dissolved carbon dioxide leaving the sample, test tube not held vertical, and evaporation of the wort.
You could purchase one of the large scale hydrometers which read from about 0.094 to 1.020. Markings are far apart and easy to read for final gravity.

 

[tcapmidnoces]

Still more accurate than my first attempt at getting an OG reading out of my mead last week. A bit of foam would gather on the hydrometer making it near impossible to read!
In hindsight I think I should've filled the tube up more and scraped off the top before I took a reading.

 

[unionrdr]

Might just be bubbles on the bottom of the hydrometer. Give it a spin when you put it in.

 

[TarVolon]

I have given it a spin in each case. Test tube has been on the same surface at all times, so that's a constant.

I had considered temperature change, but a few degrees shouldn't make a .004 difference.

Does wort evaporate that quickly to make that kind of difference? If it does, I guess the first reading is the one that's right. If it's CO2, do I trust the first reading or the later ones?

I could do a larger scale one, but it wouldn't work for OG. I've never had a refractometer, maybe that could be an answer too. People on here seem to like them.

My target OG on this one was 1.060, so I definitely ended up high either way, but high by .005 and high by .009 are pretty different.

 

[unionrdr]

I'd g with the first reading & don't leave laying about too long.

 

[BigFloyd]

In this case I was dropping the house temperature for fermentation, but according to the hydrometer temperature adjustment tables, not nearly enough to make much of a change)

You drop your house temperature for fermentation? To what temp?

 

[TarVolon]

You drop your house temperature for fermentation? To what temp?

Yeah, I don't have any temperature control setup except just my AC. So I drop the house temperature to 65 for the first two days of fermentation, and then I raise it back to 70 for the rest of fermentation (except in the winter. In the winter, my house temp is always at 65). I try to use yeasts that are comfortable around 70 if I'm brewing in the summer.

Actually didn't work as planned this time, because the yeast generated enough heat that the wort temp was 10 degrees higher than ambient (12 hours into fermentation). I usually get more like 5-6 degrees above ambient.