Effect of temperature on gravity

[vinceg]

Can anyone give me a few pointers as regards the effect of heat on my hydrometer readings? I've taken my first gravity reading on a FV of Mild I've had on the go since 2nd Oct and was surprised to see it was already down to 1.009. However I then noticed that the thermometer which I keep on the lid of the FV was reading 23 degrees as opposed to the ideal 20 degrees against which hydrometers are generally calibrated, I gather.

If the ambient temperature in the room is 23 degrees, does this mean my hydrometer reading is showing lower or higher than it would do at 20 degrees?

[Aterlatus]

Water expands as it gets hotter (very marginally, of course) as the atoms bounce around a lot more so water is less dense at higher temperatures. A "1.000" SG at 23 degrees would likely be lower than 1.000 if my physics is serving me right...

To be honest though, although hydrometers are calibrated to 20 degrees I really can't imagine +/- 3 making any difference that you'd notice :s

Of course if someone would like to correct me, please do. I'm a nub when it comes to homebrew

[vinceg]

Yeh, I see what you mean. If the warmer water is less dense, the hydrometer sinks further therefore giving a lower reading. Maybe 3 degrees might be equivalent to about 1 point of gravity, something like that, so my reading would be more like 1010, but like you say, bugger all difference. If there's any physicists out there, please chip in

[clogwog]

Yes, temperature does make a difference.

If you use a programme like BeerSmith, it has a hydrometer adjustment function built in.

If you don't want to use such a programme, have a go at this free online bit about it, including an adjustment table.

http://www.howtobrew.com/appendices/appendixA.html

[anomalous_result]

Also it's a sticky here.