Temperature to measure Gravity

[Shaft]

Hi

I'm somewhat new to this, having only made a couple of brews. However, I wanted to check something regarding measurements of OG and FG.

I was taking an OG just before kicking the temp up to a boil, then taking it again after the boil, for my OG reading. However, I realised that the reading would change massively depending on how hot the wort was. For example yesterday I took a post boil sample which was about 1.041 when it was probably still 85c ish or more, then realised after it had cooled down to fermentation temperature that this had got up to 1.051 - so what's the right reading to use? Also - my post mash temperature (pre-boil) will always be higher then fermentation temperature as I maintain it. So won't this invalidate the reading for the pre boil SG if it's taken at like 65c?

Sure this is all obvious, but couldn't find the right thread that answered this.

 

[Mungri]

Most hydrometers are calibrated to give an accurate reading @ 20c. So you should use the 1051 reading, assuming your fermentation temp is around 20c.
When you are taking a pre boil gravity, let the wort for the sample cool to 20c first.
I personally wouldn't put my hydrometer in 85c wort for fear of it breaking the glass.

 

[the_quick]

Here you go, hydrometer temp adjustment , https://www.brewersfriend.com/hydrometer-temp/

For next time.

 

[Shaft]

Ah - ok, makes sense. I'll have to work out what it was calibrated at. I haven't let the wort cool that low after sparging before, would add an extra 45 mins or so I reckon to wait that long. Do I need to use a pre-boil OG to determine brewing efficiency, or can I use post boil and just skip the pre-boil measurement?

 

[Mungri]

Ah - ok, makes sense. I'll have to work out what it was calibrated at. I haven't let the wort cool that low after sparging before, would add an extra 45 mins or so I reckon to wait that long. Do I need to use a pre-boil OG to determine brewing efficiency, or can I use post boil and just skip the pre-boil measurement?

You just need to take enough out to cool for the sample, no need to cool the whole of your wort.
Sorry if I didn't make that clear.
I just use a pyrex jug to take enough out of the pre boil wort to use as a sample.
As @the_quick says you can use a temperature adjustment. At 65c though I doubt it would be accurate.

 

[Shaft]

Haha - I should have thought of that rather obvious approach!!

Thanks all. First brews were good, regardless of not having accurate alcomohol readings However, think the batch yesterday will be close or over 6% based on what I've just found out! Looks like a hearty pilsner alright.

 

[Mungri]

If you don’t have a trial jar for your sample you should get one. You only need a small amount for the sample then.

 

[Shaft]

If you don’t have a trial jar for your sample you should get one. You only need a small amount for the sample then.

I have a rather feeble plastic holder which came with the hydrometer, which can be used as a trial jar. It's not perfect, as a little slim but seems to do the job.

 

[the_quick]

Wilko trial Jar £2.75 - believe me much better than what you have now

https://www.wilko.com/en-uk/wilko-plastic-trial-jar/p/0022576

 

[Mungri]

I have a rather feeble plastic holder which came with the hydrometer, which can be used as a trial jar. It's not perfect, as a little slim but seems to do the job.

That's all you need. As long as the hydrometer can move freely it'll be fine.
That pilsner sounds like it's going to be a bit feisty.

 

[scomet]

get a cheap refractometer and stop throwing beer away!

 

[Duxuk]

All the calculator does is to add 0.001 to the gravity for every 3 degrees celcius you are over 20 celcius, or take it away if you're under. So if your sample is at 29 degrees and your reading is 1.047 then your corrected gravity will be 1.050. Easier than logging on, I reckon.
Beware if your temp is way above 20. The calculator is only approximate. It's only certain if you measure at 20.