Pre boil variation in brix

I have been having issues with my efficiency lately and one thing I'm suspecting is my refactometer. So today I tried again and after my sparge I stirred a bit and took a sample. It showed 8.4 which is way off the 12 target. I then put it on the burner and when it was close to boiling I took another sample. I let this one sit a long time and then got a reading of 10 brix. Still off target but since the wort had not boiled yet the readings should be the same. Any ideas?

temperature compensated?

It's supposed to be (one of the reasons why I bought it) but I begin to wonder if it's not. I did calibrate again with distilled water but I don't have anything else to tell me if it's even working correctly. But I do try to let the samples cool a bit before I test them since I've read it's best to have the refractometer at room temperature and the sample not be over that temperature too much.

pretzelb said:

It's supposed to be (one of the reasons why I bought it) but I begin to wonder if it's not. I did calibrate again with distilled water but I don't have anything else to tell me if it's even working correctly. But I do try to let the samples cool a bit before I test them since I've read it's best to have the refractometer at room temperature and the sample not be over that temperature too much.

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The temperature compensation is for the refractometer its self not the liquid so much. Walker has a recent thread about this where his refractomter readings were all over the place. So, the question is, what was the temp of your refractometer (I think above 80 or 85 they are unreliable.

Edit... Also, I think you are going to have to stir very thoroughly to get an accurate pre-boil reading.

+1 to both of Ohio-Ed's suggestions. I've also found I had to minimize evaporation to get accurate readings of hot wort. I draw samples with a syringe and let them sit for a couple minutes to cool before using the refractometer.

could break out the hydrometer for the time being until you can get your refract process figured out. it would at least give you an idea of if you can trust it or not.

I already take readings inside the house so that should be constant. I did not expect that I'd have to wait a long time for the wort to cool but perhaps I do. I did try to stir a bunch before taking a sample but maybe just waiting before boil is more accurate.

I would use a hydrometer but I got the refractometer because I didn't trust the readings on the hydrometer. I'd hate to get another gadget to try and figure this all out but maybe a temp correction hyrdrometer is the next step. My other thought is to just get a round cooler mash tun kit with false bottom and give up worrying about the readings but that would cost a bit more.

make a sugar water solution at a known gravity concentration (e.g. 1lb of sugar in 1gal of water gives something like 1.036...reduce it to an 8 oz sample, then use the refractometer and see if its reading right).

You do wanna make sure you calibrate it with 'room temp' distilled water, and a good light source to read it by, then make sure your wort sample gets to cool off a bit. I'll just grab a half teaspoon and let it sit a few minutes...seems to suffice.

That said, if you batch sparge, especially a double or triple sparge, that wort on top is likely to be thinner than the first runnings (non-homogenous mixture). So I actually don't do pre-boil readings, just pre-pitching so my wort is already thoroughly mixed.

stir it really well while sparging and that might help your readings become more consistent. maybe...kinda thinking aloud here, as i Know how hard it is to mix a partial extract boil into the top off water.

But the most likely thing IMO is that you didn't stir well enough before taking the first reading.

-a.