Pre-boil refractometer readings?

[David R]

I've done half a dozen or so brews since buying a refractometer to measure my pre-boil gravity. The last 3-4 of them the pre-boil sample has come up with a reading that has been a good 4-5 points higher than it should have been based on the OG when tested post-boil. Eg; today's brew measured 1.052 pre-boil, post boil OG was 1.052 and not the 1.057 I was expecting, and yes I did test the post-boil with the hydrometer and refractometer side by side.

What I've been doing is waiting for the hot break, then drawing a small sample and placing it in a thick glass I've pre-chilled in the freezer to cool it down. After ~5 minutes I'll take a small sample of the cooled wort with the pipette and test that with the refractometer. Given that the side by side test with the hydrometer sample confirmed it is accurate, I'm wondering if the pre-boil sample temperature is causing the discrepancy. However, the refractometer I bought (http://www.trademe.co.nz/home-living/wine/wine-accessories/other/au...) supposedly has some sort of automatic temperature correction?

What am I doing wrong? Do I need to chill the sample of a more accurate temperature?

[Scott H]

Have you calibrated the refractometer with de-mineralised water?

Are you checking the tempreture of your sample and adjusting the hydrometer reading to suit as most hydro meters are calibrated for 20c?(it should say this at the top of the part with the measurements)

The sample size your taking for the refractometer should cool to measurable temps within a matter of seconds(the pipette one). Also with a preboil reading make sure you give your kettle a good stir to mix the runnings together before taking your sample.

[David R]

No I haven't calibrated the refractometer, probably a good idea I guess!

The hydro sample is usually around 18-22C as it's taken straight out of the grainfather chiller.

And part of the reason I wait until after the hot break is to make sure the boil is rolling so there's no layering.

[Peter Smith]

Mine is all over the place, I take three readings and average and even then 3-4 points is about as accurate as it ever seems to get.  That said I always gain points during the boil....   Funny I often test out of the trub when finished and find OG is 3-4 more then I expected.

I always try to do an OG hydrometer reading with recovered wort.  I only paid about $25 via aliexpress. Still its one of the most useful tools on brewday, especially at the end of sparge and if I am doing a second batch sparge for starter wort.

[Kevin Sharp]

Pre-boil is the one time I don't trust my refractometer. I find for all other uses (including estimating FG) it agrees with my hydrometer to within a point, or occasionally two. I calibrate it each brew day although the tap water here is soft enough  that I don't need to use deionised.

I think the mechanical ATC in refractometers is pretty much worthless, but this doesn't matter if the refractometer is at the same temperature when you take the measurement as it was when you calibrated it.

I used to monitor how many points I had in the kettle as I was lautering. I've changed my process now, so that I just collect the volume of wort suggested by the recipe. I chill a sample and take a gravity reading with a hydrometer. Calculate how much wort to remove from the kettle to hit my pre-boil target gravity and top up with boiling water. If there's any danger of over-spargeing I'll acidify the sparge water. The wort I take out of the kettle usually gets frozen and later used for yeast starters. This way I only need to take one hydrometer reading and that doesn't seem too onerous to me.

[Peter Smith]

Kevin - what FG conversion calc are you using and have you had to do much calibration.

[Kevin Sharp]

I use the brewersfriend calculator - http://www.brewersfriend.com/refractometer-calculator/ . I always calibrate for temperature, but I've never done what they call a Brix WRI calibration. My refractometer reads in SG and Brix. If I plug the Brix reading into the calculator the FG I get agrees closely with a hydrometer reading.