James Spencer and I did another interview for Basic Brewing Radio, and this time it was even in person, which was a lot of fun. It deals with gravity measurements and some of the intricacies of using refractometers, and concludes with a call for additional data that will hopefully corroborate my final gravity correlation.
Just wondering why the alcohol at fg doesn’t cause an error with the hydrometer? When i ferment a sprit wash for distillation (7kg white sugar in 25 litres), i typically get fg’s less than 1.000 (~.980). the alchohol content is much higher than beer (16+%), but i cant help wondering if the error you are seeing is not to do with the refractometer, but the hydrometer. What do you think? I have just ordered a refractometer last week so your interview with james was good timing for me. I’ll be collecting data for you.
Thanks for helping out, Ben.
Alcohol in the beer does affect the specific gravity, but since SG is what we want to know anyway (whether using a hydrometer or refractometer) I wouldn’t really consider it an error. What the hydrometer reads is the FG – assuming it’s calibrated properly. A refractometer doesn’t measure SG, though; it measures the refractive index. That’s why a correlation between RI and FG is needed.
How do refractometers go when measuring the likes of an imperial stout? Is the sample small enough that the dark colour has minimal effect?
Short answer: yes. The color of the sample doesn’t make any difference, at least not that I can see. It’s spread so thin by the cover plate that it’s almost totally transparent.
A very useful project. I will start collecting data using both refractometer and hydrometer, but 5 samples might take a while.
Thanks Gustav. Don’t worry too much about timing. It will be quite a while before I can get data from everyone.
Sean
My results are here:
https://spreadsheets.google.com/pub?key=0Aq7ztarwtfBidG1mbW51TFZSVWhzSnRVRmVoVzY3WWc&hl=no&output=html
Thank you Gustav!