…. and have been busy on reading up on how they work. Pretty clever I must say. The ATC function, for example, is simply done by placing the scale on a bi-metal strip. This shifts the scale up and down based on the temperature of the refractometer itself.
The measurement of refractive index is based on “total internal reflection”. Depending on the refractive index of the test solution you are either looking at the blue background of the prism (total internal reflection) or through the sample and lid at the light source. The refractive index changes at what angles that happens and that’s why you are seeing a transition from white to blue. That also allows a very simple design.
Kai
Sorry for my stupidity! But what’s a refractometer? I have been looking for a digital hydrometer - would that be similar to a refractometer. Please could you post a link where I could see one - and maybe, buy.
Thanks David
Generally speaking a refractometer is a device that measures he refractive index of a substance. The refractive index is a measure of how much light is bent when it travels from air (or another substance) into the particular substance being measured.
We home brewers use refractometers that are designed to measure the refractive index of sugar solutions. Since that index and the sugar content are related one can determine the sugar content simply by measuring the refractive index. That is done though a scale that is calibrated to the Brix scale. Another property that is strongly related to sugar content is specific gravity and that is measured with a hydrometer. For these devices too the scale can be marked as sugar content (Brix, Balling, or Plato).
Refractometers are not necessarily more precise than hydrometers but they allow sugar content (or specific gravity) measurements on very small samples (few drops) and don’t require cooling a large sample. The sample quickly takes on the temperature of the refractometer which is the temperature that is used in the temperature correction of the reading. This is necessary since the refractive index of a sugar solution also changes with temperature.
I recommend a refractometer only as a supplement to a hydrometer and not as a replacement. Determining the specific gravity of fermented beer is a bit tricky since the alcohol affects the refractive index differently than it affects the specific gravity.
Fair enough. The rating system they have in place though seems pretty good. If someone has had 10,000 positive reviews I’d say you have a pretty good chance of getting what they say they’re selling. And my experiences have been good. There are a lot of online merchants that use ebay as another outlet so it’s kind of similar to Amazon in that respect. I agree that buying from any old Joe that posts an auction entails a great deal more risk.
I appreciate the information. I know I need a hydrometer and definitely plan to buy one, I was just hoping to gain some insight on what to watch for as a clue fermentation is complete. The reason I ask is that I have been told in a different post that I underpitched my yeast. I thought the Wyeast smackpacks were for 5 gallons as they advertise, but others have said they are for 2 gallons. My batch is about 2.125 gallons so I pitched about half the swelled smackpack (no starter). I would say that fermentation seemed to be immediate and vigorous (within a day for sure), but I’m not speaking from experience. Now at day 10, the foam (krausen?) subsided about 2 days ago, but I definitely am still making CO2 and I can see bubbles and debris churning around using a flashlight. Can I expect things to be totally still at 14 days, or will there still be a small amount of activity?
