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Refractometer Discussion
RefractometerBeer Engine's refractometer calculator approximates specific gravity from refractometer readings. Approximate is stressed because there is not a direct correlation between hydrometer readings and refractometer readings when measuring brewers' wort and the disparity is even worse once fermentation is under way. There are many contrary issues which make it difficult to use a refractometer accurately for brewing, particularly if trying to relate refractometer readings to specific gravity.One issue with using refractometers is that different types of sugar have different refractive indices. The refractometers commonly used by home brewers are scaled in degrees Brix, the percentage by weight of sucrose (ordinary household sugar) in solution, but wort is mostly maltose which has a different refractive index and is not equivalent as far as the refractometer is concerned. Indeed, wort contains of a multitude of complex sugars all of which have different and sometimes competing refractive indices. Unlike, say, wine, the sugar balance of a wort is not a fixed, accurately predicable value, but is dependent upon recipe formulation and mashing regime. Thus the values given by refractometers are unreliable in absolute terms in wort, but can give useful relative values if the limitations are understood. The difference between the refractive index of sucrose and the more complex sugar composition of wort requires that a calibration factor needs to be determined and then applied to correct for the disparity in refractive indices. It is plain that the calibration factor is not a fixed figure, but varies according to wort composition. Different types of beer may require different calibration factors. Also, a hydrometer responds to all dissolved solids, solids such as protein, but the refractometer does not. This implies that different calibration factors may be required at different stages of the brewing process. Things get rather more complicated during and after fermentation. Apart from the handicap that refractometers are difficult to read in the presence of yeast cells and carbon dioxide bubbles, refractometers are not affected by the presence of alcohol in the same way that hydrometer readings are. Alcohol has a lower specific gravity than water and the hydrometer gives progressively lower readings as alcohol percentage increases. Various assumptions have to be made when using a refractometer during and after fermentation if one is trying to relate Brix to specific gravity. A guesstimate has to be made for the proportion of sugar used for yeast growth and metabolism during the aerobic phase, which does not produce much alcohol, and thereby how much is left for the subsequent anaerobic phase which produces copious amounts of alcohol. The ratio of the sugar consumed during the two phases affects the difference between refractometer and hydrometer equivalent readings. Obviously, the more sugar that is used up for yeast growth, the less will remain to produce alcohol. This ratio of total sugar consumed, to the amount alcohol produced, is affected by yeast pitching rates and dissolved oxygen levels, among other things. The process of fermentation will also affect the previously-mentioned calibration factor. A typical all-malt wort will contain about 65% maltose and other simple sugars, and 35% more complex sugars that are collectively referred to as dextrins. Prior to fermentation the wort has a particular ratio of sugars and the refractive indices of the various sugars combine to produce a net refractive index for the wort. This requires a specific calibration factor to convert to the equivalent sucrose. During fermentation the maltose is fermented into alcohol, but the dextrins, being mostly non-fermentable, remain. Thus the ratio of maltose to dextrin changes as fermentation progresses, necessitating, in theory at least, a different calibration factor at the low end to that derived for the top end. Home brewers tend to use the terms Balling, Brix and Plato interchangeably and it is often asserted that they are identical, but they are not the same thing when equated to specific gravity. This is mainly due to the fact that the original Plato and Balling tables were derived at different temperatures. There are also differences between published tables, depending upon source. Americans tend to use Brix, Europeans tend to use Plato. There are differences between official tables of American and European origin. There are errors in the original Balling measurements which find their way into the Brix tables and so into the American official tables. Furthermore, the official American tables use a strange figure for the density of water at 20°C, used in the conversion from density (which is what the Balling and Plato tables actually use) to specific gravity (which is what we are interested in). This again adds to the differences between tables of American origin and those originating elsewhere. In the Beer Engine software there is the conundrum as to whose damn tables does one attempt to match. However, the differences are sufficiently close for them to be regarded as equivalent as long as they are measured by the same method. A typical 0-32% Brix optical refractometer, for example, can not be read to a resolution of much better than 0.2% Brix, which, when coupled to the accuracy of the instrument (mine is +/- 0.2%), is fairly imprecise and would mask any differences or errors in the Balling or Plato tables. Traditionally, brewers would measure Balling, Brix and Plato using Balling, Brix or Plato hydrometers, not refractometers. Hydrometers do not only measure specific gravity. Hydrometers are available that are scaled in Baume, Brix, Balling, Plato, density or specific gravity. However, like a specific gravity hydrometer, a Brix or Plato hydrometer will give a different reading than a refractometer in the presence of alcohol or dissolved solids. For this reason, in brewing at least, it is wrong to quote Balling, Brix or Plato without specifying how it has been measured because confusion can arise, but the measurement method is rarely quoted in home brewing. One sort of unofficial standard seems to be to use Brix to refer to direct refractometer measurements and Plato for everything else. Calibration FactorThe primary calibration factor is used to account for the difference in refractive index between ordinary sucrose and the sugars present in wort. It is performed at the original gravity stage, before yeast is added. A measurement is made simultaneously with both a refractometer and a hydrometer. The hydrometer reading is converted to Plato, and then the refractometer reading is divided by the Plato figure derived from the hydrometer. The result is the calibration factor. There is a tool in a pop-up accessed from the refractometer tool in Beer Engine that does the sums for you. Simply enter your hydrometer and refractometer readings in the relevant boxes and the calibration factor is calculated. 1.04 is a typical value, apparently. Hit the save button if you want to save the factor as a default for future use. Without saving, the figure will remain valid only for the current session.It is frequently stated on the Internet that the calibration factor is 'system dependant' and implies that when you have found the calibration factor for your system you are done and dusted and have no need to worry about it anymore. This strange assertion has, worryingly, found its way into Beer Smith and Promash. It is not system dependant. There is no earthly reason why if you produce a beer to a given recipe on your system, and I produce a beer to the same recipe on my system, that we should get different refractometer readings. It simply does not compute, as Mr Spock used to say. It is, however, recipe dependant. The most obvious illustration is that an all-malt beer will have a different refractive index to a beer with a high level of ordinary sugar in it, and thus would require a different calibration factor.
Using The Refractometer ToolThe refractometer pop-up consists of two panels; 'pre-fermentation measurement' and 'secondary measurements'. The top panel, pre-fermentation measurement, as the name suggests, is for use under circumstances prior to fermentation; mash tun run-off for example or the equivalent of original gravity. It simply adjusts the refractometer reading that is put into the refractometer box by the calibration factor and displays the results in S.G. It can be used stand-alone (without using the second panel). It will also work backwards by putting a number in the original gravity box and it will fill in the refractometer and Brix boxes.The bottom panel, 'secondary measurements', is for occasions after yeast has been added. It cannot be used stand-alone; the top panel, 'pre-fermentation measurement', must be filled in with the original gravity figure first, because it needs to adjust the reading for alcohol content which can only be determined from original gravity. The bottom panel works in the same way as the top panel; that is, enter the refractometer reading in the appropriative box and the equivalent specific gravity, corrected for both alcohol and calibration factor, is displayed. The calculation will also work backwards by putting a figure into the SG box.
Text copyright (C) 2009 Graham Wheeler. |