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Old 03-07-2013, 03:15 AM   #1
nstnate
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Default Refractometer/light

Does anyone know what using a Refractometer with fluorescent light will do? Everyone says to use natural light only and especially NOT fluorescent light!

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Old 03-07-2013, 04:24 AM   #2
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the refractive index of water and wort are dependent on the wavelength of the light source. But the difference is small and you should not notice it. Sometimes I can see a thin rainbow at the transition between blue and white. But it doesn't create a false reading.

I don't see how the wavelength of fluorescent light is that much different than incandescent or sunlight. They all have light in the visible spectrum and that's the only part we see.

ColorpHast strips should not be read in fluorescent light. But refractometers are fine.

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Old 03-07-2013, 04:53 AM   #3
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That would depend on the design of the instrument. The refractive index of a 12 °P sucrose solution is 0.7% greater at 430 nm than it is at 700 nm so presumably if you had an instrument designed for 700 nm it would read 0.7% higher if you used it with a 430 nm light source. But a sensible design would include a filter in the cover or coating on the prism surface to pass only a relatively narrow bandwidth of light i.e. one centered at the wavelength for which the instrument is designed.

The main difference between natural light, tungsten light and fluorescent light is that the first two have continuous spectra whereas fluorescent lights have line spectra (most of the energy is centered at a few wavelengths). Thus the possibility exists that you would have a refractometer designed for one end of the spectrum being illuminated with a light source which has lots of energy at the other end. This would broaden the perceived transition beween light and dark but I don't see how the error would get bigger than 0.7% (e.g. 12.08 °P for a 12 °P solution).

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Old 03-07-2013, 05:59 AM   #4
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I thought the blue prism was used in most refractometers to limit the wavelength of light. Blue is easy to see and also short in wavelength.

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Old 03-07-2013, 11:51 AM   #5
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Little doubt in my mind that this is the reason the prism is coated or filtered. Going to the blue end of the spectrum would make the instrument more sensitive than if the yellow sodium or green mercury lines are used but only by a fraction of a percent.

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Old 03-07-2013, 02:07 PM   #6
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Now thats the kind of responses I like to see! Thanks! A little off topic, but what other things will throw off a refractometer readings? Besides alcohol and temp

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Old 03-07-2013, 02:33 PM   #7
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We've already mentioned scattering (by yeast and trub) especially in intruments which measure actual refraction (as opposed to reflection in a digital instrument). Anything in the solution which isn't sucrose (which is what they are calibrated for). In #58 at
http://www.homebrewtalk.com/f163/don...27/index6.html I've listed refractive index for sucrose, glucose, fructose and inverse sugar. As those data show the change is small but other dissolved things like salts, hops oils... would also have an effect. Can't think of anything else at the moment.

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