Odd behavior of refractometer

Organic compounds always take longer to dissolve or otherwise react. Bob

And, presumably, that lack of being fully dissolved messes up the reading.

It'd be interesting to know exactly how. I may never need the knowledge, but it'd be a fun tidbit.

Tim Wescott fired this volley in news: snipped-for-privacy@giganews.com:

I'm betting it's microscopic air bubbles messing with the reading. They would have even more diffraction effect than bubbles of oil, which are closer to the refractive index of water than is air.

Water-based coolants have enough surfactants in them to pretty much completely emulsify on contact with water. There's no "organic compounds dissolving", except for the surfactants themselves, which are perfectly transparent, and which go into solution essentially instantly. By themselves, they wouldn't mess with the reading, anyway.

Some coolant vendors caution (on the label) about entraining air into the mix when blending, presumably for that reason; some don't.

Lloyd

It is not the air bubbles. Oil has a higher refractive index than water and air has much lower index.

I think there might be some interesting science going on here. As you note, the surfactant does not actully make the oil dissolve in water; it breaks it up into fine globules to form an emulsion. If you have an emulsion of globules whose size is significantly larger than the wavelength of light, a handheld refractometer will give a reading of whatever component has the higher index, ie the index that gives the highest angle of total internal reflection at the prism. You might even see division inside the bright area on the reticle indicating the index of the water with surfactant. However once the size of the particles in the emulsion approach the wavelength of light, the refraction of the rays will depend on the combined refractive index of both components. What Eric is probably seeing is that is takes some time for the oil globules to break down from several microns in size to several nanometers. If there is a large range in globule sizes, you might see a blurry line.

"anorton" fired this volley in news:aZWdnfMFgNb6BMTPnZ2dnUVZ snipped-for-privacy@earthlink.com:

True enough, but it's the _difference_ in refractive index of the two (any two) which creates bending of the light. Air has a MUCH smaller refractive index than water, and oil less more than water than air is less.

Lloyd

"Lloyd E. Sponenburgh" fired this volley in news:XnsA257A5A20B0C3lloydspmindspringcom@216.168.3.70:

BTW... that's hard to parse, but it's worded correctly .

Lloyd

The above statement makes perfect sense. When the coolant is contaminated with tramp oil it does indeed appear as a fuzzy line in the refractometer. In fact, I use the width of the fuzzy line to give me a rough approximation of amount of tramp oil in the coolant. I can also see how it would take time for soluble oil to form into tiny droplets. The action of breaking up into smaller and smaller droplets has to take some time, especially when the water and oil are both cold. Also interesting is the fact that dissolved sugar and soluble oil seem to have the same refractive index, seeing as how the instrument I have is actually made for measuring sugar concentration. Eric

You think? I still haven't figured it out (parsed it), but I've only tried 4 - 6 times - maybe it will come to me yet.

Bob

A handheld refractometer essentially compares the index of the liquid to that of the high-index prism that you apply it to. The liquid is illuminated at all possible angles by the ground glass, but there is a maximum angle of light that can enter the prism, and this depends on the index of the prism and the liquid that happens to be in contact with the glass. Any refraction or scattering due to bubbles before that does not matter. If there are bubbles in contact with the glass, the light that passes through them will have only a very small maximum angle (probably off scale on the low end). So bubbles will divert some of the light and make the view less bright, but they should not affect the main shadow line.