In my search for a simple way to calculate ethanol concentrations in beer (of all things!) I remembered back to my college days - in one biochemistry lab, we used a spectrophotometer to determine enzyme reaction rates. Those Shimadzu spectrophotometers cost about $10,000 each, and had an on-board printer to print the resulting graph of transmission vs. wavelength.
Oh, look, one can be built nowadays for cheap using off-the-shelf parts:
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I noticed one part is a 3140 op amp. Is this the one?
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Is there an easy way to do away with the requirement for *two* 9V batteries and use just, say, +6V (from a wall wart, or 4-pack AA)?
Any input would be appreciated. Just imagine what could be done for high schools, using/making one of these in labs!
The problem for you is that that particular spectrometer covers only the visible spectrum, 450-700 nm, and ethanol doesn't absorb anything over that range. Do some research and see if there is anything useful in the near-infrared, where ethanol absorbs and water doesn't, or maybe see if you can use refractive index instead. [pause] Okay, I found some NIR info for you:
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and
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Of course water also absorbs in the nir, here are two references (you will have to handle the wavenumber to wavelength conversion to compare with the ethanol spectra):
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and
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Good luck.
-- Regards, Carl Ijames carl dott ijames aat verizon dott net (remove nospm or make the obvious changes before replying)
Yes, absolutely, ethanol solutions are colorless (except for the added barley and hops). I had thought using the bottom of an AOL CD as a prism would also produce some near IR, but I'd better double-check that assumption.
My browser can't open the pictures for some reason. I'll try it from a friend's computer later this week. The prices are all given in pounds sterling but look cheap.
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This page says that Mouser is going to stop selling this one. Also, doesn't mouser have a minimum total price per order? Anyway, I guess the thing to do is to study the schematics and see exactly how it works and then consider how to modify the design so that different parts can be used.
--
Ignorantly,
Allan Adler
* Disclaimer: I am a guest and *not* a member of the MIT CSAIL. My actions and
* comments do not reflect in any way on MIT. Also, I am nowhere near Boston.
There is no good reason why this opamp is better than many others for this application. I would use half of an LM358 dual. But I would also probably replace the light dependent resistor with a silicon photo diode. This would improve the linearity and still give you a spectral range from about 300 nm to 1000 nm. If you use a gallium arsenide photo diode you can extend the IR end down to something like 1700 nm.
A less complicated one can be built using filters. For a few bucks you can buy a little booklet of transparent plastic filters with known characteristics from Edmund Scientific. Use those to filter the incident light and avoid the mechanics of prism and slit.
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The op amp is pretty generic. There are several that will substitute, like the LM324. If you can't get it from Mouser, try Jameco. Mouser and other wholesalers get pissy when you want to buy one op amp, one battery clip, etc, from them. Jameco's a one-at-a-time kinda place.
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A photodiode will work better than a LDR (cadmium sulfide cell), whose response time will be slow.
A spectrophotometer can be pretty simple or highly complicated. For simple one wavelength measurements, it can be wonderfully cheap, but don't expect Shimadzu quality from the measurements.
The Edmund Scientific ones tend to be a little less sharp in cutoffs than other ones (GAM, Rosco, Lee) in my experience. Also, in general, dye-based filters have gradual cutoffs at the long wavelength end of a passband, making a narrowband slight source with dyed filters close to impossible.
Furthermore, all of these filter gels (Edmund, Rosco, GAM, LEE) have a high tendency to pass infrared. I once took a Rosco booklet, separated the transparent filters from the diffusing filters and the paper pages, fired an infrared LED through them at a phototransistor - a fair amount of the IR made it through all of them!
Most dyes are IR-passing. Furthermore, in stage lighting, it is desirable to minimize absorption of IR to reduce heating of the filters
(all too often only from "burn up soon" to "last a reasonable amount of time before getting brittle and/or changing color").
One can easily do better at spectral analysis with a diffractive "clear CD" that comes in some spindle packs of recordable optical discs, or with LEDs of known peak wavelength. (Bear in mind that some LEDs are rated by "dominant wavelength", which is different and usually displaced slightly towards green-yellow from the peak wavelength.)
How about DigiKey? They merely have a $5 surcharge for failing to make minimum order, as well as many items with pricing down to 1 unit.
Keep in mind a couple of things:
Cadmium sulfides ain't that slow - milliseconds to a fraction of a second.
LDRs and phototransistors have nonlinearities. Photodiodes tend to be linear.
DigiKey helps again here - it helps to have a spectral response of your device. It appears to me that nothing easy to get has flat spectral response. Closest to flat that I noticed so far is in "blue enhanced" silicon photodiodes, which have response through the visible spectrum being roughly proportional to wavelength (a bit less than that aooproaching UV and IR, falling more rapidly in UV as wavelength decreases, and gradually flattening as wavelength increases towards their peak response in IR around 900-950 nm or so). (Makes me maybe glad to not remember what the spectral response of a non-blue-enhanced silicon photodiode looks like!)
At the Tulane chem department, they had an IR absorption spectrometer that they used to check for benzene in the lab ethanol supply. If they got a safe batch, they'd have a party.
Hey, I've probably got a few CA3140's here in cans if you want:-). ISTR I used them as I needed the very high input impedance that was difficult otherwise to obtain in 1975.
SOIC's aren't that hard to solder - you mainly need good light and magnification.
It would real nice if we had an updated, beer, alcohol, and calories, test. The last documented test was many many years ago, which still circulates the internet. I would settle for just the alcoholic content.
Years ago, I was the co-inventor of a simple colorimeter. The reference is Simple Ultraviolet Photometer. R. E. Thiers, M. Margoshes, and B. L. Vallee Anal. Chem. 31, 1258-61 (1959)
If you can't get the publication, I can probably send you a copy. It was designed specifically to measure the coenzyme, NADH. Ralph Thiers developed a list of chemistries for the instrument, which was on the market for some time as the Coenzometer, along with packaged reaction mixes called Determatubes.
There are many enzyme reactions that can be used for various analytes. For ethanol, an assay with yeast alcohol dehydrogenase will work. It is an inexpensive enzyme that is easy to work with. Your students will be painlessly introduced to biochemical analyses.
At the time, Harvard University (where I worked in the Medical School) had a policy against patenting inventions in the field of human health. There is no patent, but there is the reference I gave.
Why not just use specific gravity or polarimetry to measure sugar content and calculate alcohol from converted sugar?
That link you show is somewhat idiotic. An LED is not a white light source but a mixture of primary colors (which you can see with a compact disk used as a diffraction grating).
And he shows in the diagram the light passing through a diffraction grating - that isn't how it is done.
A SCANNING spectrophotometer or just an adjustable wavelength spectro is still a fairly complex instrument and I don't think the average DIY device will do it. They use dual beam balanced instruments to compensate for spectral response of the source and detector and subtract them from the total absorption.
More goes on in the UV range than the visible range when it comes to analyzing chemical compounds.
Build a liquid chromatograph - spectro with a flow cell and separation column added - much more accurate when you can single out the alcohol from the rest of the mixture.
It may be a valid device for explaining the operation of black boxes to chemistry students - demystify the instrument - but a few drawings on a black board and popping the cover on a real instrument will achieve the same thing with better understanding and accuracy.
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