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Author: Subject: Simplest Colorimeter
aga
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[*] posted on 28-5-2015 at 12:33


Why is path-length important ?

If the LED and sensor are fixed, then the path-length will be the same each time between a Blank sample and the actual sample.




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smaerd
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[*] posted on 28-5-2015 at 12:44


The path length is important in accordance to beer's law.

A = e * b * c where b is the path-length. It only really matters when it comes down to comparing results, or finding e (the molar extinction coefficient).

Don't worry, I won't be publishing in nature any results I find taken by two LED's and a multimeter lol.

I don't really see how it's deceptive though, I mean, I half the concentration I get half the absorbance. I quarter the concentration I get a quarter of the absorbance. It can't be off by that much. Actually I just used a little web app to calculate the P value of the result from the correlation coefficient, the output says,
Quote:
The two-tailed P value is less than 0.0001. By conventional criteria, this difference is considered to be extremely statistically significant.

So I guess my P Value is okay as well. Physical Laws hold today!

[Edited on 28-5-2015 by smaerd]




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[*] posted on 28-5-2015 at 12:45


Quote: Originally posted by aga  
Why is path-length important ?

If the LED and sensor are fixed, then the path-length will be the same each time between a Blank sample and the actual sample.


It improves inter-apparatus reproducibility, among other things.




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aga
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[*] posted on 28-5-2015 at 12:49


So there must be a Standard path-length for a Colorimeter.

Anyone happen to know what that is ?




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[*] posted on 28-5-2015 at 12:51


It doesn't have to be standard, it just has to be known. Most people use 1cm... C'mon Aga, look up the equation and understand it its really easy.



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[*] posted on 28-5-2015 at 12:53


Maths ! Eeek !

I'll just build one and measure the path length.




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[*] posted on 28-5-2015 at 12:57


Quote: Originally posted by aga  
Maths ! Eeek !



Laziness! Eeek!

This is well within your capabilities, Igor. Don't put Master to shame, please?




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[*] posted on 28-5-2015 at 12:59


Egh ! Ugh ! Egh. Ooooh.

Yarm mahrster.

I shall see to it on the morn.




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[*] posted on 28-5-2015 at 14:26


So I made a little amplification stage. It's stable but has a pretty big offset.

It's voltage goes from 3.42V-4.97V which is about a 5x gain on signal size but a considerable off-set. It at least would give decent values for a microcontroller and could easily be improved on. Considering the op-amps cost about 0.25$ USD it's a pretty insignificant cost to factor in.

Also theres an error in my diagram, one of the 7uF cap's should be a 560pF cap.

amp stage.png - 13kB

[Edited on 28-5-2015 by smaerd]




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[*] posted on 28-5-2015 at 15:58


Take back my previous amplification circuit. I changed the op-amp and added a new resistor from ground to the positive input. It now goes from 0-4.56V

Seems stable, should be good for a microcontroller. I'll probably use it for my analog only circuit.

Edit - the R? is 270 ohm

LM358N amp circuit.png - 19kB]

[Edited on 29-5-2015 by smaerd]




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[*] posted on 29-5-2015 at 09:20


I know this kit is more expensive than what is being discussed here, but I thought this link might be relevant in this thread. They have complete plans available as well: http://www.iorodeo.com/content/educational-colorimeter-kit
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[*] posted on 29-5-2015 at 09:41


Ah cool they are using one of the Light To Frequency detectors (http://www.mouser.com/ProductDetail/ams/TCS3200D-TR/?qs=07cg...). I wanted to buy one of those a while back. They are also using the RGB LED idea which is pretty cool.

I like their design and the fact that they give premade enclosing materials and everything else. Very solid, I don't think I'd pay 85$ + an arduino cost for one though. It's cool that they made a shield for the arduino to use their sensors for someone who wants to sort of lego something together.

Unfortunately I feel like a lot of the things they have in their design are just for appeal. Then again look at their soft-ware, its very feature rich and cleanly made. The 85$ price tag is definitely fair, it's cool that someone at home made this. It's cheaper then many other commerical options. Nice link.

I'm going for a 10-20$ build :). It won't have nearly as many output/processing features though.




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[*] posted on 5-6-2015 at 05:14


Quote: Originally posted by blogfast25  
Quote: Originally posted by aga  
Why is path-length important ?

If the LED and sensor are fixed, then the path-length will be the same each time between a Blank sample and the actual sample.


It improves inter-apparatus reproducibility, among other things.


Isn't inter-apparatus reproductibility influenced by the electronic behind and thus by each specific led, electronic part and circuit wiring specificity?
--> Are 2 leds really identical in properties and reactivity?

I think the best is to get standard procedure for calibration for each individual apparatus.

I wonder what would be the result with a UV-Led...a friend of mine showed me one about 8-7 years ago (was expensive at that time)...must be much more democratic by now.
This may require SiO2 cuvettes instead of glass for full potential.

By definition of smaerd principe exposed in the top of this tread UV-Led could be used with any of the 3 Led as detector (RGB) and maybe with a combination of the 3.

If you make a window path perpendicular to the inital beam; then not only absorbance but also emission might be analysed ;).

[Edited on 5-6-2015 by PHILOU Zrealone]




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[*] posted on 5-6-2015 at 05:33


Quote: Originally posted by PHILOU Zrealone  

Isn't inter-apparatus reproductibility influenced by the electronic behind and thus by each specific led, electronic part and circuit wiring specificity?
--> Are 2 leds really identical in properties and reactivity?

I think the best is to get standard procedure for calibration for each individual apparatus.



Inter-apparatus reproducibility is indeed affected by the whole apparatus, so path length included.

Each apparatus has to be calibrated for each specific analysis and frequently too.





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[*] posted on 5-6-2015 at 06:40


PHILOU Zrealone - You are correct in a lot of ways. Not every LED has the same characteristics. So my LED colorimeter may be slightly more or less sensitive in a range of concentrations then yours is. However, if both people take a measurement or a beers law plot on two different instruments, in ranges that are proportional it doesn't matter. Ex, whether the first data point is 0.878mV and 0.645mV doesn't matter so long as concentration is proportional to voltage. Absorbance units are dimensionless because they are only ever used as relative measurements. In fact this is common for all spectroscopy.

As an undergrad one day I was tasked with testing colorimeters that general chemistry students reported as malfunctioning. So I devised beers law experiments with 4 or five kinds of food coloring. Each colorimeter had slightly different values but ultimately functioned acceptably.

I do like your fluorescence idea. I was thinking along the same lines and that is why I selected the bracket that I did to hold the light sources. I was going to use my 9W UV lamp as a source rather then a UV LED though. Edit- Actually the cuvette you see me using in the OP is a quartz cuvette I purchased a long time ago to experiment with fluoresence and light scattering.


I wish I could edit my latest post in this thread. The schematic I drew up there isn't working as intended. It amplifies, but is insensitive to change or really any kind. So I'll be updating that once I get time.

[Edited on 5-6-2015 by smaerd]




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[*] posted on 6-6-2015 at 11:16


Quote: Originally posted by smaerd  
So I made a little amplification stage. It's stable but has a pretty big offset


Try using the AD8221, and a higher quality capacitor in the feedback loop, it may improve your circuit.

Quote: Originally posted by aga  
Why is path-length important ?

If the LED and sensor are fixed, then the path-length will be the same each time between a Blank sample and the actual sample.


The sample itself will alter the actual path length compared to an empty cuvette by virtue of its Refractive index. Also this factor can bend light out of the path lowering the incident light on the detector. I would rather use something like the BPW34 instead of a red LED as a detector in the circuit. Since total flux incident on the detector is important I wonder if putting the detector at the focus of a parabolic reflector would improve the system. I was thinking about the differing refraction of various samples shifting light away from the detector (by differing amounts depending upon sample) on this last point.




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[*] posted on 6-6-2015 at 11:18


Build one IrC !

Build. Build. Build.




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[*] posted on 6-6-2015 at 11:29


http://www.ebay.com/itm/Fire-Starter-Lighter-Parabolic-Solar...

What do you think, could this reflector be hardware hacked to work?




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[*] posted on 6-6-2015 at 11:54


Thou jesteth surely ?



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[*] posted on 6-6-2015 at 12:39


I was considering the dual use since the apocalypse is almost upon us. How long can we trust the power grid we use to cook what we catch while living in the wild. When not performing experimental measurements we could be fishing.




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[*] posted on 6-6-2015 at 13:11


Farming on a small scale can be done in all situations.



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[*] posted on 6-6-2015 at 14:12


Quote: Originally posted by aga  
Farming on a small scale can be done in all situations.


One turnip at a time.




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[*] posted on 6-6-2015 at 14:38


Anyway aga I was thinking of a large cell using a better detector, higher precision circuitry with lower offset. The parabolic reflector would work for that. Say in a situation where steady flow of high volumes through a (large) cuvette section which measures any changes in production. Gasoline refinery or something similar. That reflector could be taken apart and used to focus light on a BPW34 detector where monitoring of any changes in composition was required. Whats wrong with that idea?




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[*] posted on 9-6-2015 at 13:20


IrC there is nothing wrong with that idea. I just gave your idea shot using a LTC1050 op-amp and a BPW-34 photodiode with ~10x gain in TIA mode. (No parabolic reflector or long sample path though). My +/-1% accuracy digital voltage display(~3$) gave me another nice linear fit (R^2 = 0.997) with a populace of 4 samples! The voltage output is a little low(0-1.25V), but the BPW-34 isn't too sensitive to green light (mostly for red light).

I'm gonna build this thing a little tupper-ware container house, put in a trimmer potentiometer(so I can raise the gain a liiitttle more or lower it for longer wavelength light sources), and solder up the circuit

Do you know of any cheapy photodiodes like the BPW34 that cater more to the mid-visible range? I'd really like to increase the sensitivity for non-red sources.

Edit - also I had a really hard time reverse biasing the LED to do anything other then be a schmitt trigger(0V or 5V almost only).

[Edited on 9-6-2015 by smaerd]




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[*] posted on 9-6-2015 at 20:40


If I had to choose I would try the S5821Si PIN photodiode from Hamamatsu.

http://www.hamamatsu.com/eu/en/product/category/3100/4001/41...

From the above link the spectral response:

341-667-k_s5821_sr_xx.jpg - 35kB

From the graph the green is roughly half the IR but still a decent choice.

This one may be even better, S10784.

http://www.ebay.com/itm/Hamamatsu-S10784-Silicon-PIN-Photodi...

Attachment: s10783_s10784_kpin1079e01.pdf (2.7MB)
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Attachment: HamaNews_0208.pdf (4MB)
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http://hamamatsu.advante.ru/catalog/997/1001.html

This page is useful since you can quickly scan specs and device number.



[Edited on 6-10-2015 by IrC]




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