Calibrate DIY chlorine color tester

[DaveOB]

I have almost completed an electronic water color tester ( DIY unit using an Arduino board, LCD display and color sensor ).

Once done, I will need to calibrate it with a few known control samples, so that I can get accurate readings at different levels, and then use these to calculate the reading of the water being tested.

I will be using the local HTH tablets that get dissolved into a set volume of water, and changes color based on the FC level. I have never been very good with comparing the water color to the printed color chart, and I feel that the background light, etc have a great effect on the visualization and personal interpretation of the color - not to mention that everyones eyesight is different.

Once I have this mastered, I hope to be using it to accurately read the pH as well.

So I want to prepare 6 different FC control samples ( 1ppm, 2ppm, etc )

Does the following calculation look correct :

The online pool calculator tells me to add 16g of 65% cal-hypo to 10,000 liters of water to raise the FC to 1ppm.

This makes the ratio :
16g : 10,000 L
= 0.16g : 100 L
= 0.0016g : 1 L

Naturally I want to make a small sample ( 1 L ) for the calibration tests, but I can not simply measure 0.0016g of cal-hypo - too small an amount to measure accurately.

My thinking is :
16g cal-hypo into 1 L ( 1000 ml ) water ( all water used is reverse osmosis water as I assume this should have the least contaminates ).
Ratio is 16g : 1 L

Take 10ml and add to 990 ml water ( to make 1000 ml )
Ratio is now 0.16g : 1 L

Take 10ml and add to 990 ml water ( again to make 1000 ml )
Ratio is now 0.0016g : 1 L

If this is correct, the FC reading should raise by 1ppm for every 10ml of solution added to the 1 L of calibration water.

 

[jblizzle]

While I appreciate the entertainment value of working on this and creating your own tool, you would be able to get a much better idea about your FC and CC levels (which you made not mention of, but are important to watch for) by obtaining the FAS-DPD chlorine test. This does not require color matching ... just counting drops until the water turns clear.

There is going to be entirely too much error in trying to create a sample to accurate calibrate your tool ... and the solution sample will not last. You will have to make it every time you want to try to calibrate.

 

[DaveOB]

Hi Jason

FAS-DPD is not available in our country, except for 1 or 2 companies that import, usually at about 500% import mark-up.

I do realise that my idea is not as good as FAS-DPD, but it is the best that I can think of at the moment.

Also, at a future point, I want to fully automate the color reading as the Arduino will be in full control of my pool maintenance.

 

[jblizzle]

Also, at a future point, I want to fully automate the color reading as the Arduino will be in full control of my pool maintenance.

Even the big pool companies have not been able to get this to reliably work. They try to measure ORP in the water to be a substitute for measuring the chlorine level, but it does not work when you add the stabilizer levels that are needed for outdoor pools.

I just have my doubts about ANY automated color matching. Some pool stores have very expensive testing computers that try to do this, but if they are not maintain and calibrated, they are not accurate ... and with any of them, there is no real way to know if it is giving you the right level or just giving you an arbitrary value.

I saw today that someone in Zimbabwe was just able to import a test kit ....

I am interested in seeing what you are able to come up with though

 

[DaveOB]

Thanks for the reply.

I fully appreciate and respect your opinion, and do agree with you with regard to the complexities and challenges that I am likely to face to get this to work reasonably well.

But as you said earlier : appreciate the entertainment value of working on this and creating your own tool

and to a large degree, that is exactly what this is about - and also the fact that a few times in the past 30 years of my career I have been told many times that what I want to do is simply not possible and will never work. And many times they were right. And many times I tried ( which 'they' did not do ). And many times I failed. But just a few times I succeeded in such spectacular fashion that the experts in the chosen field had to apologize and admit they were wrong.

My point : I will always try. I will fail often, but it will not stop me from trying something else.

So, in order to give me a reasonable chance, and to later be able to say that you were part of the solution, do the numbers in the original post look correct ?

 

[Pommezimbo]

Howzit DaveOB from just a few "clicks" north of you in Harare.

Let me start by saying first off, I feel your pain!! Trying to get a reliable way to test pool water "round these parts" for a reasonable price is nigh on impossible.

However, I managed it! I am the guy jblizzle is mentioning. So there is hope!! It required some truly Awesome help state-side, and was still a bit of a PITA to be honest for both parties involved, but, it can be done. The only real cost for me, was postage. It will most likely be a bit more pricey for you however sadly, due to the $ to R conversion, but still MUCH cheaper than what's currently on offer. Of course Zim, as I am sure you know, uses USD as it's primary currency, so it didn't sting toobadly in total . My reasoning was, I had tried ALL other methods short of making my own system like you are attempting. Therefore I knew in the long run it would pay for itself-Both in saved chemicals and less headache's!! Of course I made sure I got as many refills of reagents as I could, to make it worth while. The whole process took about maybe 6 weeks from TFTestKits HQ, to my poolside. Not bad at all.

I know the HTH kit you have, I have had several. Sadly I have to tell you, in my experience and opinion, the thing is a pile of you-know-what for the chlorine test. On the flip side at least it's pretty good (probably the best available here) for PH and TA. I have confirmed this now with my TF kit. I don't mean to gloat or anything about having a decent kit, I just remember the months of frustration with what's available on the Southern African Market. The main problem with the HTH kit in particular is the color block (again in my opinion) just doesn't relate to the actual colors the tablets produce. As I say, I feel your pain! Add that to the fact it bleaches out when the Chlorine level gets to high and well..... I don't think I need to go any further! I found I had much better luck with the OTO based tests available. At least with that one you can sort of get higher readings by diluting the pool water, as described on this site, to achieve higher readings. AND once you get good at it, the OTO shows you Free Chlorine (Albeit a very rough indicator) for a few seconds before drifting up to Total Chlorine.It's not great, but its not terrible either.

Hey, at least you guys get stuff cheaper in Durban than here! EVERYTHING pool related in Zim comes on a truck from South.... As I am sure you probably know! This results
in a simple Ph ONLY Drop based test costing upwards of $15.....! I have good friends and in-laws in Durban, all over SA in fact, so trust me whenever my Wife and I go visit we have a very very long shopping list (and an empty suitcase!) Of course the opposite applies when people come visit us. Things are certainly better here than 2008 for sure, but there is still a way to go before true economies of scale start taking effect for non-staple grocery items.

If you want to know more about how got the TF100 here, please don't hesitate to PM me :-D (It's breaking TFP rules to publicly post methods of getting TF kits from outside the states as can get the site owner into trouble...)
I will very happily help/ advise you on how to get one, if you wish to take that route.

I know your main goal is to get your Arduino working. From the info you have posted however it seems this will be FAR easier a task to calibrate reliably, with a trustworthy set of numbers. And then hey, you never know, you may well end up being the first person to make a reliable device of this nature for a domestic pool! I too have been told by companies and people in the past, many many times, that my ideas are going to fail. And like you I have also experienced those people being sadly correct many times. But also like you I have experienced success in the face of adversity.... I wish you the best of luck! Sadly I can't help you with the scaling down of the DPD test to smaller volumes.

P.S. Another option is to make your own FAS/DPD test to calibrate with..... Actually not such a hard thing to do as it turns out. It's something I am currently playing around with as a side project. I am looking to one day post a full step-by-step guide on here, on how to do this with pics. My way of hopefully giving back! Especially to the people such as You and I who are in the same situation around the world. At the end of the day it's FAR easier not to mention cheaper to buy one from TFTestKits or Taylor direct if you are so lucky as to be on their shipping list. BUT if you are not so lucky or don't have access to an affordable ready made equivalent, the "home-made" version is a viable option. The raw materials needed are readily stocked by many chemical plants, and the process not overly complicated. Other members on this site have done so with success. Chemgeek has kindly helped me so far with this, by advising me of the process and the materials needed. Again if you are interested in the advice he has sent me so far, you can either check my threads on my page, or I can copy and paste to you in a PM. Being the sort of person who is undertaking the project you currently are, I am sure you will have no problem making your own FAS/DPD!

P.P.S. Just in case it might interest you, my pool is fully automated on the chemical dosing side of things with an extremely cheap, low-tech device I made by using the info on this thread:

http://www.troublefreepool.com/homemade-acid-or-chlorine-injection-system-t4174-40.html

I aim to post pictures when I have cleaned up my pad (currently in the middle of a pressure side re-plumb, my latest unexpectedly expensive headache ), this setup works perfectly at keeping my FC within range for 30 days. Currently I am fine-tuning the dosing with my TF100. Add that together with my pool having 50ppm borates and stable TA to buffer the Ph (meaning I only need maybe 250ml of acid every six weeks) my pool is virtually maintenance free!!
Sorry to ramble on, :blah: I just had a feeling when I saw this thread title come
Up on recent posts this morning, that you would be based on the same continent!
As I say, please PM me if you would like further info, I hope I have helped. Us TroubleFreePooler's on this continent are a rare breed and need to help each other with the "unique" difficulties we face when we can! At least thats what i think :-D .Otherwise please feel equally free to ignore everything I have written if you feel so inclined!
Cheers from Zim.

 

[duraleigh]

If you want to know more about how got the TF100 here, please don't hesitate to PM me :-D (It's breaking TFP rules to publicly post methods of getting TF kits from outside the states as can get the site owner into trouble...)

Well, that's not very accurate. TFTestkits simply does not have the resources (regulation compliance, labeling) to ship products anywhere but within the 50 states and US territories. Period.

There is no collusion on the part of TFTestkits to facilitate international shipping.....we simply don't do it. Everything we ship goes to a valid address within the states or territories and TFTestkits has no control whatsoever as to what that addressee might or might not do with it.

 

[Pommezimbo]

Oh, ok cool. Well that makes things easier then! I could have sworn I read a post on here somewhere, where a guy was asked not to discuss getting a kit outside of the US. Specifically the way's in which one needs to split the kit up to avoid paying ridiculous import tax/ having the whole thing being confiscated, because that country had huge restrictions on foreign products to force only local products to be purchased... My bad
Sorry for posting incorrect information, I will make sure I check and re-check next time.

 

[Pommezimbo]

Ok after doing some searching, come to think of it I can't think where on earth I got the idea that shipping options prices methods etc are not allowed to be discussed on the forum!!! Honestly I can't even think which forum I read that on.....
Must be losing the plot as I was seriously convinced of this!! Once again sincere apologies. All I know is that to get them into Zimbabwe itself without problems, was an ordeal. But then it was an ordeal to get my renewed driving license here in the mail let alone anything else.... I won't bore any one with the details!! Dave, I am sorry for doing you guys a dis-service. Third hand all my dealings with TFTest kits and first hand with TFP and its members have been nothing short of excellent. Anyway this is now off topic, and feels like hi-jacking the topic. Best of luck with the Calibration, I can highly recommend a FAS/DPD kit to do so.

 

[DaveOB]

Well, it's been a few days since I last posted, and I finally found some time tonight to get the test wiring completed.

As a side part to this water test project, I have added the following :
I am using an Arduino Uno R3 board ( with 16 x 2 LCD screen ), and connected 2 one-wire temperature sensors ( DS18B20's ). The sensors themselves are in waterproof piping and plumbed into the 'feed' and 'return' pipes for my solar coils. The aim here is to use stepper motors to adjust the ball valves to restrict water passing through the coils when the coils are not producing any heat ( raining, overcast, etc ). Accuracy of the sensors is 2 decimal places so I am very pleased with the results. I have the sensors buried in clear 6mm ID fish tank hosing and encased in waterproof silicon. The sensors obviously only detect a temperature change when the silicon changes temperature, but the few seconds that they take to register any change is certainly not critical for this application. Now just to add the stepper motors to the ball valves.

On the chlorine color tester, I am really excited about what I think is positive progress :

I found some glass test tubes online that fit almost perfectly in some cheap PVC electrical conduit ( 20mm ID conduit ). So I have made a 'cross' of pvc piping. The test tube fits in the vertical section. The left side tube ends in a rubber cap that contains an RGB LED. The right side ends with a cap containing the color sensor.
Once the test tube is in, and a third ( loose ) cap placed over the top, the complete unit is light-tight, which I think will make the reading a lot more easier to use.

As a test, I have used a test tube of clear water and run the sensor calibration ( takes less than 1 second ) so that any slight discoloration in the base water does not affect the reading.

Next I added a few grains of my favorite ground coffee to the test tube and pressed the 'read' button. LEDs turn on, and a reading is displayed within half a second.

The readings were : RGB : 580 : 403 : 181

Now considering that the reading range ( from the sensor ) is 0 : 1023, I had to scale them back to the conventional 0 : 255 range to be able to check / verify the color on my PC.

Red : 255 / 1023 * 580 = 145
Green : 255 / 1023 * 403 = 100
Blue : 255 / 1023 * 181 = 45

Looking at my color picker on my PC, the RGB value of 145:100:45 is amazingly close to my tube of cold coffee. ( obviously affected by PC monitor settings and human perception of the displayed RBG color on the screen )

So the next stage of the project is to create a set of control solutions, from 0 to 10 ppm, in 0.25ppm increments. Once I have a color reading from each of these ( with the HTH chlorine tablets in each ), and added to the code, it should be very easy to have my LCD on the Arduino display an accurate chlorine reading, with an accuracy of around 0.25 ppm.

Next step would be to get some control samples for different pH levels, and repeat the process as above. I think that the local scientific labs might be able to help with this. My thinking is to still use the HTH test kit for the Chlorine Neutralizer, and pH test solution, but to use the new color sensor instead of the 'human eye to printed color chart' method.

If anyone is interested, I will provide more feedback once I have progress.

 

[jblizzle]

Interesting progress. I am still not sure you will be able to differentiate to 0.25ppm. Getting different readings for clear vs brown vs red, etc is possible. But, I am not sure how well it will reliably differentiate between shades of pink (DPD test) or yellow (OTO test) which are imperceptible to the human eye.

Actually that raises a question ... is your HTH chlorine test the pink DPD test?
If so, here is a problem you may run into. As the FC level gets higher and higher, the pink color can start to bleach out and go back to clear. So for a given shade of pink, there would be 2 possible FC levels. And if it is clear, that means either you have 0ppm of FC of very high FC levels.

Using a yellow OTO test might work a little better as the color goes from clear to yellow to darker yellow to orange to darker orange to brown at high FC levels.

But, if you are limiting it is 10ppm, then maybe this won't be a problem.

BTW this is not correct:
"Accuracy of the sensors is 2 decimal places so I am very pleased with the results"
2 decimal places is more like the precision of the sensors ... you have no idea of the accuracy as compared to truth.

 

[DaveOB]

Actually that raises a question ... is your HTH chlorine test the pink DPD test?
If so, here is a problem you may run into. As the FC level gets higher and higher, the pink color can start to bleach out and go back to clear. So for a given shade of pink, there would be 2 possible FC levels. And if it is clear, that means either you have 0ppm of FC of very high FC levels.
Using a yellow OTO test might work a little better as the color goes from clear to yellow to darker yellow to orange to darker orange to brown at high FC levels.
But, if you are limiting it is 10ppm, then maybe this won't be a problem.

BTW this is not correct:
"Accuracy of the sensors is 2 decimal places so I am very pleased with the results"
2 decimal places is more like the precision of the sensors ... you have no idea of the accuracy as compared to truth.

Hi jblizzle

The HTH test tablet does produce a pink color in the water, so I am assuming it is DPD.
I am going to use the results to adjust the ( next project ) automated bleach feeder ( based on the color test results ) so am not expecting FC levels to be too far away from my target of 4ppm ( CYA level at about 35ppm ). If I do get a clear result from adding the DPD tablet, then maybe I need to get my test strips out to confirm if the reading is zero or too high, 'cos then something has gone seriously wrong.

The temperature sensor actually produces much more decimals ( I think it was 4 or 6 decimals ) but I have rounded it to 2 decimals. You are however correct that I can not verify the accuracy of the reading, but then I suppose it is often not possible to verify certain readings from electronic devices - we just assume that they are 'accurate'. I have been using these sensors for other projects in the past, and have found then very reliable. A test of 4 sensors at the same location gave me identical readings ( identical to 3 decimal places ) so they seem to be reasonably stable. My main usage is that if coils are cooler than the feed, then divert the feed directly back to pool and do not pass thru the coils. For this purpose, the accuracy of the sensors will be more than adequate.

For the calibration tests, I am thinking to start with distilled water, and create samples with known ppm values. Once I have the first 2 samples, say 2ppm and 2.25ppm, I will be able to take readings and see if there is sufficient difference in the readings to be able to accurately identify the relevant sample.

 

[jblizzle]

Careful as I think test strips can bleach out at high FC levels as well ...

 

[dschlic1]

Several issues you should be aware of when trying to calibrate a chlorine analyzer. First you should use a GREEN light source, a green LED is fine. The reason is that green color is absorbed by the red of the DPD. Much more sensitive measurement system. The second issue is that the light transmission is not necessarily linearly related to the chlorine level. Because of this you will have to program in a curve to fit the transmission curve. The third issue is that chlorine calibration solutions are very unstable. A possible alternation is to use potassium pomegranate instead of chlorine.

 

[DaveOB]

Several issues you should be aware of when trying to calibrate a chlorine analyzer. First you should use a GREEN light source, a green LED is fine. The reason is that green color is absorbed by the red of the DPD. Much more sensitive measurement system. The second issue is that the light transmission is not necessarily linearly related to the chlorine level. Because of this you will have to program in a curve to fit the transmission curve. The third issue is that chlorine calibration solutions are very unstable. A possible alternation is to use potassium pomegranate instead of chlorine.

Sincere Thanks for the advice. I had read about the green light issue before, and think that I have taken that into account in the procedure below.

To test the green light theory, I will be making 4 sets of readings for each stage of the calibration tests. First a reading ( actually all readings are to be an average of 10 readings ) with all RGB colors turned on for the LED, and thereafter the average with each R, G & B individually.

At the end of the calibration testing, this should give me sufficient data to determine the best lighting method, although I also suspect that the green alone will be the best result.

The curve should also not be a problem as I was thinking to calibrate in 0.25ppm increments. Any reading that falls between the values of, say 2ppm and 2.25ppm can be calculated at being 2ppm + a percentage of the difference in the calibration color readings for 2ppm and 2.25ppm. This way I should never be out by more than 0.25ppm.

The control samples will be made with distilled water, and I used the following below to run some initial tests this evening.

The result of the 0.5ppm and 3ppm control solutions were remarkably similar to the printed color chart on the HTH test kit ( using the HTH DPD no 1 tablets ).

What I did not like was the fact that a solution with a 'known' value of 2ppm looked visually very close to the printed chart's 3ppm color. As I am in control of the formula for the control samples, I have to seriously doubt the accuracy of the printed color chart - and coming from a commercial printing background many years back, I have good reason to doubt the printing.

I made the control sample as follows :

Pool Calculator says use 3.5% ( our household brand strength ) bleach : 278 ml : 1000 L water = 10 ppm.

1000 L = 1,000,000 ml

legend:
W = Distilled Water
B = Bleach ( 3,5% )
C = Chlorine
X = Solution 1 ( first intermediate solution )
Y = Solution 2 ( second intermediate solution )

1,000,000 ml(W) + 278 ml(B) = 10 ppm

I need to reduce the water to a smaller volume, but have the bleach easily measurable, so :
500 ml(W) + 55.6 ml(B) = 2000 ppm
= 0.002(C) parts per 1 ml(W) ( this is now solution X )

250 ml(W) + 4 ml(X) = 0.008(C) parts in 250 ml(W) = 32 ppm ( solution Y )

250 ml(W) + 2 ml(Y) = 0.000064(C) parts in 250 ml(W) = 0.256 ppm
250 ml(W) + 4 ml(Y) = 0.000128(C) parts in 250 ml(W) = 0.512 ppm
250 ml(W) + 6 ml(Y) = 0.000192(C) parts in 250 ml(W) = 0.768 ppm
.. .. every additional 2 ml(Y) increases the control sample 0.256 ppm

If the volume of (Y) is reduced to 1 ml increments, the control sample changes to increments of 0.128 ppm, but I think this is simply of no real use.

I really can't see the stability of the control solution being an issue. It is being made, and used, only once in a short period of time ( 30 minutes ). Once I have the data into the program, it should not need recalibration unless either the LED or the color sensor is changed. The fact that I an zero'ing the sensor with a water sample before adding the DPD, and the color sensor range is 0 : 1023, it should take into account any minor fluctuations in variables.

 

[dschlic1]

What colorimeters actually read is % transmission. So you have a light source and a sensor, and with no color you have 100% transmission, and with total blockage you have 0% transmission. While 100% transmission will correspond to 0% chlorine, and say 10 ppm chlorine corresponds to 50% transmission, 75% transmission may not correspond to 5 ppm chlorine. It my very well be 8 ppm chlorine. The relation between % transmission and ppm chlorine will not be linear and you will have to fit that curve in you instrument.

 

[DaveOB]

What colorimeters actually read is % transmission. So you have a light source and a sensor, and with no color you have 100% transmission, and with total blockage you have 0% transmission. While 100% transmission will correspond to 0% chlorine, and say 10 ppm chlorine corresponds to 50% transmission, 75% transmission may not correspond to 5 ppm chlorine. It my very well be 8 ppm chlorine. The relation between % transmission and ppm chlorine will not be linear and you will have to fit that curve in you instrument.

Hi dschlic1

I fully understand and agree 100% with your advice and info.

However, I am thinking that my pre-measured control samples and data will give enough accuracy for this application.

For example : ( simple numbers used to keep the example easy for others ) :

The color sensor can output values in the range : 0 : 1023

My known value controlled reads are :

2,00 ppm = 500
2,25 ppm = 550
2,50 ppm = 610
2,75 ppm = 670
3,00 ppm = 730

Once I have all this data in my code, and I do a pool water test with a reading result of : 595

I can than use the following calculation :

595 - 550 = 45
610 - 550 = 60
45 / 60 = 0.75
2.25 + ( 0.75 x 0.25 ) = 2.4375 ppm

Agreed that the real value, because of the curve effect, may be 2.41 ppm, but for what I intend using it for, it really does not need to be that accurate. I just want to know the 2ppm is at 2ppm, and not at the 3ppm that the printed color reference chart says it is.

Based on the above, I think that the key to success of this method is using known control values at small increments.
If my control values were only measured at, say, 0.5ppm , 2 ppm , 5 ppm and 10 ppm then I would have to admit that the curve would have a big effect on the results.

 

[DaveOB]

Update on progress

Today I completed the enclosure ( light tight for the sensor section ) and coded the Arduino and LCD screen.

I used distilled water and household bleach to create a control solution, and created 18 sample strengths between 0.256 ppm and 6.678 ppm.

After calibrating the color sensor with the test tube of distilled water, I started taking readings of the different strength samples, each with a DPD 1 tablet in 13 ml of solution ( time from adding the tablet to taking the readings was same for each sample measured ).

I took readings with all 3 colors of the RGB LED on ( all on at the same time ), as well as readings with each color of the LED on separately.

In all cases, the recorded value was an average of 10 readings with a 100ms delay between readings. This process was repeated 3 times for each sample, and then the 3 readings again averaged. So each reading is an average of 30 separate readings.

My findings are that the color with the best result for the DPD1 color was the Green LED ( as previously mentioned earlier in the thread by dschlic1 ), and that it was best to read with only the Green LED lit by itself.

Results were great until sample 13 ( 4.479 ppm ). After this point, the Green color readings seem to become erratic and I assume that there must be either :
1. some bleaching of the color, or
2. limitation of the DPD tablets used, or
3. it is outside the range of the sensor that I am using.

Whatever the reason, I am not overly concerned as I am looking at keeping the FC levels at around 4ppm, so am expecting readings to always be below that value.

My results, if anyone is interested, are :

#___Green Val__ppm
1___749.00___0.00000
2___578.00___0.25600
3___455.33___0.49869
4___326.00___0.85676
5___255.67___1.19620
6___191.67___1.51800
7___160.00___1.95107
8___113.67___2.36161
9___108.33___2.75081
10___91.33___3.11976
11___86.67___3.59754
12___65.33___4.05047
13___53.33___4.47984

After coding the board, I was able to run a test with pool water late afternoon.

A Green reading of 390 gave me 0.68 ppm. Not unexpected as we ( 4 ppl ) have been in the pool for around 3 hours today, air temp 30 C and water 31 C. Very hot sunny day so expected the chlorine to be burnt out.

I then added 200g cal-hypo and waited an hour before testing again. A new Green reading of 64 = 4.098 ppm, which is just about spot on what I expected to see.

On a side note : when I was doing the calibration, I was also looking at the printed color chart on side of the HTH test vial, that is usually used to visually compare the color of the water. The color of the 1.5ppm water was almost the same as the printed 3ppm color, and visually there was very little change in the color of the samples from 2.5ppm to 6.5ppm. There is absolutely no way that a manual comparison to a printed color chart can be made accurately. This has re-enforced my belief that the visual color comparison method is completely useless ( the 'visual' part, not the DPD part ) and about as much good as a test strip. The electronic color sensor is far more accurate at seeing color than my eyes are.

 

[Mike_W]

Sounds like a cool project.

Color matching need not depend on light transmission. My wife recently took a cushion from our sofa to a local paint store. They used their color matching machine to create a custom colored paint, that for all practical purposes, was an exact match to the cushion. I not familiar with how their technology works, but you might want to look into it.
Mike

 

[DaveOB]

Thanks Mike_W

Reason for using the transmission lighting is that the color sensor board only has a white LED for reflective lighting, and I needed a green light for this application.

I also think that the transmission thru the solution in a light tight container will give more accurate / consistent results than light reflecting off the glass of the test tube. Just a gut feeling I had ( done no scientific research to back that up ).