KH Guardian(KHG) Alk calculation

[Jonty]

There is another thread on the forum discussing a new automated Alkalinity monitor/controller https://www.reef2reef.com/threads/would-you-invest-a-grand-for-a-kh-guardian.276668/
. The premise of the unit is the ability to calculate Alk from Ph. I would like to know how this works given Ph swings in our aquarium with constant Alk given Co2 swings ext?

 

[Randy Holmes-Farley]

If that device works like the Mindstream, it measures CO2 and pH, so can properly calculate alkalinity.

 

[JonasRoman]

There is another thread on the forum discussing a new automated Alkalinity monitor/controller https://www.reef2reef.com/threads/would-you-invest-a-grand-for-a-kh-guardian.276668/
The meof the unit is the ability to calculate Alk from Ph. I would like to know how this works given Ph swings in our aquarium with constant Alk given Co2 swings ext?

The KH-guardian uses the titration-method with acid. the other methods of measuring CO2 and pH and calculate is expensive and inaccurate.

 

[Randy Holmes-Farley]

The KH-guardian uses the titration-method with acid. the other methods of measuring CO2 and pH and calculate is expensive and inaccurate.

It may be expensive, and may be technically hard to implement at hobbyist prices, but measuring alkalinity via CO2 and pH is a well established scientific technology that works very well.

 

[Randy Holmes-Farley]

If it works by acid titration, then changes in the CO2 level won't have a big impact on the measured alkalinity. No more so than it does with an alkalinity test kit.

 

[JonasRoman]

It may be expensive, and may be technically hard to implement at hobbyist prices, but measuring alkalinity via CO2 and pH is a well established scientific technology that works very well.

Yes i know that and agree...my comment was correlated to this hobby-scenario..i ment by inaccurate according to the price...you can hardly not for a reasonable price do a machine which will measure pH and CO2 with required accuracy for this price...only that was my point. When i was calculating on this, please correct med if i remember wrong, is that with that method you have to measure CO2(aq) with 5 ppm accuracy and pH with 0.02. Do you agree?

 

[Randy Holmes-Farley]

Yes i know that and agree...my comment was correlated to this hobby-scenario..i ment by inaccurate according to the price...you can hardly not for a reasonable price do a machine which will measure pH and CO2 with required accuracy for this price...only that was my point. When i was calculating on this, please correct med if i remember wrong, is that with that method you have to measure CO2(aq) with 5 ppm accuracy and pH with 0.02. Do you agree?

To get what accuracy? To match or beat a pH titration kit using a dye, you certainly do not need 0.02 pH accuracy. The curve of pH with added acid is quite steep at a pH in the low 4's in seawater, so the exact pH endpoint is not very sensitive.

I discuss those issues here:

A DIY Alkalinity Test: By Randy Holmes-Farley - REEFEDITION
https://www.reef2reef.com/blog/a-diy-alkalinity-test-by-randy-holmes-farley

In my tests, an uncertainty of 0.7 pH pH units in the endpoint caused a 7% change in determined alkalinity (or 0.5 dKH out of 7 dKH). Most pH meters can get a lot closer than that.


Figure 1. A pH titration of pure water and water from my reef aquarium using 0.1 N HCl.

 

[JonasRoman]

Randy, we are talking about two complete different things! I totally agree with you, I have done a lot of titration exp and find the same as you, if i will "pass" the endtitrationspoint with about 0.05 pH unit this will only lead to a false high dKH of about 0.05dKH. And this is of course due to the steep in the curve around pH 4.2, so long as we are talking to the situation when we titrate with a strong acid. Completely agree that we do not need a more accuracy in pH-meeting than 0.1 maybe, when we are talking about titration method.

I ment the method when you are measuring CO2(aq) AND pH and the with math caclulate. THEN the difference of pH will affect KH calculation a lot. Look at this equation which is the base of calculating true KH based on CO2(aq) and pH. You can see that there needs no much alter in pH to give big difference in KH. Therefore i think this method is out of choice concerning us. That was my point. we were talking about different methods Randy.

 

[Randy Holmes-Farley]

Thanks, Jonas. I agree, fairly precise measurement is needed in that case.

I haven't worked through that one in detail to see the relationship between pH measurement and the alk this way. Did you plug that into a spreadsheet and look for the variation with pH changes? Assuming the left side predominates at tank pH (where there is less alk due to carbonate), the alk is inversely related to H+, so a change of 0.3 pH units is very roughly a factor of 2 in alk.

 

[JonasRoman]

Thanks, Jonas. I agree, fairly precise measurement is needed in that case.

I haven't worked through that one in detail to see the relationship between pH measurement and the alk this way. Did you plug that into a spreadsheet and look for the variation with pH changes? Assuming the left side predominates at tank pH (where there is less alk due to carbonate), the alk is inversely related to H+, so a change of 0.3 pH units is very roughly a factor of 2 in alk.

I agree, very smart way to do an estimation. I did some plots yes, thus, with a constant CO2(aq) plotted different pH and found according to this equation that very small alterations in pH reading got quite big difference in alk(as I said and if I remember correct I think it was around 0.02 you had to achieve to get reasonable accuracy...i cannot find my diagrams right now) , and also found that the accuracy of measuring CO2(aq) was important, where I found(if i now remember correct) that you at least need an accuracy of 5ppm of CO2 reading. When I checked the market that type of sensors was very expensive. Only the sensor costs around 1000Euro (maybe some know more cheap but i found none).

 

[JimWelsh]

The spreadsheet found here might help: http://cdiac.ornl.gov/ftp/co2sys/CO2SYS_calc_XLS_v2.1/

And Jonas is right that you need to know both pH and CO2 with great accuracy in order to be able to calculate alkalinity this way. A change in pH of only 0.001 pH units causes a change of about 0.03 dKH, and a change in CO2 of 1 PPM causes a change of about 0.025 dKH (assumptions: S=35 T=25C pH=8.20 CO2=400).

 

[JonasRoman]

The spreadsheet found here might help: http://cdiac.ornl.gov/ftp/co2sys/CO2SYS_calc_XLS_v2.1/

And Jonas is right that you need to know both pH and CO2 with great accuracy in order to be able to calculate alkalinity this way. A change in pH of only 0.001 pH units causes a change of about 0.03 dKH, and a change in CO2 of 1 PPM causes a change of about 0.025 dKH (assumptions: S=35 T=25C pH=8.20 CO2=400).

Thanks Jim for the more precise Numbers and the spreadsheet

 

[reef_ranch]

How accurate are the lab grade pH meters available in the hobby?

After posting that I did a bit of research, looks like our probes are accurate only to .1 pH, not nearly good enough to use to calculate Alk (even assuming an accurate CO2 measuring device).

 

[JimWelsh]

At the laboratory where I work, where pH in wine is one of our most common analyses, and considering that the wine matrix is so heavily buffered that it is very easy for our clients to determine pH on their own with great accuracy, our laboratory control limits are in a warning condition at +/- 0.02 pH units, and the controls are considered "out" at +/- 0.03 pH units. Saying this another way, a very professional laboratory with very strict standards has a measurement uncertainty of +/- 0.03 pH units when measuring the pH of a substance that is much easier to measure the pH of than saltwater. Just sayin'.....

 

[reef_ranch]

So I can pick one up at BRS then, right?

 

[Elementalj]

So I can pick one up at BRS then, right?

I just LOLOLOLOL

 

[JonasRoman]

How accurate are the lab grade pH meters available in the hobby?

After posting that I did a bit of research, looks like our probes are accurate only to .1 pH, not nearly good enough to use to calculate Alk (even assuming an accurate CO2 measuring device).

I agree with Jim.

The so called lab-grade in our hobby is more concerning longer intervall between calibration because of more junctions, and sometimes more than one reference-electrode (more Ag), etc thus last longer.
Jim or other, please correct me If i am wrong

 

[JonasRoman]

How accurate are the lab grade pH meters available in the hobby?

After posting that I did a bit of research, looks like our probes are accurate only to .1 pH, not nearly good enough to use to calculate Alk (even assuming an accurate CO2 measuring device).

yes, definitely you are correct.

 

[SCSI]

I better start posting something on R2R to begin to earn some credibility. Considering all the relevant folks are on this thread already (hello Randy, Jim and Jonas).

I've been working on alkalinity machines on and off for 10 years or so but didn't brag much about the progress. For me it became a hobby of its own and looks like 2017 is a going to be an interesting year for us all.

Now, here's what I wanted to say on the topic of pCO2/pH alkalinity method. First of all, Randy is definitely correct saying that the method is well established and feasible provided all the variables are measured well. Jim's estimates of Alk/CO2 dependency are on par with my calculations. Jonas is right saying that CO2 determination is not cheap but it does not end there. In fact it's much more than $$$ for a decent NDIR detector...

In my experience, the biggest challenge in pCO2 determination in tank water is inherent sensor drift and calibration difficulties. Last summer I have spent several months and a few $$$$ playing with everything related to CO2 and came to realization that it's virtually impossible to build a low-cost CO2 detector which can be left for years without routine calibration. While short-term measurement repeatability can be improved by using thermal stabilization, dual-beam detectors and modern front-end circuitry, the long-term heat source and detector drift and contamination need to be dealt with by other means. In my experiments with various NDIR detectors I was able to achieve 3-5 ppm accuracy and linearity in air but measurement drift was 0.5 to 2 ppm per day! This was very close to the results I was getting from my research grade Li-cor LI-6252. In other words, some sort of automatic calibration is necessary to achieve long-term stability in the field and that is the biggest challenge. Here I'm not talking about many other factors interfering with CO2 measurements such as water vapor, gas pressure and leaks and diffusion barrier fouling.

We can use buffers to calibrate pH sensors fairly reliably and virtually eliminate effects of drift. It's not so easy with CO2. We can establish zero point rather easily by scrubbing ambient air of CO2 and use it as zero gas, but we also need a second point for calibration of the slope of the sensor (span gas). They supply calibration gas mixtures in nice shiny cylinders which cost a few hundred bucks per bottle. They also make dynamic gas blenders which cost like a BMW. I ended up building one myself just to validate linearity and stability of my detectors and it was a lot of fun. By the way, the instruction manual for Li-cor recommends to perform daily calibration in order to achieve stated performance and still it costs like a Toyota.

In conclusion, while pCO2 by itself is an important subject to measure and it's quite possible to build a device for hobby use for well under $1k, the drift is an issue. Calibration with outdoor air (400ppm) once a week/month can get us to ~10-20 ppm accuracy which is acceptable for most applications, but determination of alkalinity via pCO2 demands an order of magnitude better stability and sadly we are not there yet. I wonder how's Mindstream doing over there...

Regards,
Kirill

P.S.

Jim - congrats on the deal and best of luck getting it to market! Those (Takasago?) pinch valves look awesome in star arrangement
Jonas - it's great to see yet another machine popping up! Now from Scandinavia.
Randy - thanks for your valuable input and for having us all here in your forum.

P.P.S. I'm sure you've seen it already, but here it is again. My first prototype from 9 years ago

 

[JimWelsh]

@SCSI - It's good to see you active online again, Kirill! I followed your thread over on another reef forum that petered out some time ago. I have also looked into using NDIR CO2 sensors, but quickly came to the same conclusions you just described, and abandoned that approach before I had sunk very much money into it, because the futility of it was apparent (at least with current technology with a hobbyist budget).