Do I need to know absolute pH if all I care about is pH drop?

[gjcarew]

I'm lazy and don't want to calibrate my pH meter. I know that means the pH measurement will be off, but wouldn't it be off by the same amount every time? So if all I care about is relative pH change for CO2 (as in dropping pH by 1) do I really need to calibrate my pH meter first, or can I just use it as is?

 

[Yugang]

don't want to calibrate my pH meter

A fixed drop in pH results in multiplying CO2 ppm by a certain factor. This is the nature of the logartithmic relationship (inverse of exponential) between pH and CO2 ppm. For example a 1 pH drop will bring an initial 2 ppm CO2 to 20 ppm, or an initial 10 ppm to 100 ppm. The risk with not calibrating is that you don't know your initial CO2 ppm, and this may have a big impact, as it multiplies by 10, on your CO2 ppm after a 1 pH drop.

As a tank will not be fully degassed early in the morning, it is not impossible that still 10 ppm CO2 remains, and with a 1 pH drop you would expose your livestock to a likely lethal 100 ppm CO2 while you may have thought you brought it from 3 to 30 ppm.

Now if the argument is that you will always take a measurement on a fully degassed sample ...

CO2 pH reference - quick degassing

CO2 users may measure the pH drop, and use that as an estimation for the CO2 ppm. The reference level should be the pH of fully outgassed water (in equilibrium with ambient air), and is usually checked on a glass of tank water that has been left open for about 24-36 hrs. That's a long time. I...

scapecrunch.com

.... then it is important to realise that pH calibration also includes the slope of the curve. So what you assume a 1 pH drop measured on your device does one time, may be what a 1.1 pH drop did when you calibrated a rather long time ago.

I'm lazy

Then you may consider to use a reactor in overflow mode, as that will stabilise your CO2 independent on water chemistry (pH) or instabilities of your regulator. My pH meter broke down months ago, and I never felt the need to buy a new one as I am as lazy as you and don't want to care for my probe calibration, KH measurement or pH/KH/CO2 tables anymore.

 

[gjcarew]

If I'm understanding correctly, are you saying that if my pH meter thinks my starting pH is (let's say) 5.5, it will need to register 10x more OH- ions to see a drop to 4.5 pH than if it thought the initial value were 6.5 and the pH was decreasing to 5.5?

 

[Yugang]

If I'm understanding correctly, are you saying that if my pH meter thinks my starting pH is (let's say) 5.5, it will need to register 10x more OH- ions to see a drop to 4.5 pH than if it thought the initial value were 6.5 and the pH was decreasing to 5.5?

What I am saying is that pH is only a proxy for CO2 ppm. It is only reasonably reliable if

1. you measure pH correctly (ie calibrated instrument)
2. you know your KH, or water chemistry (pH drop method can help mitigate for unknowns)
3. trust pH/KH/CO2 tables (only work when you know your stable KH)

Indeed a pH drop from 5.5 to 4.5 corresponds to 10 fold increase in CO2 ppm, as well as 6.5 to 5.5 corresponds to a 10 fold increase but just from a different basis.

Maybe it is helpful and practical to share my own experience, I used a pH controller for many years. From experience I found that my pH probe drifts just a little, and I could get away with monthly calibration that took me perhaps 20 minutes. The other thing is that I change 65-70% water weekly, so my tank's chemistry would never drift far from what comes in from the tap. So I knew pretty well what I wanted my target pH to be, and just kept it stable over time. Sometimes, out of pure curiosity I would check my degassed water pH (which was always the same within a few hundredths), and could look up the estimated CO2 ppm I was working with.

In summary I believe it is risky to not calibrate a pH probe, especially if you don't know yet how far it could drift since last calibration. Also Ph probes can have unexpected behaviours due to electrical issues or aging. Then pH drop method only makes sense if you know the starting pH, and mistakes are quite unforgiving as any deviation multiplies rather than adds up. When you're too lazy for calibration or monitoring water chemistry (including KH drifts) then I would be cautious not using this technology and just stick with more basic tools.,

 

[gjcarew]

Thank you! I appreciate your expertise

 

[Zeus.]

do I really need to calibrate my pH meter

Yes and No.

Yes, you should calibrate to start off with, how often you need to calibrate it depends on many factors, like quality off the probe and where it is stored between use.

No, If you are also using a DC (Drop Checker) and watching your fish during the pH drop the need for calibration is reduced IMO/IME

pH controller

Used once for quite some time, there good but found that through out the CO2 period the pH varied as it switched the CO2 on/off. So the need to calibrate the probe for a pH controller is higher, after all its in the tank all the time.
I found doing a pH profile and controlling the Injection period gave me a more stable pH during the CO2 period. I did have a PLC and duel CO2 injection. Twin injection for pH drop then single injection for photo period. Made getting a stable pH easier IMO/IME

you know your KH, or water chemistry

measured it once never found it helpful

rust pH/KH/CO2 tables

Found them a waste of my time.

I change 65-70% water weekly, so my tank's chemistry would never drift far from what comes in from the tap.

But what's in the Tap ?. where I use to Live in the UK the tap in that location varied over the year, it was very hard water, Nitrates increased when farmers was fertilising their crops, the water company 'blended the water at certain times of year to meet European Laws to keep it within certain parameters. So what's actually in the tap changed with the seasons.

Obviously this will vary in all parts of the world, the North West of UK gets it water (soft) from the lake district (Big lakes with good rainfall), Folk who use the tap water often find good results in their tanks easier IMO.

stable KH

The only way to get a stable kH IMO is to use RO water, remineralize it and use Inert rocks. Even the aquasoil (AS) last longer (As with hard/very Hard water AS Exchange capacity soon runs out as Ion and Cations with a higher bonding affinity render the AS inactive.)

 

[Burr740]

I'm lazy and don't want to calibrate my pH meter. I know that means the pH measurement will be off, but wouldn't it be off by the same amount every time? So if all I care about is relative pH change for CO2 (as in dropping pH by 1) do I really need to calibrate my pH meter first, or can I just use it as is?

You can still accurately measure the drop itself, so yes. But the problem is when one gets "off" it doesnt stay at the same degree of "off" very long.

The baseline degassed number you come up with now is only good as long as the pen remains off to the same degree. Soon as it gets a little more off the degassed number is no longer good, because it wouldnt be the same if you checked it again

And since there's no way to know how off one is, or when it becomes more off, besides sticking it in some calibrateing solution. Its best to keep it calibrated all the time so that youre always working with the right degassed value

Short answer is still yes. Just know that youll need to frequently re-check the degassed number too

^Good stuff from the guys above btw

 

[Zeus.]

did this little article elsewhere
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Dialling in the CO2 injection Rate and CO2 Profiles​

Why do we need a stable Carbon Dioxide (CO2) level?
A stable Carbon Dioxide (CO2) level or concentration is key to successfully growing plants in a CO2 injected tank. The CO2 concentration needs to be stable throughout the photoperiod, usually anything from 6 to 8 hours long, and consistent from one day to the next. If the CO2 level is not stable during this period, plants will have to constantly adapt and may not grow as well. A fluctuating CO2 concentration may also give algae an advantage and it could start to become a problem.

Stabilising CO2 during the photoperiod can be tricky and time consuming, and can sometimes take weeks of trial and error, especially for folk new to CO2 injection. Also key to success is good water flow. This is often overlooked but is essential to distribute CO2 evenly throughout the tank. CO2 travels 10,000 times slower in water than in air, so without good flow CO2 concentration will not be the same throughout the tank. Plants waving gently in the flow is a good indicator that it is adequate. Good surface agitation is also essential. This ensures adequate levels of gas exchange and helps to keep the tank water oxygenated.

What level of CO2 to aim for?
Many folk aim for a CO2 concentration of 30ppm. This is regarded as the upper safe limit for livestock. A lower CO2 concentration than 30ppm CO2 can also greatly benefit plants and is perhaps kinder to livestock, so don’t feel the need to hit the 30ppm mark. When tank water has 30ppm of CO2, the pH will be approximately 1.0 unit lower than it was before CO2 was injected. To find a tank’s CO2 level or baseline before injection starts, take a clean glass of tank water and leave it at room temp for 24 hrs after which take a pH reading. This will be the baseline or degassed pH of your tank water. For example, if the pH is 7.2 after fully degassing, and your target CO2 concentration is 30ppm then the target pH will be 6.2.

Watch your fish when Injecting CO2
It is best to adjust CO2 levels before adding any livestock to avoid accidentally stressing fish and other aquatic critters. If this is not possible it is essential to keep a close eye on your fish and shrimp etc when first injecting CO2 especially if you are new to the concept. Watch for any signs of abnormal behaviour. If the fish come to the surface and start gasping for air or show any other signs of distress, stop CO2 injection immediately. If this happens a large water change will be necessary as a temporary fix to reduce CO2 concentration and hopefully introduce more oxygen. Next check your equipment and don’t start CO2 injection again until you are confident the cause has been eliminated. If you are unable to get to the bottom of the problem seek advice or guidance, never be afraid to ask.

Injecting CO2
After starting to inject CO2 the pH of the tank water will drop quickly at first, then slowly until it reaches equilibrium with CO2 uptake by plants and CO2 loss from the water surface. If the injection rate is too high, you will overshoot your target CO2/pH which could be fatal to the tanks inhabitants so small steps at first are essential to avoid this. If the injection rate is too low, your tank water may take many hours to reach the target CO2 concentration and may not hold that concentration and remain stable once the lights come on. It’s often a case of trial and error, taking the greatest care to get it right especially when livestock is present.

Drop Checker (DC)
A drop Checker (DC) is a great tool for checking the concentration of CO2 in tank water. A working DC is filled with an indicator solution, usually a reagent in deionised water calibrated to a value of 4dkH. This solution changes colour according to the concentration of CO2. The colour will change from blue when tank water is fully degassed of CO2, to lime green when tank water has reached a CO2 concentration of 30ppm. DC’s can often change colour slowly; the rate varies with design and can take unto 2hrs. A DC can be used by itself to gauge CO2 concentration, however for the purposes of establishing a stable CO2 concentration it’s easier to use the combination of a pH profile and a DC

For further information on measurement using a drop checker, see Clive’s excellent article linked below

https://www.ukaps.org/forum/pages/co2-measurement-using-a-drop-checker/

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Time it takes to get your target CO2 level
The time it takes to reach a CO2 concentration of 30ppm varies with tank size and injection method. About 2hrs is normal for a 50 litre tank, but bigger tanks can take 4 to 5 hours plus.

Doing a pH profile
Doing a pH profile is a great way to achieve a stable CO2 concentration and is relatively simple to do, ‘if’ a little time consuming. Firstly, decide on your target CO2 level. Then find the baseline or degassed pH of your tank water as outlined above under the heading - What level of CO2 to aim for. Set the injection rate, or Bubbles Per Second (BPS) using a bubble counter, to a suitable level for your tanks size and CO2 setup. Take the pH of the tank water as CO2 injection starts or just before.

Thereafter, take a pH reading every 30 mins and note it down. As the pH change slows down start to take the pH a little more often. If the pH drops more than 1.0 unit reduce the injection rate (BPS). If it is taking too long to get the desired pH drop increase the BPS. Once the target pH is reached, turn the lights on, then keep taking the pH every 30 mins, repeating the BPS adjustment to keep the pH as stable as possible. After 4 to 5 hours of lights on turn the CO2 off and call it a day.

Next day take the pH as CO2 comes on and thereafter every 30 mins and note the time. As the target pH is reached turn the lights on and again note the time. Some fine tuning will probably still be needed to get a stable pH (I personally call it stable when there is less than 0.1 pH drift from lights on till CO2 off). So it’s a case of ‘rinse repeat’; check the pH and adjust BPS if required to suit, until you have a stable pH from lights on and all the way through to lights off. Then the hard work is over. DO NOT adjust your BPS rate again to try and get the pH to drop faster, it is not possible. The time it takes is the time it takes. You will have already got this time noted down. It may be 2hrs it may be 4 to 5hrs with bigger tanks. The length of time a pH drop takes is always worth a final check. You will already have a rough idea from the previous days pH profiling but some further fine tuning may be required.

The whole process can take days or even weeks to perfect, especially if you need to increase the water flow to ensure adequate CO2 distribution throughout the tank, or to increase surface agitation and therefore gas exchange. Other commitments, such as work and household chores, can interfere with the process so make sure you have at least a few days free to start with, any distraction may prove fatal to fish and other aquarium livestock, again another reason why it’s best to dial CO2 in before adding livestock.

If in doubt ask
Finally, there’s no such thing as a stupid question, especially where CO2 injection and the health of fish and other livestock is concerned. So if in doubt always ask. There are many experienced and knowledgeable members who will be only too happy to help.

Zeus.

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Which my help or provide topics of interest/discussion

 

[Yugang]

What I am saying is that pH is only a proxy for CO2 ppm. It is only reasonably reliable if

1. you measure pH correctly (ie calibrated instrument)
2. you know your KH, or water chemistry (pH drop method can help mitigate for unknowns)
3. trust pH/KH/CO2 tables (only work when you know your stable KH)

measured it once never found it helpful

I assume you mean that a detailed water chemistry test did not help you in the context of fertilisation?
In the context of CO2, are you saying you don't want to know KH when using pH as a proxy for pH? I agree that for some specific circumstances you don't need to, or can have a workaround, but in general kH knowledge is essential for relating CO2 to pH.

Found them a waste of my time.

Why? For many people, especially beginners these table have done a lot of good for decades. Yes, one needs to be cautious, just like with all the alternatives, but after initially being sceptical and researching the origins of these tables I actually think they are pretty good.

We may want to be more specific whether we are giving advice for beginners or advanced users.

• Beginners can do a good job with pH probes, and also controllers do generally work well, but at a minimum need to check KH and KH stability in the tank. For them CO2/pH/KH tables will generally work well, but again KH measurement is necessary.
• Advanced users like to emphasise the limitations and need to be cautious with pH measurement or CO2/pH/KH tables, but this is not to say that these methods haven't had, and still have value for many thousands of users.

I found doing a pH profile and controlling the Injection period gave me a more stable pH during the CO2 period.

How did you keep your injection rate constant over time? And was that indeed better than a good pH controller would do?

 

[Art]

Great question and great discussion. This is a topic that I think does need more conversation and I also think there are many ways to achieve what works for you.

For me, I've settled on the following. Would love your comments on it as I'm always open to improving.

Basic concept - use a pH controller to deliver the needed CO2 in the tank. The controller also serves as a safety mechanism to shut off CO2 if anything goes wrong and alerts me.

What is needed - a KH reading of zero or close to it, a pH controller and a handheld pH meter.

The process - as I run with zero KH, I assume pH and CO2 are correlated similar to the infamous table we all know. I calibrate the controller's probe and the handheld. When they align, I use them to control the CO2 in the tank shooting for about a 1.3 pH drop from start through finish of the light period. The controller is set to automatically keep the pH drop steady throughout the period and the Apex Neptune system allows for that without too much on and off switching.

I calibrate the handheld every week or two because it's easy. I then check the controller's probe against my handheld to make sure they still align. It usually works well for a few weeks before the controller's probe starts to drift. Once the drift becomes material, I calibrate the controller's probe.

The above system has worked well for me and I don't use drop checkers or other means to determine CO2. Following @GreggZ, I do have an airstone that comes on for 5 minutes every hour around the clock just to make sure there is gas exchange at the surface and O2 levels stay high.