Issues with Input Configuration: ADS1115 Generic Analog pH/EC

Hey Guys,

I am experiencing issues with a low-cost pH probe connected to an ADS1115. The probe has been calibrated with two points and verified. However, I am encountering problems with the pH value calculations, which yield implausible results.

I am using the Input module ADS1115 Generic Analog pH/EC, with EC and temperature compensation currently disabled.

When I deactivate and reactivate the input, the displayed pH value is correct for a while, but then it starts making large jumps. Suddenly, it shows a pH of around 20 or a pH of about -8. I’ve tried several things but haven’t found a solution.

Here is a picture showing the voltage at A0, where the pH probe is connected, in relation to the pH value. The power supply might not be perfectly optimal as there is some ripple and very slight fluctuations, but these can’t be the cause of the problem. The chart shows that the fluctuations at A0 are at most about 0.05V, but the pH value moves into an impossible range. When the input is restarted, I get plausible fluctuations, but after some time, large jumps in the pH value occur again.

Does anyone know what might be causing this? There is nothing unusual in my log related to the pH measurement.

Is your power source isolated? Do you have any other electrical devices touching the water?

No, I haven’t implemented an isolated power supply or signal transmission yet. That’s definitely something I still need to add. Currently, I have everything set up in a test environment. The pH probe is placed in a plastic container with water, and nothing else is in there.

When you look at the voltage trend compared to the pH trend, you’ll notice that at times the analog input has the “same” voltage but a completely different pH value.

I’m not sure what caused it, but my pH fluctuations have stopped. I had accidentally set an input to update once per second, which might have been causing the issue. However, the main thing is that the problem has been resolved for several days now. My entire setup is up and running.Additionally, all my analog signals are now galvanically isolated.

The EC probe, however, cannot be calibrated correctly. I suspect the issue is that my cheap EC probe cannot accurately measure a conductivity of 3.0 mS (calibration solution) as it is below its range. It is a TDS meter from DFRobot, officially measuring up to 1000 ppm, which is below EC 3.0. For now, I’m using an external meter to manually adjust the EC value.

Which EC probes do you use? I don’t want to spend too much money and think that less precise probes would be sufficient for my needs. I’m looking to spend around 50€.

You really don’t need to constantly measure EC or TDS.
The only thing that measurement is telling you is if your nutrient solution is too concentrated.
If you follow the nutrient manufacturer’s mixing ratios on whatever product you are using, then you shouldn’t ever have to worry about the EC or TDS levels being too high.
The only time you may want to test the EC or TDS is whenever you mix a new tank of nutrients just to make sure you haven’t gone over the nutrient manufacturer’s recommended TDS.
You will also need to top-off the water level in your nutrient tank on a daily basis to prevent the EC levels from getting too high.
As water evaporates, it leaves any dissolved substances behind… this means that usually you will almost always see constantly rising EC or TDS levels over time… the way to prevent this is to simply re-fill your nutrient reservoir with fresh water every day to keep the solution from becoming too concentrated.
You really don’t need a meter to tell you this.
NEVER add more nutrients to an already mixed reservoir! You will completely throw off the correct balance of nutrients and cause TDS levels to rise above the nutrient manufacturer’s recommended levels!
Instead of constantly monitoring EC/TDS levels with a meter, it would be better to install a water level sensor on your nutrient reservoir and an electrically controlled valve that would automatically refill your nutrient reservoir when the water level drops below a certain point.

I manage water refilling using an optical water level sensor that controls a small DC pump. Additionally, I have installed an overflow float switch as a backup in case the main sensor fails, and another float switch in the deionized water reservoir to detect if the tank is empty. However, I have not yet integrated these sensors into the control system.

I’ve been learning about this field. I’m new to plant cultivation and hydroponics, but I’ve been reading and researching a bit over the past few months. One thing that confuses me is the advice not to add any further nutrients to a nutrient solution. My fertilizer manufacturer recommends daily fertilization and to prepare a new solution every 14 days.

I also already have a flow sensor installed for measuring the amount of deionized water added to the hydroponic tank. My plan for later was to use the flow sensor to measure the amount of deionized water added to the hydroponic tank and then adjust the fertilization accordingly. Additionally, I read that adjusting the pH increases the conductivity, and I was concerned that this might significantly affect the EC value. However, I have no idea how much I need to adjust the pH.

Even though I don’t necessarily need it, I would like to have an EC probe for continuous measurement. It’s a bit of a tech obsession thing for me. I’m considering buying the DFROBOT DFR0300
Do you not continuously measure the EC value?

When you mix a new tank of nutrients, they are all at specific ratios as recommended by the nutrient manufacturer. If you add more nutrients to the already balanced mix ratios in your tank, then you are going to end up with mix ratios above what the manufacturer recommends (higher TDS) and you will most likely cause a nutrient overload which will harm your plants (over-fertilization).

There is absolutely no reason to spend extra money on deionized water for your hydroponics system. It is not going to have any noticeable effect on the final yields of your plants! Tap water is perfectly fine. In fact, tap water usually has enough calcium and magnesium in it already that you don’t have to add Cal-Mag to your nutrient regimen. Tell me how many farmers or gardeners you know that use deionized or RO water on their outdoor crops? The answer is none, the costs of filtering all the irrigation water would make growing the crops unaffordable to the farmer as well as too expensive for most consumers to buy. I live where the water is very “hard” (high mineral content) and I have no problems using it in my hydroponics or in my outdoor garden. I have been outdoor gardening for over 40 years and growing vegetables and cannabis indoors in hydroponics for about 10 years and I have always used tap water with no problems.

That being said, I used to test my EC/TDS every time I mixed a new tank of nutrients. Because there is an already high mineral TDS in my tap water, I measured the TDS of the water before I added nutrients and after to make sure I wasn’t going over the recommended TDS levels. Even with our hard water, I never saw TDS levels go above the nutrient manufacturer’s recommended levels… as long as I followed the exact mixing instructions for the nutrients! This is why I no longer bother with measuring the EC/TDS, even when I mix a new tank… because as long as I keep following the exact same mix ratios as recommended by the nutrient manufacturer, I know that my TDS levels are never going to be too high. In all the time I have been growing in hydroponics, I have never seen any indications of nutrient-excess in my plants. This means I am not going over the recommended TDS levels.

No, there is no need. You only need to check it once after you mix a new tank of nutrients. Most EC meters are not designed for continuous use, the minerals in the nutrients will cause the sensors in these meters to corrode rather quickly under continuous use (even if you are using a sampling box). If you look at the warning on the DFROBOT web page for that meter, it tells you this…

You will end up having to replace that meter at least once a year, if not more… not to mention the time and cost of having to re-calibrate it on a regular basis (which kind of defeats the purpose of having an automation system in the first place).
You are only wasting your time and money trying to continuously measure EC/TDS.

Yes, any time you dissolve something in water, you are going to increase it’s electrical conductivity, which is another reason it’s a waste of time trying to continuously measure the EC since it will constantly fluctuate even with temperature changes between day and night. Stop worrying about EC/TDS and worry about keeping your pH levels correct. PH is far more important to plants than EC/TDS. Typically, you will want to maintain a pH of about 6.0 in your nutrient tank to prevent nutrient lockout. Since I have to top-off my tank with fresh water manually, that is also when I check my pH levels… about once every other day. It takes less than a minute to test the pH using test strips or test drops and then I add the appropriate amount of pH up or down (again, no expensive pH sensor needed). Since I am using a 12 gallon tank, I always know exactly how much pH up or down I need to raise or lower the pH by one point (about 1 tablespoon in my case). This is why I don’t need an expensive pH meter or a dosing system either. A pH dosing system is just an extra cost and takes more time to calibrate than it does to simply measure the pH by hand every 2 days. If you are doing everything right, your pH levels should be fairly stable and shouldn’t need to be adjusted too often. Plants take at least 5-7 days to noticeably respond to fluctuations in the nutrient solution, this is another reason it is not necessary to constantly monitor EC/TDS or pH… your plants aren’t going to die if these measurements go out of range for a day or 2.

Don’t make things more complicated than they need to be. Plants don’t really need that much help or attention to grow happily as long as they are getting the appropriate amount of light, water, and nutrients. The point of an automation system is to do the work for you, not create more work for you.
Keep it simple :slightly_smiling_face: