I use lab grade sensor probes, mostly from “Atlas Scientific”. It will give you the most accurate and reliable readings.
The ORP sensor (know as the amperometric sensors) and EC (TDS/PPM) sensor, I strongly recommend buying quality probes from a reputable company.
For a home grower, it’s easily possible to build an advanced controller board using a $3 microcontroller like an ESP32 (a powerful beast) hooked up with $150-$450 sensor probes.
I like to add a GUI interface, utilizing a big LCD display which I can interact with and control the grow.
You can get quality high resolution 5 inch displays for around $50 and easily hook it up to a raspberry pi or ESP32.
You can also control/monitor everything through the internet, Bluetooth or WiFi from your smartphone or what not.
So here is a list of parts needed to make a quality automated home grow controller system, for monitoring, it will be based for hydroponics or aeroponics systems:
- A quality TDS/EC/PPM probe, has a whopping 10 year lifespan, for $114 from Atlas Scientific: Atlas Scientific
- You'll also need amplifier circuit, costs $11 from Atlas Scientific: Atlas Scientific
- You'll also need a Development Board for the amplifier circuit. So that the monitoring microcontroller, EC probe and the amplifier circuit can all properly and safely connect together and communicate. Costs $19 from Atlas Scientific: Atlas Scientific
- You'll also need calibrating solution to calibrate the EC probe, costs $13 from Atlas Scientific: Atlas Scientific
(Note: requires soldering the amplifier circuit to the development board. The EC probe and it's components will cost a total of $157, you're saving $48 but you could spend the $48 of a total of $205 and buy everything as a kit, all prebuilt from Atlas Scientific and avoid soldering and have a ready solution to put everything together and works out from the box. The full kit includes the EC Probe + better amplifier circuit + much better development board + Calibrating solution + extra cables: here is the link from Atlas Scientific for the kit Atlas Scientific)
- A quality ORP sensor probe, complete kit for $79 from Atlas Scientific: Atlas-Scientific
- A quality temperature probe, complete kit for $67 from Atlas Scientific: Atlas-Scientific
- A decent CO2 sensor, "Sensirion’s Photoacoustic CO2 sensor, model SCD41" has an accuracy of 40 PPM+/- with the range of 400 PPM to 4,000 PPM. It also has a built in humidity and temperature sensor which is stated on the datasheet. Costs $50 at DigiKey: DigiKey
- If you need a much higher accuracy and range for a CO2 sensor, would recommend the Atlas Scientific's "EZO-CO2" sensor, has an accuracy of 50 PPM+/- with a range of 0 PPM to 10,000 PPM. It is ready out from the box to be plugged into a microcontroller and provide CO2 readings. Costs $152 from Atlas Scientific: Atlas Scientific
- The infamous ESP32 microcontroller. You'll get 3 pieces for $16 from Amazon: Amazon
- Or if you want to solder the ESP32 to your own PCB board, you can get an ESP32 for less than $3 at AliExpress shipped from China: AliExpress
- I am assuming you have all the DuPont wires needed to connect and test everything, if not you can buy a pack of DuPont wires from Amazon: Amazon.
Well that seems like a basic and important sensors needed for a hydroponics grow system, enough to monitor and record your grow.
However this is no where near complete for a fully automated system which can automatically do: nutrient dosing, fan control, pump control, light control and etc, more components are needed, I'm keeping this post short.
It seems you are only interested with pH, TDS and CO2. So I'll give you a practical price and it will be just as good or better than the commercial stuff targeted for home growers.
pH probe= $70
Temperature probe= $67
TDS probe= $157
CO2 probe= $50
ESP32 Microcontroller= $16
That all totals out to be:
$344 
Note: You'll need few miscellaneous things like plastic housing, battery/power adapter and etc. Also need to program the ESP32 to do the monitoring and recording tasks.
The price could be reduced much more if you're designing your own PCB and integrating all these parts together. Will be complex.
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But I have a much better idea, obtaining a "Milwaukee MW804 MAX Waterproof 4-in-1 pH / EC / TDS / Temp", Pen Tester which costs $138.
Open that up and do some simple "reversing engineering" and hook it up to an ESP32, can be done easily.
Now this requires some automation to bring in some reservoir liquid sample to the Milwaukee MW804 sensor head, then have some kind of microcontroller to tell the MW804 to test the liquid sample, then extract the data needed from the MW804 to the microcontroller/database.
Lastly, need to automatically clean the MW804 sensor head with some clean distilled water. Have the waste water disposed to a waste container.
This process can be repeated automatically indefinitely, every 5 mins, 30 mins, 1 hour or 5 hours and etc.
Calibration can also be done automatically as well, requires more complexity to the physical design.
I choose the Milwaukee MW804 because it has good accuracy, reliable and best of all you can replace the sensor head when it's lifespan has reached it's limit. Thus this hacked system can be used and continued for many years.
So now the price for pH, TDS , Temperature and CO2 will cost less than $200 for a quality and reliable monitoring system.
The pros to this concept is that the pen tester probe is not always swimming in the reservoir 24/7, the pen tester will always be cleaned and calibrated automatically, so every reading should provide very accurate results and lifespan will last much longer and is cheaper than what we currently have today in the market for quality automation solutions. Maybe other lab grade "4-in-1" pen testers can be used.
If anyone is interested with this concept, I think I'll take the liberty and actually make a tutorial on how to build one.
I personally rather spend the extra money for the Atlas Scientific probes, simply because they work and save so much time from brainstorming for a cheaper viable alternative solution
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The only reason why the commercial stuff costs thousands of dollars is because they have to pay good money for hardware and software engineers to make these boards and also have the boards sent to few separate authoritative agencies (which costs a lot of money) so that it would be tested and certified to be labeled in compliance with mandatory legal regulations and safety codes to be a viable product for the end customers.
